How to Create a Simulated Custom Method in VBX Custom Control

• Professional and Enterprise Editions of Microsoft Visual Basic, 16-bit only, for Windows, version 4.0

SUMMARY

The Microsoft Visual Basic VBX custom control architecture does not provide a mechanism for creating custom methods. Custom properties and custom events are supported, but only a set of eight predefined methods can be used by VBX custom controls.

However, to simulate a custom method, a VBX developer can attribute any meaning to the standard methods. To override the meaning of a standard method, you may want to use one of the two techniques described below.

MORE INFORMATION

Technique One: Implement a Custom Action Property

The most common way to simulate a custom method is to implement a custom action property. For example, assume that the control needs a custom line method. You could set up an action property that draws a line when the property is set.

Below is a hypothetical code example:

    ' Draw line from (20,30)-(200,15)
    MyControl.StartX = 20
    MyControl.StartY = 30
    MyControl.EndX = 200
    MyControl.EndY = 150

    MyControl.Action = DRAW_LINE

This technique is very popular and is used in several of the custom controls that ship with the Professional Edition of Visual Basic version 3.0.

Technique Two: Use an Exported Function

As an alternative, you can use an exported function within the VBX itself. For example, add the following code to the Visual Basic program:

   Private Declare Function CustomMethod Lib control.vbx (X As Control, ...

   int FAR PASCAL _export CustomMethod( HCTL hctl, ...
   {
     //Perform the custom method on the given hctl.
   }