Method and apparatus for resolving divergent paths in graphical programming environments6425121Abstract The invention relates to a mechanism for detecting forks within data flow diagrams corresponding to application programs and for controlling the execution of such application programs. The flow diagram is preferably formed by interconnecting a plurality of symbolic representations of program objects with a plurality of wire constructs. The symbolic representations correspond to program objects having input properties and configured to execute pre-defined functions based on the values of its input properties. The flow diagram preferably represents a logical flow of information used to set the input property values of the program objects. According to the invention, the input properties of the program objects are first invalidated. Next, information is allowed to flow along at least a portion of the diagram, thereby setting one or more of the program objects' input property values. In response to having its input property value set, the program object validates the respective input property. The program objects are further configured to wait until all of their input properties have been validated before executing their associated functions. Claims What is claimed is: Description CROSS-REFERENCE TO RELATED APPLICATIONS
PROGRAM OBJECT PROPERTIES
Data
Name Type Description
CancelBlock Boolean If set, prevents program object from executing
or from completing execution of its function.
ControlIn Boolean When used, controls when program object
begins execution of its function.
InvalidProperty Integer Invalidates an identified property of the
program object in order to ensure orderly
execution.
PROGRAM OBJECT EVENTS
Name Description
RunBlock Occurs when program object is about to commence
executing its corresponding function.
Invalidate Occurs when program object is about to up-date one or
Group more of its properties as a result of executing its
corresponding function.
DataReady Occurs after program object has up-dated one or more of
its properties as a result of executing its corresponding
function.
RateReady Issued by program objects that perform scanning operations
upon successful completion of a scan.
StatusReady For program objects that operate in one or more modes or
states, this event occurs repeatedly while the program object
executes its corresponding function.
ControlOut Occurs when program object has completed execution.
ErrorOut Occurs if program object generated an error during
execution and may contain an identification of the type
of error that was generated. It may also occur to indicate
that no error condition was generated during execution.
where Boolean means that the property may be set to True or False and Integer refers to any integer. The GUI 400 may also include additional windows. For example, GUI 400 may include a project explorer window 416, which provides a hierarchical view of the current project. A project simply refers to the collection of files (e.g., form files, binary data files, class module files, etc.) and objects associated with the application program being developed. GUI 400 may also include a properties window 418 that displays the properties of a selected program object residing in the form window 404. The properties window 418 includes a pull-down object list 420, that contains a list of all of the program objects currently residing in the form window 404, and a property window 422, that is divided into two columns: a name column 422a and a current value column 422b. The name column 422a identifies all of the properties associated with the program object selected in the object list 420, while the current value column 422b shows the values that are currently associated with those properties. To generate an application program, the developer selects one or more icons preferably from the designer toolbar 414 that perform requisite functionality for carrying out the tasks of the application program. In response, the program-development environment 310 places corresponding symbols in the designer window 406. The developer then graphically links these symbolic representations by drawing "wires" between them in order to create a data and/or execution control flow diagram. He or she will typically do this by using the mouse 230 (FIG. 2) or similar input device to cause the cursor 240 to move from one symbol to the next, although other graphical or even keyboard inputs may be used to perform the "graphical input". In response, the graphical designer system 314 of the program-development environment 310 generates an event handler procedure to be run as part of the application program being developed. In accordance with the invention, the development environment 310 also includes in the same resultant application program other event handlers, which the developer optionally specifies textually by entering commands and other information in a code window that the development environment 310 also provides on GUI 400. That is, the development environment 310 gives the developer the option of using textual inputs in order to specify event handlers that might otherwise be impossible or more difficult to represent graphically. Suppose, for example, that the developer wishes to create a simple software program in which the position of a vertical scroll bar is displayed in a label. From the User Interface designer sub-toolbar 414c, the developer first selects the vertical scroll bar icon 424. To select icon 424, the developer uses the mouse 230 (FIG. 2) to position the pointer 240 over the vertical scroll bar icon 424 and activates (e.g., "clicks") the mouse button 234. This mouse click is a conventional windows event that is received by the operating system 302 (FIG. 3) in a conventional manner. Since the mouse click occurred over the designer window 406, operating system 302 passes this window event to the graphical designer system 314 of the program-development environment 310 by a communication mechanism represented by arrow 326, and the designer system 314 treats the windows is event as a selection of the vertical scroll bar class by the developer. As shown in FIG. 4B, in response to the selection of icon 424 from the User Interface designer toolbar 414c, the graphical designer system 314 causes a symbolic representation 426 of the program object corresponding to the vertical scroll bar class to be displayed in the designer window 406. The designer system 314 also issues a call to the visual programming system 312 through its extensibility object 316 as represented by the communication mechanism of arrow 320. This call directs the visual programming system 312 to instantiate a program object from the vertical scroll bar class and add that program object to the container application represented by the form window 404. That is, form window 404 may maintain a linked list of pointers to program objects that are considered to "belong" to the form, and in this list is placed a pointer to the vertical scroll bar program object that was instantiated. Since the vertical scroll bar is a user interface element, the visual programming system 312 also causes a vertical scroll bar image 428 to appear in the user form object 408. Vertical scroll bar image 428 basically corresponds to the way in which the vertical scroll bar user element will appear in the respective user interface at run-time of the application program being created. Vertical scroll bar image 428 may be moved and/or re-sized by the developer in a conventional manner. As part of the process of adding a program object to the form window 404, the visual programming system 312 also assigns a name to that program object. The name may consist of the object's class followed by an integer, e.g., VScrollBar1 for the first vertical scroll bar added to form window 404. The name uniquely identifies the program object within the form 408. Upon adding the program object to the form window 404, the visual programming system 312 preferably returns the assigned name to designer system 314 by a communication mechanism represented by arrow 318. The program-development environment 310 may then display a name 426a as part of the symbolic representation 426 of the object in the designer window 406. The name displayed in designer window 406, e.g., Form1.VScrollBar1, may be derived by concatenating the name of the program object, e.g., VScrollBar1, with the name of the form window in which it resides, e.g., Form1. As indicated, the symbolic representations appearing in designer window 406 are used by the developer to create a data and/or execution control flow diagram that logically corresponds to the application program being developed. To facilitate the generation of such diagrams and the creation of corresponding event handlers by the program-development environment 310, each symbolic representation in designer window 406 preferably includes one or more terminals disposed about it. These terminals, moreover, are associated with some pre-defined combination of the properties, methods and/or events of the respective program object that is symbolically represented. Vertical scroll bar 426, for example, has four terminals 430a-d. In order to facilitate a generally left to right data flow direction and a top to bottom execution control flow direction, the terminals of all symbolic representations appearing within the designer window 406 preferably conform to the following general rules. Terminals on the left side of a given symbolic representation, such as terminal 430a of vertical scroll bar 426, preferably correspond to a property used as an input by the respective program object. Terminals on the right side of a symbolic representation, such as terminal 430c of vertical scroll bar 426, preferably correspond to (i) an optional property generated as an output and (ii) an event of the respective program object. Terminals on the top of a symbolic representation, such as terminal 430b preferably correspond to a property which, when changed to a new value, triggers execution of the respective program object, and terminals on the bottom of a symbol, such as terminal 430d of vertical scroll bar 426 preferably correspond to an event that occurs when the respective program object has completed execution of its respective function. The vertical scroll bar program object, for example, has a plurality of pre-defined properties, methods and events. In particular, the properties of the vertical scroll bar program object include: Enabled, Height, Width, Minimum, Maximum, Value, etc. The methods associated with the vertical scroll bar program object include Move, Drag, Set-Focus, ShowHelp, Refresh, etc. The events associated with the vertical scroll bar program object include RunBlock, DataReady, ControlOut, etc. Terminal 430a at symbol 426 is preferably associated with the vertical scroll bar's Value property. Terminal 430b is associated with the scroll bar's ControlIn property. is Terminal 430c is associated with the vertical scroll's Value property and its DataReady event. Terminal 430d is associated with the object's ControlOut event. The association of properties and events to terminals is preferably maintained in a plurality of terminal data structures stored at memory 214 or 220. In particular, for each type or class of program object represented by an icon on the designer toolbar 414, there are one or more corresponding terminal data structures, depending on the number of terminals supported by the respective program object