To accomplish the actual drawing use drawing operations from the Common Graphics subsystem. By restricting yourself to these drawing techniques, you ensure that the code you write is portable across all the operating systems that VA Smalltalk supports. To learn more about the Common Graphics class hierarchy and the drawing process, refer to the Programmer Reference.

To draw the circle, write three new instance methods in MyCwShape. The first of these methods is drawFilledCircle.

Before drawing the circle, the preceding method clears the shape's drawing area, using the clear method:

These methods call createGC which is defined as follows:

Together, these methods create a graphics context (an object that holds information about colors, line width, line style, and so on) and then draw a filled circle with an outline around it. The circle is filled with the widget's foreground color (inherited from CwDrawingArea) and the outline is drawn in the widget's background color.

After drawing, the graphics context is freed because it is no longer needed.

To make the shape part visible when it is dropped into the Composition Editor, you must set its default size by implementing the class method defaultEditSize in MyShape. This method returns a point value, indicating the width and height of the part.

Along with the defaultEditSize method, you also implement attachmentSpecAt: and positionSpecAt:, unless you already inherit a suitable implementation of these two methods.

To specify an icon representing the part, add the following class method to MyShape:

Move the above class methods to your edit-time application, MyEditSamplePartsApp.

Next, you must ensure that the draw method is called whenever the part needs to be redrawn, such as when the part is first displayed, or when it is covered by another window and then uncovered. To do this, implement a special callback method for the part. Callbacks are methods that respond to events from the operating system. The callback that is called when the part needs to be redrawn is known as the expose callback. If you are familiar with Presentation Manager or Windows programming, this is similar to handling a WM_PAINT message.

To use a callback method, the part must register its interest in a specific event, and give VA Smalltalk the name of a method to call whenever the event occurs. The shape's superclass (AbtDrawingAreaView) has already registered its interest in the expose callback, so all you need to do is override the callback handler method itself. To register a callback, you would use an instance method like this one. In this example, you do not need to write this method because the superclass already implements it:

Before registering any callbacks, check your superclasses to see which callbacks they already register. If you register a callback more than once, you might find that the callback handlers are called more often than you intended. For example, if you registered the expose callback again, your shape part would be drawn twice each time it needed to be redrawn.

Now create the following instance method in MyShape, which is the expose callback handler:

To learn more about the different callbacks you can implement, refer to the Programmer Reference.

Before you begin testing your work it is probably worthwhile to review the classes and methods you have implemented in your runtime application.

The MyShape class should have the following instance methods:

The MyShape class should have the following class methods:

The MyCwShape class should have the following instance methods:

Now you're ready to try testing your shape part.

Reuse the MyTestSamplePartsApp Application to test your work and create a new part named MyShapeTestView for testing.

Open the Composition Editor on MyShapeTestView and drop a shape part, MyShape, inside a Window part.

You should see a filled black circle. If not, go back and check each of the methods you wrote.

Now try two other tests:

In the first test, you should have seen that the shape was redrawn correctly after it was uncovered.

In the second test, you should have seen that the shape's size was changed so the shape just fit around the push button. In the next section, you will learn how to change this behavior.