Hackers Guide to Visual FoxPro 7.0

2017-07-07 02:10:07

This, ThisForm, ThisFormSet

The documentation refers to these three as "object references" and we can't think of anything better. They're not commands nor functions; neither are they properties, events nor methods. And they're not exactly variables, either. What they are, in an informal sense, is handles. They let you grab onto objects without knowing the names of the objects.

Usage

When you're writing method code for an object, you often need to refer to properties and methods of that object and of other related objects. But you don't know what the name of the object will be when it's instantiated. So you often can't write something like MyWonderfulObject.Caption.

Even when you do know the name (for example, when you're designing a form that will have only one instance), it's not a good idea to refer to the object by name. Doing so limits the reusability of the object.

Similarly, in a method of a form or formset, you don't want to refer to it by name since you may instantiate it more than once, with a different name for each instance.

The three object references are the way to avoid the problem. THIS refers to the current object. When you're in a method of a control, THIS is the control itself. In a method of a form, THIS is the form. In a method of a formset, THIS is the formset, and so on. The same idea applies to non-visual objects as well. THIS is always the object whose method you're in.

ThisForm, then, is the form containing the object. In a method of a control, ThisForm refers to the form containing the control. In a method of a form, ThisForm and THIS both refer to the form. Finally, ThisFormSet refers to the form set containing the object whose method you're in.

Example

oTest=CREATEOBJECT("TestForm") oTest.SHOW() READ EVENTS DEFINE CLASS TestForm AS Form ScaleMode = 0 && Foxels just to make it easier && to position things ADD OBJECT txtUpdate AS Updater ADD OBJECT cmdClear AS ClearButton ADD OBJECT cmdQuit AS QuitButton PROCEDURE Init * set up Clear button to clear text box This.cmdClear.cWhatToClear="txtUpdate" * position things This.Height=10 This.Width=30 This.AutoCenter=.T. This.txtUpdate.Left=5 This.txtUpdate.Top=2 This.cmdClear.Left=5 This.cmdClear.Top=5 This.cmdQuit.Left=15 This.cmdQuit.Top=5 ENDPROC ENDDEFINE DEFINE CLASS Updater AS Textbox Width=20 Height=1.5 PROCEDURE InteractiveChange * change the form caption ThisForm.Caption=This.Value ENDPROC PROCEDURE ProgrammaticChange * revert the form caption ThisForm.Caption="TestForm" ENDPROC ENDDEFINE DEFINE CLASS ClearButton AS CommandButton Width=7 Height=1.5 Caption="Clear" * add a custom property cWhatToClear="" PROCEDURE Click LOCAL cClearWhat cClearWhat=This.cWhatToClear ThisFORM.&cClearWhat..Value="" ENDPROC ENDDEFINE DEFINE CLASS QuitButton AS CommandButton Caption="Quit" Width=7 Height=1.5 PROCEDURE Click CLEAR EVENTS ENDPROC PROCEDURE Destroy CLEAR EVENTS ENDPROC ENDDEFINE

The example includes several class definitions, three of them controls (two buttons and a text box) and one for a form class. In the form's Init method, we can change properties of the form using THIS.property (for example, THIS.Height=10). But, in the Text box's InteractiveChange event, to change the form's caption, we need to use ThisForm.Caption, indicating that FoxPro should look outside the current object (the text box) to see the containing form.

We should point out that this sample does something we never actually do—it uses foxels. We always work in pixels. But for a hand-coded sample like this, foxels make things much easier.

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