class. FIG. 5 is a highly schematic block diagram of a preferred terminal data structure 500. The terminal data structure 500 has at least four fields. A first field 502 preferably contains the name of the event, if any, that is associated with the particular terminal. A second field 504 preferably contains the name of the property, if any, that is associated with the particular terminal. If there is no event or property associated with the given terminal, then respective field 502 or 504 is set to null or de-asserted. A third field 506 preferably contains a code that identifies the particular type of terminal. In the illustrative embodiment, there are four types of terminals: data input, data output, control input and control output, and each type has a corresponding code. To the extent the data structure 500 corresponds to a data output type, a fourth field 508 is preferably used to store a group identifier. For a given type or program object class, the group identifier associates multiple data output type terminals whose corresponding properties are related to one another. For example, a joy stick object may have separate data output terminals for its "x" and "y" locations. Nevertheless, subsequent program objects should probably treat these two values as a single data point. Accordingly, the data output terminals associated with joy stick's "x" and "y" locations would preferably have the same group identifier. A fifth field 510 preferably contains a tool tip. A tool tip is a piece of descriptive text which is displayed to the developer when the cursor lingers over the respective terminal (e.g., "control input", "error output", and so on). The program-development environment 310 preferably maintains or otherwise has access to pointers to these various terminal data structures 500 within memory 214 (FIG. 2) (e.g., as a linked list). The pointers, moreover, may be mapped by the program-development environment to the names of the corresponding object classes so that, given the name of some object class, the program-development environment 310 can access the terminal data structures for each control or program object that has been instantiated from that class. Symbolic representations appearing in the designer window 406, including the terminals, are preferably generated by the program-development environment 310 from respective bit maps stored in one or more image files within memory 214 (FIG. 2). The program-development environment 310 preferably maintains an association of bit maps to icons on the designer toolbar 414 so that when a developer selects a particular icon, the program-development environment 310 can direct the window manager 306 to draw the corresponding image from the appropriate bit map. Symbolic representations can also be moved about the designer window 406 by dragging them around with the mouse 230. The developer then selects the next program object or control for use in the application program being created. Suppose that the developer selects the label icon 432 (FIG. 4B) from the User Interface sub-toolbar 414c. As shown in FIG. 4C, the program-development environment 310, in response, causes a symbolic representation 434 of a label program object to appear in designer window 406. Symbolic representation 434 also includes a plurality of terminals 436a-c. The program-development environment 310 additionally directs the visual programming system 304 to add a label program object to form window 404. Since the label program object is also a user interface element, like the vertical scroll bar, the visual programming system 304 additionally causes a label image 438 to be drawn on the user form object 408. The label program object has its own pre-defined properties, methods and events. For example, the properties of the label program object include Height, Visible, Font, BackColor, Caption, ControlIn, CancelBlock, etc. Its events include RunBlock, ControlOut, etc. Data input terminal 436a of symbol 434, moreover, is preferably associated with the label's Caption property. Terminal 436b is associated with the ControlIn property, and terminal 436c is associated with the ControlOut event. Note that symbol 434 does not have any data output terminals. Generation of Event-Handler Code Through Graphical Inputs At this point, the developer has two program objects residing in the form window 404. With the prior art systems, such as the Visual Basic.RTM. programming system from is Microsoft Corporation, the developer would now have to write one or more textual event handlers in order to have the position of the vertical scroll bar displayed in the label. As described above, the need to learn the keywords and syntax governing such textual event handlers has been a drawback to the use of Visual Basic by non-programmers, including scientists and engineers. With the program-development environment 310 of the present invention, the developer may cause the development environment 310 to generate corresponding handler procedure by simply graphically linking the symbolic representations of the program objects in the designer window 406 with one or more novel wire constructs. The developer need not generate any text-based code at all. Unlike the prior-art systems that only enable the user to graphically provide event handlers, though, the program-development environment 310 of the present invention also affords the developer the ability to provide or modify event handlers textually. It thereby frees the developer of the constraints and limitations imposed by such prior-art graphical programming tools. To cause the position of the vertical scroll bar image 428 to be displayed in the label image 438 at application run-time, the developer graphically links the symbolic representation 426 of the vertical scroll bar program object to the symbolic representation 434 of the label program object using a wire construct, rather than writing a textual event handler. To connect symbols 426, 434 with a wire construct, the developer moves the cursor 240 (FIG. 2) to terminal 430c (FIG. 4C) at symbol 426 using the mouse 230. As described above, terminal 430c is associated with both the DataReady event and the Value property of the respective vertical scroll bar program object, i.e., VScrollBar1, which resides on the form window 404. With the cursor 240 over terminal 430c, the developer preferably executes a mouse click using mouse button 234. Since this mouse click occurred in the designer window 406, the operating system 302 (FIG. 3) passes the respective windows event to the designer system 314 by the communications mechanism represented by arrow 326. In response, the designer system 314 directs the operating system 302 to switch the mouse 230 from "cursor mode" to "line drawing mode" through a call via arrow 328. In particular, designer system 314 directs the operating system 302 to modify the appearance of the cursor 240 and to begin tracing subsequent mouse 230 movement with a line, whose first end is anchored to terminal 430c. Thus, as the developer drags the mouse 230 away from symbolic representation 426, a line emanates from terminal 430c following the movement of the mouse 230. The developer preferably extends this line to terminal 436a of symbolic representation 434, which corresponds to label program object Label1 residing on form window 404. When the free end of this line reaches terminal 436a, the developer preferably executes a second mouse click. Again, the corresponding windows event is passed by the operating system 302 to the designer system 314 and it, in response, causes the free end of the line to become attached to terminal 436a. Designer system 314 also directs the operating system to stop tracing mouse movement with a line and to return the cursor 240 to its original appearance. FIG. 4D is an illustration of the GUI 400 with a wire construct 440 extending between the two symbolic representations 426, 434. In response to graphically connecting or linking two symbols in the designer window 406, the program-development environment 310 creates event handler program code that sets the label object's Caption property to the value of the vertical scroll bar object's Value property when the vertical scroll bar object's DataReady event occurs. Clearly, there are several ways in which this can be accomplished. For example, Visual Basic code for handling the indicated event (e.g., DataReady) and affecting the designated property (e.g., Caption) could be generated and added to the application program, and that event handler program code could then be compiled or interpreted in the normal manner at run-time. Preferably, though, the program-development environment 310 instantiates a new control or program object, a wire program object, adds this new object to the form window 404 and sets its properties in a predetermined manner. The basic function of the wire program object is to retrieve the Value property from the vertical scroll bar object in response to the DataReady event and to set the Caption property of the label program object to that Value. That is, this new object basically provides event handler functionality for other program objects residing in the form window 404. Specifically, the graphical designer system 314 directs the visual programming system 312 through calls to its extensibility object 316, as arrow 320 indicates, to instantiate a wire component control or program object from the wire object class and to add this object to the form window 404. That is, form window 404 adds a pointer to the wire is program object to its linked list of controls. It should be understood that the wire construct 400 appearing in the designer window 406 is preferably just a symbolic representation of the wire program object added to the form window 404. The visual programming system 312 also assigns a name to this program object, e.g., Wire2, which it returns to the designer system 314. As described below, as part of its initialization procedure, designer system 314 preferably directs the visual programming system 312 to instantiate and add a wire program object, which may be named Wire1, to the form window 404. Thus, the "first" wire that is drawn on the designer window 406 by the developer actually corresponds to the second wire program object to be instantiated and added to the form window 404. Therefore, this wire program object is typically assigned the name Wire2. The wire control or program object is itself a program module having its own pre-defined properties, methods and events. In the illustrative embodiment, each wire control or program object has the following properties, methods and events:
WIRE CONTROL PROPERTIES
Data
Name Type Description
Name Text Specifies the name of the wire program object.
Beep Boolean Determines whether the wire program object
emits a "click" sound whenever it is triggered.
Cancel Boolean Determines whether the wire program object
executes upon being triggered or invoked.
Enabled Boolean Determines whether the wire program object
executes in response to its triggering event.
Index Text Distinguishes between two or more wire program
objects having the same name.
Left Integer Specifies the x-coordinate position of an image
of the wire program object appearing on the user
form object.
OneShot Boolean If Enabled property is False, determines whether
Enabled the wire program object should nonetheless
execute one time.
Sink Text The name of the sink program object and its
respective property to which the wire program
object is graphically connected.
Source Text The name of the source program object and its
respective property to which the wire program
object is graphically connected
SourceGroup Integer Used to organize related properties of the source
program object.
Tag Text Assigns an additional identifier to the wire
program object, typically for use by the
application program.
Top Integer Specifies the y-coordinate position of an image
of the wire program object appearing on the user
form object.
Trigger Text The name of the program object and its respective
event, the occurrence of which causes the wire
program object to execute.
Value Variant A data store, the contents of which can be copied
from the source, modified, if desired, and passed
to the sink by the wire program object.
Name Description
WIRE CONTROL METHODS
Run Causes the wire program object to execute.
WIRE CONTROL EVENTS
Action Occurs in response to the wire program being triggered or run.
(Value) The argument corresponds to the current value of the wire's
Value property prior to any event handling routines.
Done Occurs once the wire program object has finished propagating its
Action event and setting the specified sink property, provided
that the Cancel property is still false.
where Boolean means that the property may be set to True or False, Text means that the property is an alpha-numeric string, Integer means that the property is an integer, and Variant means that the property can take any of the data formats specified by the corresponding variant structure definition. After the visual programming system 312 has added the wire program object to the form window 404 and returned its name, the designer system 314 next sets the various properties of this wire program object. The wire's properties, moreover, may be displayed in the property window 422 (FIG. 4D) of property window 418, as indicated by rows 442a-n, by selecting the wire program object, e.g., Wire2, from pull-down object list 420. The particular values to which the wire's properties are initially set depends on the particular program objects that have been logically connected by the wire construct 440 within designer window 406. For each wire control or program object, the designer system 314 identifies three corresponding program objects: a "source" program object, a "sink" program object and a "trigger" program object. Designer system 314 also examines the terminal data structures 500 that are associated with the graphically linked terminals 430c and 436a. Designer system 314 then uses this information to set the properties of the respective wire program object, i.e., Wire2. It should be understood that attempts by the developer to wire a first input terminal to a second input terminal or a first output terminal to a second output terminal are rejected by the program-development system 310. To identify the source, sink and trigger program objects, designer system 314 determines the names of the program objects that have been linked by the subject wire construct 440, the form window(s) on which those program objects reside, and the particular types of terminals that have been graphically linked by wire construct 440. As indicated above, information regarding the names of the graphically linked program objects and the form window(s) on which they reside is returned to the designer system 314 by the visual programming system 312 when system 304 adds those program objects to the form window 404. Thus, designer system 314 already has this information in its allocated portion of memory 214. Information regarding the types of terminals that have been linked is derived by the designer system 314 from the terminal type code fields 506 for the terminal type data structures 500 associated with the respective terminals, i.e., terminals 430c and 436a. The designer system 314 uses this terminal type information to determine which of the linked program objects should be considered the source object, which program object should be considered the sink object, and which program object should be considered the trigger object. In the preferred embodiment, the program object whose linked terminal is either a data output or control output type is treated as the source object, while the program object whose linked terminal is a data input or control input type is treated as the sink object. Here, linked terminal 430c at symbolic representation 426 is a data output terminal, while terminal 436a at symbolic representation 434 is a data input terminal. Thus, the designer system 314 considers the VScrollBar1 program object to be the source object and the Label1 program object to be the sink object for respective wire object, i.e., Wire2. After identifying the source and sink control objects, the designer system 314 is ready to set the Sink, Source and Trigger properties 442h, 442i and 442m of Wire2. The wire program object's Source property is preferably a concatenation of the following information: the name of the form window 404 on which the source program object resides, e.g., Form1, the name of the source program object, e.g., VScrollBar1, and the property associated with the linked terminal at the source program object, e.g., Value. The Source property may further be concatenated with the event associated with the linked terminal at the source program object, e.g., DataReady. The designer system 314 preferably obtains the source event and property parameters for use in setting the wire's Source property from the event field 502 and property field 504 from the terminal data structure 500 associated with linked terminal at the source program object, i.e., terminal 430c. For data output type terminals, such as terminal 430c, system 314 similarly obtains the SourceGroup property parameter 442j from the group identifier field 805 from the corresponding terminal data structure 500. The wire program object's Sink property 442h is preferably a concatenation of the following information: the name of the form window 404 on which the sink program object resides, e.g., Form1, the name of the sink program object, e.g., Label1, and the property associated with the linked terminal at the sink program object, e.g., Caption. Again, the designer system 314 preferably obtains the sink property parameter from the property field 504 of the terminal data structure 500 associated with linked terminal at the sink program object, i.e., terminal 436a. The wire program object's Trigger property 442m is preferably a concatenation of the following information: the name of the form window 404 on which the source program object resides, e.g., Form1, the name of the source program object, e.g., VScrollBar1, and the event associated with the linked terminal at the source program object, e.g., DataReady. As described above in connection with setting the Source property, this information may be derived from the name of the source program object and also from the contents of the event field 502 of the terminal data structure 500 associated with linked terminal at the source program object, i.e., terminal 430c. It should be understood that the designer system 314 may derive and set the Source property 442i first and then strip off the specified property of the source (e.g., Value), which was obtained from field 504 of the corresponding terminal data structure 500, to set the Trigger property 442m. The wire program object preferably includes built-in functionality that automatically sets its Beep, Cancel and OneShotEnabled properties 442b, 442c and 442g to FALSE, and its Enabled property 442d to TRUE. The Value property 442n is preferably set, at least initially, to null or is otherwise de-asserted. In the preferred embodiment, wire program objects are not intended to appear in any of the user interfaces that may be generated at run-time of the application program being developed. Accordingly, the Left and Top properties 442f, 442l of all wire program objects, which specify where on the user form object 408 an image of the object should appear (and, hence, where on the run-time user interface those images should appear), are set to default values (e.g., "20000") that are sufficiently high so as to "place" the image of the wire program objects off of the user form object 408. Thus, at run-time, no image appears on the user interface corresponding to any wire program object that may nonetheless reside on the corresponding form window. Additionally, or alternatively, the wire object's Visible property may be set to FALSE. Each wire program object instantiated and added to the form window 404 in response to graphical inputs of the developer includes at least some program code that may be called upon to execute when the respective application program is run. This program code, which is generated solely in response to the developer having graphically linked the symbolic representations of two program objects, basically causes the sink program object, e.g., Label1, to execute or otherwise take some action in response to an event generated by a trigger program object, e.g., VScrollBar1, and using some property of the source control object. That is, the wire object represents event handler procedures or code incorporated within the application program. FIGS. 6A and 6B are a flow diagram of the steps corresponding to the preferred event handler procedure or code generated by the program-development environment 310 in response to such graphical inputs from the developer. This procedure may be called upon to execute during run-time of the application program. Running of the graphically generated event handler procedure may be initiated in one of two ways. First, it is initiated when the trigger control component, as identified in the wire's Trigger property 442m, e.g., VScrollBar1, issues the particular event also identified in the wire's Trigger property 442m, e.g., DataReady, as indicated by block 602. In order to learn of the occurrence of this event (e.g., DataReady), the wire program object preferably registers with the trigger program object using an Event_Advise_Notification( ) method having the desired event as an argument. In response, the VScrollBar1 object notifies Wire2 whenever its DataReady event occurs. Alternatively, the event handler procedure may be initiated by invoking the wire's Run method, as indicated by block 604. Following initialization, the next step is to determine whether the wire program object's Enabled property 442d is TRUE, as indicated at block 606. If the wire's Enabled property 442d is FALSE, the code preferably ends, as indicated by first end block 608. As explained above, when the wire program object is first instantiated, it sets its Enabled property 442d to TRUE. Thus, unless the Enabled property 442d was subsequently set to FALSE at some point during run-time, as explained below, or was re-set by the developer, the response to decision block 606 is typically yes. As indicated at block 610, the event handler procedure next retrieves the value of the property specified in the wire's Source property 442i, e.g., Value, from the source object, e.g., VscrollBar1, also identified in the wire's Source property 442i. To do this, a Get( ) method may be invoked on the source program object. A separate Get( ) method may be invoked for each readable property. The Get( ) method is a conventional method that is preferably supported by all of the component controls or program objects utilized by the program-development environment 310 of the present invention. As an argument to the Get( ) method, the code inserts the name of the property, e.g., Value, the value or setting of which is to be returned. Suppose the current setting of the VScrollBar1's Value property is "15". Then, in response to the Get( ) method, the VScrollBar1 returns "15" to the wire program object. This value may be returned to the wire program object through either a Pass_By_Value or Pass.sub.--By _Reference communication method, both of which are well-known to those skilled in the art. The wire program object next copies this value, i.e., "15" to its own Value property 422n, as indicated at block 612. Upon copying the value into its Value property, the wire program object preferably issues its Action event, as indicated at block 614. Other elements or processes of the application program, including other component controls or program objects, may register as "observers" with the wire program object using the Event_Advise_Notification method described above so as to be notified of the wire's Action event. These observers may respond to the wire's Action event in any number of ways. At decision block 616, the wire program object waits until all of these "observers" have indicated that they have finished processing the wire's Action event. Next, the wire program object queries whether its Cancel property 442c (FIG. 4D) is FALSE, as indicated at block 618. As explained above, when the designer system 314 first sets the properties of a wire program object, it sets the Cancel property 442c to FALSE. In response to the wire's Action event (or some other event), however, another process, control component or program object may change the wire's Cancel property 442c from FALSE to TRUE. If the wire's Cancel property 442c is TRUE, then execution stops as indicated by second end block 620. Assuming the wire's Cancel property 442c is still FALSE, then the wire next up-dates the Sink property 442h, i.e., Caption, with the current value of its own Value property 442n, as indicated at block 622. This may be accomplished by invoking a Set( ) method on the sink control identified by the wire's Sink property 442h, i.e., Label1. A separate Set( ) method may also be invoked for each settable property. The Set( ) method is another conventional method supported by all of the component controls or program objects utilized in the program-development environment 310 of the present invention. After setting the sink's property, the code corresponding to the wire program object issues a Done event, as indicated at block 624. Observers may similarly register with the wire program object, again using the above-described Event_Advise_Notification method, so as to be notified of its Done event. These observers may be configured to take any number of different actions in response to the wire's Done event. At this point, the wire program object has finished executing as indicated by third end block 626. FIG. 7 is a flow diagram of steps preferably executed by a typical program object, such as the Label1 program object, incorporated in the application program being developed during application run-time. The program object begins execution in response to one or more of its properties being up-dated by a corresponding wire object as indicated at block 702, such as when the Wire2 object up-dates the Caption property of Label1. Next, the program object sets its CancelBlock property to FALSE as indicated at block 704. The program object then issues its RunBlock event as reflected at block 706. As with the Action and Done events issued by the wire program objects, observers (including wire program objects) may register with the program object using the Event_Advise_Notification mechanism so as to be notified of its RunBlock event. These observers may interact with the program object by, for example, changing its properties etc. As indicated by decision block 708, the program object waits until all such observers have returned from its RunBlock event. Next, the program object determines whether its CancelBlock property is still FALSE as indicated at decision block 710. One or more of the observers could have set the program object's CancelBlock property to TRUE in response to processing the RunBlock event. If its CancelBlock property is still FALSE, the program object executes its corresponding functionality and up-dates its own corresponding properties as warranted as indicated by block 712. Upon up-dating its properties, the program object issues its DataReady event as indicated by block 714. To the extent a wire program object is connected to one of this program object's data output terminals, the issuance or occurrence of the DataReady event may trigger that wire program object to begin operation. After issuing its DataReady event, the program object next issues its ControlOut event as indicated by block 716. To the extent the program object's control output terminal is connected to a wire construct, the corresponding wire may begin operation. Execution of the program object is now complete as reflected by End block 718. If, in response to decision block 710, the program object's CancelBlock property is TRUE, then processing stops at that point as indicated by No arrow 720 leading from decision block 710 to End block 718. It should be understood that a given program object may execute its corresponding functionality, as described at step 712, and then issue a RunBlock event, as described at step 706. This may be implemented by objects that perform mathematical operations, for example, and are thus less likely to cause erroneous data propagation problems in the corresponding application program. It should be further understood that, depending on the type of program object, other events besides DataReady may be issued. For example, program objects that operate in discrete or determinative modes or states, such as the For Loop, Do Loop and Wait objects, described below, or an Analog In Scan object, may issue one or more StatusReady events in place of the DataReady event. Program objects that perform scanning functions, such as Analog In Scan or Analog Out Scan, may issue a RateReady event in place of the DataReady event. Those skilled in the art, moreover, will recognize that other such events may be defined and implemented by the program objects utilized with the program-development environment 310. Generation of Event-Handler Code Through Textual Inputs A significant advantage of the present invention is its ability also to generate event handler procedures or code in response to textual inputs by the developer. In some circumstances, for example, it may be more efficient to specify an event handler textually rather than graphically. In particular, following the example of FIGS. 4A-D, suppose the developer wishes to have the background color of the label image 438 turn red during run-time whenever the value to be displayed exceeds 15000. Although the label object has a BackColor property, in the absence of a specific terminal on the corresponding symbolic representation 434 for the Label1 program object that is associated with this property, it would be difficult to specify this functionality graphically. Indeed, with the prior art graphical program languages, such as HP VEE and LabVIEW, it would be extremely difficult, if not impossible, to provide this functionality, because the graphical images for the label program object provided by these prior art systems do not have a terminal or pin for setting the object's background color in response to the value of its Caption property. With the present invention, the program-development environment 310 allows the developer to switch to a textual programming paradigm in order to specify an event procedure or other functionality that is more easily described textually as opposed to graphically. To specify an event handler textually, the developer directs the program-development environment 310 to call-up and display a code window in which textual inputs may be entered by the developer. More specifically, the developer, using mouse 230, moves the cursor 240 (FIG. 2) over the symbol of interest, e.g., Label symbol 438 (FIG. 4D), as displayed in the designer window 406 and executes a double mouse click. Since the cursor 240 is over the designer window at the time of the mouse click, the operating system 302 (FIG. 3) preferably passes the respective windows event to the graphical designer system 314. In response, the designer system 314 issues a call to the visual programming system 312, via arrow 320, causing it to display a code window on GUI 400 (FIG. 4D). FIG. 8A is a preferred illustration of the GUI 400 of FIG. 4D further including a code window 800. Code window 800 includes a pull-down object box 802, which contains a list of all of the program objects currently residing in the form window 404. By default, the object box 802 initially displays the program object selected by the developer, e.g., Label1. Code window 800 further includes a pull-down procedures/events box 804, which contains a list of all of the procedures and events supported by the selected program object of object box 802. Selecting a particular procedure or event from box 804 positions the entry point for subsequent textual inputs at the first line of the respective procedure or event. The procedures/events box 804 may initially display the first event supported by the corresponding object, e.g., the Change event, which is issued when an object's Value property changes. Code window 800 further includes an input area 806. Within the input area 806, the developer can write, review and edit program code for the respective application program using the keyboard 224 to generate textual inputs. In the preferred embodiment, the developer enters one or more statements within input area 806. A statement is basically a syntactically complete unit that expresses some action, declaration or definition. A statement generally occupies a single line, although a first designated symbol, e.g., the colon (":"), may be used to include more than one statement on a line, and a second designated symbol, e.g., the line-continuation character ("_"), may be used to continue a single logical line onto a second physical line. FIG. 8B is a preferred representation of the GUI 400 after the developer has written a series of statements 808a-g into the input area 806 of the code window 800 following the selection of the RunBlock event from the procedures/events box 804. As indicated above, statements 808a-g comply with the keywords and syntax defined by the programming language supported by the visual programming system 312 of the program-development environment. In the illustrative embodiment, this programming language is Microsoft's Visual Basic. Statements 808c-g specify the functionality for turning the background color of the label image 438 red if its Caption property (which is set to the Value property of VScrollBar1) exceeds 15000. Statements 808a-b are simply comment statements that describe the functionality to be carried out by the subsequent statements. In response to entering one or more statements in the input area 806 of code window 800, the program-development environment 310 generates constituent program code for insertion in the corresponding application program. That is, at run-time, the statements 808a-g are compiled or interpreted and executed as required, thereby implementing the functionality of the corresponding statements. Those skilled in the art will understand that the code window 800 may be called-up in other ways. For example, the developer may choose the "Code" option (not shown) from the View command of menu bar 410. It should be understood that a developer may also display and edit the properties of a wire program object, thereby causing the program-development environment 310 to modify the corresponding event handler procedure. As described above, the developer may cause the properties of a wire object, e.g., Wire2, to be displayed in the properties window 418 of GUI 400. By selecting one of the properties listed in the property window 422 of window 418, typically through a mouse click, the developer can edit the selected property. For example, although the wire program object preferably sets its Beep property 442b to FALSE upon instantiation, the developer may re-set this property to TRUE through textual inputs entered in the property window 418. In response, the event-handler procedure generated by the program-development environment 310 causes the computer system 200 to sound a tone each time the wire program object executes. The developer may also change a given wire object's trigger property 442m to a different event and/or a different program object. More specifically, as described above, the program-development environment 310 sets the trigger property 442m of a wire program object based on the particular source terminal, e.g., terminal 430c, to which the wire construct 440 of the corresponding wire program object, e.g., Wire2, is connected. The wire program object, moreover, executes in response to the occurrence of the event specified in its trigger property 442m. By editing the trigger property 442m, a developer may cause the program-development environment 310 to modify the corresponding event handler procedure such that the wire program object now executes in response to some newly identified event and/or program object (e.g., an object other than the wire's source object). To prevent developer-induced errors, the program-development environment 310 may be configured to block the display (and thus the editing) of wire program object properties through property window 418. Although the program development environment 310 of the present invention involves graphical event handler code generation, some implementations may not provide that capability for all available control components or program objects that may be incorporated into a given application program. Or, they may provide different toolbox icons or elements for the same control components, some of which enable the developer to program the control's event handlers graphically and others that do not. In such implementations, the toolbox 402 (FIG. 4A) may be divided into two areas. A first area 402a contains a plurality of icons corresponding to program object classes that can only be used in the form window 404. The program objects corresponding to these icons do not have a corresponding symbolic representation for use in the designer window 406. A second area 402b contains a plurality of icons that can be used in both the form window 404 and the designer window 406. That is, the program objects corresponding to these icons include symbolic representations capable of display in the designer window 406. Detecting the Presence of Branches in the Data/Control Flow Diagram and Ensuring Synchronous Execution of Program Objects at Run-Time In creating the data/control flow diagram within designer window 406 (FIG. 4D), a developer may connect more than one wire construct to the same terminal of a given program object so that the same data or information may be provided to two different program objects in response to the same event. Similarly, the developer may connect two wire constructs to two separate but related or complementary output terminals of the same program object. The resulting data/control flow diagram thus has a fork defining two or more "branches" or "streams". If these two streams subsequently converge at some point downstream, e.g., some other program object, problems may arise if this downstream program object reacts (e.g., executes its associated function) in response to new data or information on only one stream. More specifically, since many computer systems only include a single processor, the program steps corresponding to the branches must be executed in some order. That is, the steps of one branch are typically executed before the steps of another. Such serial execution can cause problems if a downstream object executes in response to new data or information from only one stream. That is, by executing when only part of its input data or properties have been up-dated, the program object and thus the application program in which it is incorporated may produce un-intended results. To avoid such problems, the program-development environment 310 of the present invention preferably includes a mechanism that causes such program objects to wait until the data or information from both streams has been up-dated before acting (e.g., executing its associated function). In the illustrative embodiment, the wire program objects incorporated into an application program are further configured to detect the presence of such forks in the respective data/control flow diagram, and to ensure that "stale" data or information is invalidated by the appropriate program objects. In general, an initialization process is run before run-time of the application. The initialization process determines which wire program objects are connected to the same output terminals of the same program objects and, therefore, exist at a fork in the data/control flow diagram. The initialization process also identifies and informs those program objects that will be receiving control input information during run-time (i.e., those objects whose ControlIn properties may be changed). At run-time, the wire program objects execute a branch-detection and data/control flow coordination process. More specifically, when a wire program object is about to set the associated property of its corresponding sink program object, the wire first determines whether it exists at a fork of the data/control flow diagram. If so, the wire program object causes its sink program object to first invalidate the current property that is about to be up-dated and directs all other wire program objects also located at the fork to do the same, and the sink objects cause the wire objects connected to their output terminals to invalidate their own sink objects and so on until the end of the flow diagram is reached. As a result, all of the program objects located downstream of the fork, invalidate the "stale" data currently associated with their data input terminals. After the downstream program objects have invalidated their input values, the wire program object then sets the sink property, thereby validating the sink property. Only when all of the data input terminals of the downstream program objects are validated do the program objects execute their associated functionality and up-date their output properties, thereby ensuring proper coordination. FIG. 9 is an exemplary illustration of the GUI 400 generated by the program-development environment 310 (FIG. 3) having a branched data/control flow diagram 900 graphically represented within the designer window 406. In particular, diagram 900 includes a command button symbolic representation 902, two analog input symbolic representations 904, 906, a comparator symbolic representation 908, an LED symbolic representation 910 and a digital output symbolic representation 912. The analog input symbols 904, 906 correspond to program objects that can obtain indoor and outdoor temperature measurements, respectively. Symbolic representations 902-912 are interconnected by a plurality of wire constructs, each of which corresponds to a respective wire program object that resides in the form window 404 and is thus incorporated into the application program. Specifically, a first wire construct 914 connects a data output terminal 916 of button symbol 902 to a control input terminal 918 of outdoor temperature symbol 906. A second wire construct 920 connects the data output terminal 916 of button 902 to a control input terminal 922 of indoor temperature symbol 904. A third wire construct 924 connects a data output terminal 926 of the outdoor temperature symbol 906 to an "x" data input terminal 928 of the comparator symbol 908. A fourth wire construct 930 connects a data output terminal 932 of indoor temperature symbol 904 to a "y" data input terminal 934 of the comparator symbol 908. A fifth wire construct 936 connects a data output terminal 938 of comparator symbol 908 to a data input terminal 940 of the LED symbol 910. A sixth wire construct 942 also connects data output terminal 938 of comparator symbol 908 to a data input terminal 944 of the digital output symbol 912. Each wire construct 914, 920, 924, 930, 936 and 942, moreover, corresponds to a wire program object residing on form window 404. As shown, the data/control flow diagram 900 includes several forks. In particular, a first fork exists at data output terminal 916 of button symbol 902 since both wire construct 914 and 920 are connected to this terminal. A second fork exists at data output terminal 938 of comparator symbol 908 since both wire construct 936 and 942 emanate from this terminal. Furthermore, appearing within the user form program object 408 of GUI 400 are a button image 950, that has been labeled "Take One Measurement", and an LED image 952 that has been labeled "Heat Indicator". In response to the developer having "drawn" the wire constructs 914, 920, 924, 930, 936 and 942, on the designer window 406, the program-development environment 310 generates corresponding event handler procedures or code in a manner as described above. The event handler procedures for wire constructs 914 and 920, for example, cause the indoor and outdoor temperature program objects of symbols 904, 906 to acquire a temperature measurement in response to user selection of the button image 950, which measurements are then provided to the comparator program object of symbol 908. Preferably, however, the comparator program object of symbol 908 only executes when new temperature data has been received at both terminal 928 and terminal 934. If the comparator program object were to react when just one of its data input terminal 928 or 934 is up-dated (thus rendering the information associated with other data input terminal stale), it may be making an un-intended comparison. Since the program-development environment 310 (FIG. 3) of the present invention may be running on a single processor computer system, such as computer 200 (FIG. 2), true parallel operation is not possible and thus only one of the data input terminals 928 or 934 will be up-dated at a time. To prevent such un-intended operating characteristics, the program-development environment 310 employs a novel synchronization process. In particular, the program-development environment 310 preferably incorporates program instructions or code corresponding to one or more initialization processes into each application program that is developed. These initialization processes, which are executed at application run-time, first identify all wire program objects whose corresponding constructs reside at a fork of the corresponding data/control flow diagram, e.g., diagram 900. To do this, the wire objects compare their respective source and SourceGroup properties 442i, 442j (FIG. 4D). If two or more wire objects have the same Source and SourceGroup, then they are either connected to the same data output terminal and thus exist at a fork, or they are connected to different, but nonetheless related or complementary, output terminals of the same source object, and thus should be treated as if they exist at a fork. Next, each wire program object invalidates its sink property to prevent unintended or spurious execution of the program objects of the application program. More specifically, each wire program object sets the InvalidProperty property of its sink program object, as identified in its respective sink property 442h, to the particular identifier that is associated with the sink property. That is, each property of a program object, in addition to having a name and a value, also has an identifier, which may be a numeric or alpha-numeric string. Other program objects, including the wire program objects, can obtain these identifiers by querying the object (e.g., with a standard COM mechanism). In response to having its InvalidProperty set to the particular identifier for one of its properties, the corresponding sink program object sets a flag associated with the identified property to indicate that the value of this property is now invalid, and thus should not be used by the sink program object. In this way, program objects learn whether their control input terminals are connected to any wire constructs within the flow diagram, e.g., diagram 900 (FIG. 9). In other words, wire objects connected to control input terminals set the InvalidProperty of their sink objects to the identifier for the sink's ControlIn property. Thus, if a program object's InvalidProperty is set to the identifier for its ControlIn property, then the object "knows" that its control input terminal is connected to a wire construct, and that information may thus be received from his wire construct. At this point, the initialization process is preferably complete and the application program may be run. It should be understood that the initialization process may also identify all root blocks within the data/control flow diagram. A root block is any program object whose symbolic representation does not have a wire construct connected to either its data input or control input terminals. In addition, all variable program objects, which are described in detail below, are treated as root blocks even if one or more of their data or control input terminals are connected to corresponding wire constructs. In flow diagram 900 of FIG. 9, the object corresponding to command button symbol 902 is a root block. During run-time of the application program, data and/or control flow typically begins at one or more of the root blocks of the corresponding flow diagram, and proceeds to flow downstream of the root(s) (e.g., from source objects to sink objects). To identify the root block(s) of a flow diagram, the source and sink properties of each wire object may be examined. If a particular program object only appears as a source for one or more wire objects, and not as a sink for any wire object, then that program object is considered a root block. FIGS. 10A-10B illustrate a flow chart of the steps corresponding to the running of an application program whose flow diagram includes one or more forks, such as diagram 900 (FIG. 9). In response to the occurrence of its corresponding triggering event, a wire object that is part of any flow diagram, first determines whether it is at a fork as indicated at step 1002. As described above, by virtue of the initialization process, all wire objects know whether or not they are at a fork of the flow diagram. If it is at a fork, the triggered wire object invalidates its sink property and directs the other wire objects at the fork to invalidate their sink properties as indicated at block 1004. In response to having a control or data input property invalidated, a program object preferably issues an InvalidateGroup event as indicated at block 1006. Those wire objects connected to the data output or control output terminals of such a program object are configured to respond to the issuance of the InvalidateGroup event. In other words, these wire objects register with their source objects, preferably using the Event_Advise_Notification method, so as learn of any InvalidateGroup events. These wire objects then respond by invalidating their own respective sink properties as indicated at block 1008. As indicated at block 1009, the steps of blocks 1004-1008 are preferably repeated by the remaining wire and other program objects whose symbolic representations are located downstream of the fork relative to the root, thereby invalidating the respective data and control input terminals. It should be understood that the program instructions or code that is incorporated into the application program is preferably configured such that steps 1004-1008 are completed at each of the downstream objects before resuming processing of (i.e., returning program control to) the wire object that was initially triggered and thus initiated the invalidation of the object input terminals. Next, the wire object that was triggered in step 1002 preferably runs its event handler procedure or code as indicated at block 1010. This event handler procedure, which is generated by the program-development environment 310 and incorporated into the application program, preferably corresponds to the steps of FIGS. 6A and 6B described above. Thus, the wire object sets the respective property of its sink object with the value of the property that the wire object retrieved from its source object. In response to having its data input property up-dated, the sink object preferably transitions the state of that property, which had previously been invalidated at step 1004, to valid as indicated at block 1012. Preferably, the sink object changes the flag associated with the up-dated property to valid. In response to changing an input property from invalid to valid, the sink object next determines whether its control input terminal is connected to a wire construct as indicated by decision block 1014 (FIG. 10B). If it is, the sink object then determines whether or not the value of its ControlIn property has been changed (e.g., toggled) as indicated by Yes arrow 1016 leading to decision block 1018. That is, the sink object, which may maintain a particular flag for this purpose, determines whether or not its ControlIn property has been changed. If not, the sink object preferably returns program control to the previous wire or other program object as indicated by No arrow 1020 leading to end block 1021. Alternatively, the sink object may run a wait loop at decision block 1018. If the sink object's ControlIn property has been toggled, it then determines whether all of its data input terminals are valid as indicated by Yes arrow 1022 leading to decision block 1024. If one or more input properties are still invalid, the sink object may wait until the remaining data input terminals are validated or, in the preferred embodiment, return program control to the previous wire or other program object as reflected by No arrow 1028 to end block 1021. If the sink object's control input terminal is not connected to a wire construct, then processing moves from decision block 1014 directly to block 1024, by-passing block 1018, as shown by No arrow 1026. If (or when) all of the sink object's data input terminals are valid, the sink object proceeds to execute its associated function as indicated at block 1030. Execution of the sink object preferably proceeds as described above in connection with FIG. 7. Thus, the sink object up-dates the properties associated with its data output terminals and issues its DataReady event. This DataReady event is typically a triggering event for any wire object whose construct is connected to a data output terminal of this sink object. Accordingly, this wire object and its sink property proceed to execute the steps of 1002-1030. That is, this process is repeated by the remaining wire and program objects downstream of the fork, thereby ensuring that the program objects do not execute their particular functions until all of their data input terminals have received valid data and their control input terminals, if connected, have been triggered. For example, consider operation of the program object for the comparator symbol 908 (FIG. 9) of flow diagram 900. At run-time, a user interface corresponding to user form program object 408 is displayed on screen 235 (FIG. 2) of computer 200 to the end user. When the end user selects (e.g., mouse clicks) the command button image 950, the corresponding program object issues its DataReady event, which is the triggering event for the wire objects of constructs 914 and 920. As described above with steps 1002 and 1004 (FIG. 10A), whichever wire object first processes this DataReady event will set its sink property to invalid and direct the other wire object to do the same. The invalidation of data input terminals propagates throughout the flow diagram 900 as described above with steps 1006 and 1008. As a result, the "x" input property of terminal 928 and the "y" input property of terminal 934 at comparator 908 are invalidated. The wire objects of constructs 914 and 920 then process the DataReady event, thereby causing the objects of indoor and outdoor temperature symbols 904, 906 to take a temperature measurement and pass these measurements via wire constructs 924, 930 to the data input terminals 928, 934 of comparator 908. As each respective property at comparator 908 is up-dated with the new temperature measurement, comparator 908 changes the state of the property from invalid to valid as indicated by step 1012 (FIG. 10A) described above. Since comparator 908 does not have a wire construct connected to its control input terminal, it simply waits until both of its data input terminals 928, 934 have been up-dated (and thus validated) before executing, as indicated by blocks 1024 and 1030 (FIG. 10B). That is, as each data input terminal is up-dated and transitioned to the valid state, comparator 908 determines whether all of its other data input terminals are valid. Only after the last data input terminal is up-dated and validated does the object run its comparison function, provide an output on terminal 938 and issues its DataReady event. As shown, the branch detection mechanism of the present invention prevents objects, such as comparator 908, from executing their respective functions until all of their data input terminals have been up-dated. Accordingly, the branch detection mechanism avoids the generation of un-intended results by the application program. It should be understood that object properties associated with data input terminals that are not connected to any wire constructs preferably remain valid at all times. Recursion Blocking Mechanism In some programming situations, the developer may wish to create a data/control flow diagram having one or more circular paths. Such circular paths typically represent corresponding loop-back or feed-back conditions within the data/control flow diagram. Unless they are handled in a consistent and known manner, such loop-back or feed-back conditions can cause unintended consequences or results during execution of the corresponding application program. In addition, if left un-detected, circular paths, can also consume substantial computer resources, such as CPU and memory resources, even to the point of overwhelming the system. According to the present invention, a method is provided for efficiently handling circular paths specified within the data/control flow diagram. FIG. 11 is a preferred representation of a GUI 1100 generated by the program-development environment 310 (FIG. 3) on computer screen 235 (FIG. 2) that is similar to GUI 400 described above in connection with FIGS. 4A-4D. Within the designer window 406 is a data/control flow diagram 1102 that includes a circular path. In particular, the flow diagram 1102 includes three symbolic representations 1104, 1106 and 1108 each corresponding to a text box program object. Symbolic representations 1104-1108 are interconnected by a plurality of wire constructs. A first wire construct 1110 connects a data output terminal 1112 of symbol 1104 to a data input terminal 1114 of symbol 1106. A second wire construct 1116 connects a data output terminal 1118 of symbol 1106 to a data input terminal 1120 of symbol 1108. A third wire construct 1122 connects a data output terminal 1124 of symbol 1108 to a data input terminal 1126 of symbol 1104, thereby completing a circular path within the flow diagram 1102. Within the user form object 408 of form window 404 are images 1128, 1130 and 1132 that correspond to text box symbols 1104-1108. Without some mechanism for handling the circular path of flow diagram 1102, running of the corresponding application program may overload the computer's processing and memory resources. More specifically, suppose an end-user were to run the application and enter some information, e.g., "hello", into a user interface element (not shown) corresponding to image 1128. As described above, the program object for symbol 1104 would issue its DataReady event indicating the presence of new data associated with its data output terminal 1112. In response, the program object for wire construct 1110 passes this information to the object of symbol 1106. The program object of symbol 1106, in turn, issues its DataReady event causing wire construct 1116 to pass the information to the object of symbol 1108. The program object of symbol 1108 then issues its DataReady event causing wire construct 1122 to return the information to the object of symbol 1104, thereby completing the circle. The object of symbol 1104 assumes that it has just received "new" data and, in response, issues its DataReady event, repeating the cycle described above until the computer's resources are eventually exhausted. To avoid this problem, the program-development environment 310 of the present invention includes a novel method for efficiently handling circular paths specified within the data/control flow diagram. FIG. 12 is a flow diagram of the preferred steps of the method that are preferably performed by each program object represented in the flow diagram. First, the object initializes a "busy" indicator to some pre-determined value, as indicated at step 1202. In the illustrated embodiment, the busy indicator is a counter that may be initialized to "0". Specifically, each program object that is represented by a symbolic representation within a given flow diagram preferably establishes a busy counter (not shown) within memory 214 (FIG. 2). Next, the object waits to be triggered as indicated by decision block 1204 which has a No arrow 1206 that loops back on itself. As described above in connection with step 702 of FIG. 7, an object is triggered when one or more of its properties is up-dated typically by a connected wire object. When the program object is triggered, it first increments the busy indicator (e.g., the counter), preferably by "1", as indicated by block 1208. The object then determines whether the value of its busy counter exceeds some predetermined threshold as indicated by decision block 1210. In the preferred embodiment, the threshold is set to "1 ". If the busy counter does not exceed the threshold, the object then executes its respective function as indicated by No arrow 1212 leading to block 1214, which preferably corresponds to steps 704-718 of FIG. 7 described above. That is, the object places its output data on its data output terminal and issues its DataReady and its ControlOut events. As described above, other objects, such as wire program objects, may respond to the DataReady and ControlOut events. As indicated at decision block 1216 and No arrow 1218 which loops back to block 1216, the object waits until such "observers" have returned from the DataReady and Control out events. Specifically, upon issuing its DataReady event, program control (e.g., processing by the CPU 210 (FIG. 2) shifts from executing the steps of the program object to executing the steps corresponding to the event handler procedure that is triggered by the object's DataReady event. When this event handler procedure is finished, program control then returns to the object so that its execution may be completed. So that program control may be returned to the appropriate location within the steps corresponding to the program object, a pointer to the location is typically pushed onto a stack within memory 214 (FIG. 2). When the event handler procedure or code has finished executing, this pointer is popped off of the stack and processing resumes at the appropriate location. As described above, execution of the event handler procedure or code may result in the issuance of one or more events (e.g., Action, Done, etc.) and it may be interrupted so that program instructions or code triggered by these events may be executed. When the observers return from the object's DataReady and ControlOut events, the object then decrements its busy counter, preferably by "1", as indicated by Yes arrow 1220 leading to block 1222. Upon decrementing the busy counter, processing is complete as indicated by End block 1226. Referring again to step 1210 in which the busy indicator is tested, if the value of the busy indicator exceeds the threshold, then the object does not execute and, instead, simply decrements its busy counter, as indicated by Yes arrow 1224 leading from decision block 1210 to block 1222. That is, if the busy indicator returns a busy indication, then execution of the corresponding object is by-passed (e.g., the object is short-circuited) and the object simply decrements its busy indicator. As described above, upon decrementing the busy counter, processing by the object is complete as indicated by End block 1226. To avoid conflicts between the incrementing and testing of the busy indicator, the steps of blocks 1208 and 1210, are preferably performed by the computer system 200 (FIG. 2) as an atomic operation. An atomic operation refers to some unitary action that is essentially indivisible (i.e., the steps are not interrupted). Those skilled in the art to which the present invention pertains are aware of techniques for ensuring that particular programming instructions or steps are treated as atomic operations. For computer systems that support multiple threads, semaphores may be used to prevent a given object that was triggered by a first thread from being re-triggered by a different thread. Semaphores are well-known techniques for coordinating or synchronizing activities. Returning to the example of FIG. 11, when information, e.g., "hello", is entered by the end user into the user interface element of image 1128, the object of symbol 1104, which corresponds to this user interface element, executes the steps of FIG. 12. In particular, the entry of information triggers the object as indicated by step 1204. Accordingly, the object increments its busy counter, which was initialized to "0" at step 1202, by "1" so that its busy counter is now set to "1". Since the value of the busy counter does not exceed the threshold of "1", the object of symbol 1104 executes its functionality as indicated by steps 1210 and 1214. In particular, the object places the newly entered information on its data output terminal 1112 and issues its DataReady event. The wire object of construct 1110 responds to the DataReady event and passes this information to the data input terminal 1114 of the object of symbol 1106. As described above, this information is eventually returned to the object of symbol 1104 at its data input terminal 1126 by wire construct 1122. In response, the object of symbol 1104 assumes it has been triggered, as indicated at step 1204, and thus increments its busy counter (again by "1") and tests it, as indicated at steps 1208 and 1210. Since the busy counter is now at "2", exceeding the threshold of "1", the object decrements the counter as indicated by Yes arrow 1224 leading to step 1222, instead of executing again as might otherwise occur without the recursion or re-entry blocking mechanism of the present invention. As a result, program control returns first to the object of wire construct 1122 so that it may complete its execution, and then to the object of image 1108 so that it may complete its execution. Program control similar returns to wire construct 1116, and then to object 1106. Eventually, program control returns to the wire object of construct 1110, which completes its execution, thereby returning control to the object of symbol 1104 following the issuance of its DataReady event. In other words, the result of decision block 1216 is now yes, and thus the object of symbol 1104 can decrement its busy counter from "1" back to zero. As shown, with the method of the present invention, the re-triggering of the object of symbol 1104 (i.e., triggering before the object has completed execution from an earlier triggering) does not result in an overloading of the processing or memory resources of the computer 200 (FIG. 2). It should be understood that the steps of FIG. 12 may be executed by each of the wire program object's of the respective flow diagram as well. Symbolic Representations for Repeating Steps When creating application programs, developers often include certain programming steps that are to be repeated many times. Rather than entering such steps over and over again, many programming languages include command structures that automatically repeat certain identified steps. For example, with Visual Basic from Microsoft, developers can enter specific keyword commands within the code window to repeat certain statements. In particular, Visual Basic allows developers to create what are known as Do Loops and For Loops. A Do Loop executes certain code over and over again until some condition is met. Typically, the syntax of a Do Loop appears as follows: Do [statements] Loop Until/While condition. where "[statements]" are the particular code statements that are to be repeated and condition refers to the condition that stops the loop. A Loop Until condition checks the condition after running through the loop and repeats only if the condition is FALSE. A Loop While condition also checks the condition after running through the loop but repeats only if the condition is TRUE. Do Loops may also be configured to check the condition before entering the loop. A For Loop is used when the developer knows precisely how many times the loop should repeat. Unlike a Do Loop, a For Loop uses a variable called a counter that increases or decreases in value during each repetition of the loop. The syntax of a For Loop typically appears as follows: For counter =start To end [Step increment] [statements] Next [counter] When executing the For Loop, the application program sets the counter equal to the specified start value and tests to see whether the counter is greater than the specified end value. If so, the loop is exited. If not, the statements are executed and the counter is incremented by the specified increment value (or by "1" if no value was specified). The loop then repeats again checking to see if the counter has surpassed the end value (in either direction). According to the present invention, the program-development environment 310 (FIG. 3) is further configured to incorporate program code within the application program being developed that repeats certain statements or steps (i.e., the program includes loop structures) in response to graphical, as opposed to textual, inputs from the developer. In particular, as described below, the program-development environment 310 includes a plurality of program objects having corresponding symbolic representations that may be caused to appear in the designer window 406 of the GUI 400. These symbolic representations may be connected with other symbols within the designer window 406 using one or more wire constructs in order to define a loop. However, in order to present a simplified and refined flow diagram within the designer window, the loop preferably appears as an acyclic branch of the flow diagram. That is, although the branch terminates (i.e., there is no wire construct from the end of the branch back to the main flow diagram), it nonetheless is repeatedly executed at application run-time. At the end of the branch may be a Break symbol as described below. FIG. 13 is a preferred representation of a GUI 1300 generated by the program-development environment 310 (FIG. 3) on computer screen 235 similar to GUI 400 (FIG. 4A) described above. The Core sub-toolbar 414b of the designer window 406 includes a plurality of icons that correspond to program objects for use in performing loop-type functions, among others, within the application program being developed. In particular, sub-toolbar 414a includes icons for a For Loop, a Do Loop, a Timer, and a Break control, among others. Each of these icons, in a similar manner as described above, corresponds to an object class from which one or more program objects or controls may be instantiated. As described herein, these loop controls can generate multiple outputs from a single input. Within the designer window 406 are preferred symbolic representations of several program objects corresponding to icons of Core sub-toolbar 414b each having one or more terminals for connecting one or more wire constructs. A For Loop symbol 1302, for example, includes a start index data input terminal 1304a, an end index data input terminal 1304b, a step value data input terminal 1304c, a control input terminal 1304d, a loop index data output terminal 1304e, an error data output terminal 1304f and a control output terminal 1304g. The For Loop object of symbol 1302 preferably incorporates the ControlOut, DataReady, ErrorOut and RunBlock events described above. The For Loop object of symbol 1302 basically runs a branch of the flow diagram connected to its loop index output terminal 1304e repeatedly until the end index specified at input terminal 1304b is reached. In particular, starting with the start index specified at input terminal 1304a, which can be initialized to "0" each time the respective application program starts-up, the For Loop object issues its DataReady event and outputs the current loop index value from terminal 1304e (e.g., 1, 2, 3, etc.) using the step value specified at terminal 1304c (e.g., 1) to count up (or down) to the end index specified at terminal 1304b. A Do Loop symbol 1306 preferably includes a control input terminal 1308a, a loop index data output terminal 1308b, an error data output terminal 1308c and a control output terminal 1308d. The Do Loop object also incorporates the ControlOut, DataReady, ErrorOut and RunBlock events. The Do Loop object of symbol 1306 basically runs a branch of the flow diagram connected to its loop index output terminal 1308b repeatedly until some condition, which is preferably specified graphically by one or more symbols connected to a Break symbol, which is described below, is met. In particular, with its loop index property preferably initialized to "0" each time the respective application program starts-up, the Do Loop object repeatedly issues its DataReady event and outputs its loop index value from terminal 1308b until the graphically specified condition is met and the Do Loop object is stopped. A Timer symbol 1310 preferably includes an interval data input terminal 1312a, a frequency input terminal 1312b, a control input terminal 1312c, a loop index data output terminal 1312d, an error data output terminal 1312e and a control output terminal 1312f. Like the Do Loop object, the Timer object of symbol 1310 also runs a branch of the flow diagram connected to its loop index output terminal 1312d repeatedly until some specified condition is met. In particular, with its loop index property preferably initialized to "0" each time the respective application program starts-up, the Timer object issues its DataReady event and outputs its loop index value from terminal 1312d each time the value specified at its interval data input terminal 1312a (which may be in milliseconds) elapses. The Timer object then preferably increments its loop index property by "1". The value specified at the frequency data input terminal 1312b preferably specifies the number of timer events per second. A Break symbol 1314 preferably has a control input terminal 1316. As indicated above, a Break object is used to stop execution of a corresponding loop object (e.g., Do Loop, For Loop or Timer objects) upon satisfaction of some specified condition. The Break object preferably terminates the execution of the first up-stream loop object to which the symbolic representation 1314 of the given Break object is connected when its control input property is triggered (e.g., changed). For example, a Break object can be used to stop execution of a loop when a particular value generated during the loop sequence exceeds some threshold. In this case, the control input terminal 1316 of a Break object 1314 may be connected to the data output terminal of a symbol whose corresponding object compares two values and outputs a TRUE indication on the connected data output terminal if the first value (the value generated within the loop sequence) is greater than the second value (the threshold). This loop will continue to run until the value exceeds the threshold. At this point, the comparison symbol will output a TRUE indication that is provided to the Break symbol by a corresponding wire construct. Upon having its control input property triggered, the Break object stops execution of the first up-stream loop object (i.e., Do Loop, For Loop or Timer objects). The properties (or at least those properties that are declared public and thus may be changed by a developer) of the For Loop, Do Loop, Timer, and Break objects may each be selectively displayed by the program-development environment 310 (FIG. 3) in the properties window 418 by selecting the desired object from the pull-down object list 420. The specific properties displayed within the corresponding properties window 422, moreover, may be modified and edited by the developer, thereby changing the properties of the respective object residing in the form window 404. The use of the loop symbols and corresponding objects may best be understood through an example. FIGS. 14A-D are preferred representations of a GUI having a flow diagram incorporating a loop. Suppose that a developer wishes to create an application (or a process thereof) for summing a sequence of numbers and stopping if the sum exceeds some specified value. Using the icons of designer toolbar 414, the developer preferably creates a data/control flow diagram within the designer window 406 of the GUI 400 for performing such steps. FIGS. 14A-D are preferred illustrations of a GUI 1400 similar to GUI 400 described above showing some of the steps followed in creating such an application program. More specifically, within designer window 406 of GUI 1400 is a data/control flow diagram 1402 of the application program. The flow diagram 1402 includes a number of symbolic representations interconnected by a plurality of wire constructs. The symbols, moreover, correspond to respective program objects that have been instantiated and added to the form window 404 as described above. The symbols include a command button symbol 1404 having a data output terminal 1406, a For Loop symbol 1408 having a control input terminal 1410a and a loop index output terminal 1410b, first and second label symbols 1412 and 1414, each having corresponding data input terminals 1416 and 1418, respectively, and an addition symbol 1420 having an "x" data input terminal 1422a, a "y" data input terminal 1422b, a data output terminal 1422c and a control output terminal 1422d. Flow diagram 1402 further includes an X>Y comparator symbol 1424 having an "x" data input terminal 1426a, a "y" data input terminal 1426b, and a TRUE data output terminal 1426c that is up-dated when the value of "x" is greater than the value of "y". Comparator 1424 may also include a FALSE data output terminal 1426d that is updated if "x" is not greater than "y". Flow diagram 1402 further includes a text box symbol 1428 having a data output terminal 1430, and a variable symbol 1432 having data input, control input, data output, error output and control output terminals 1434a-e. A Variable symbol, such as symbol 1432, is typically used to read a new value on its data input terminal 1434a and, upon triggering of its ControlIn property (provided its control input terminal is wired) to pass that value to its data output terminal 1434c and issue a DataReady event. Thus, variable objects can save a data value for later use by the application program. Flow diagram 1402 further includes a Break symbol 1436 having a control input terminal 1438. In response to adding the command button, labels and text box symbols 1404, 1412, 1414 and 1428, which are all user interface components, the program-development environment 310 (FIG. 3) places corresponding button, labels and text box images 1440, 1442, 1444, and 1446, respectively, in the user form program object 408 of the form window 404. The symbolic representations within the designer window 406 are also interconnected by a plurality of wire constructs, thereby forming the data/control flow diagram 1402 of the corresponding application program. In particular, the data output terminal 1406 of command button symbol 1404 is connected to the control input terminal 1410a of For Loop 1408 by a first wire construct 1446. Loop index output terminal 1410b of For Loop 1408 is connected to the data input terminal 1416 of first label 1412 by a second wire construct 1448, and also to the "x" data input terminal 1422a of the addition symbol 1420 by a third wire construct 1450. Data output terminal 1422c at addition symbol 1420 is conn | ||||||