Drawing

Add a baseDrawOn: method

Making the pie wedge draw

Use `Show Morph' on the pie wedge object's middle-button menu to make it appear as a morph. We have not changed any behavior, so it draws as a circle.

Use `Move to Own Window' to move the morph into a window on a new world. This makes it easier to fix errors in the draw method, since the main programming window can still be used even if the morph's drawing method is broken.

Add the following slots to the object:

	baseDrawOn: canvas = (| r. |
		"X bug: zero span angle draws full circle!"
		percent = 0 ifTrue: [ ^self ]. 
		"X bug: fillArc sometimes goes one pixel out of bounds"
		r: baseBounds indent: 1. 
		canvas
			fillArcWithin: r 
			From: startAngle 
			Spanning: spanAngle
			Color: color.
		self
	).

	spanAngle = (  3.6 * percent ).

baseDrawOn: is the primary method used to render a morph (but not its submorphs). It takes a canvas as argument (an object which serves as a proxy for an X drawable).

We need a couple of lines to work around problems with the underlying X graphics operations. First, if we try to draw a filled arc with span of zero, X interprets this as a full circle. Second, the X server we tested this code on occasionally drew slightly outside the specified bounds.

Pick up the pie wedge to make it redraw. It should show as a third of a circle.

If something goes wrong...

If you write an erroneous baseDrawOn: method, e.g., one that results in a run-time error, then you are likely to crash the world containing the object being drawn. Under these circumstances, Morphic usually detects that the display loop is failing, and creates a debugger on the failing process, placing the debugger in another, new world. However, it is tedious to repair the problem in a completely new world (because of the lack of context). Hence, when working on a critical operation in a morph, such as baseDrawOn:, it is better to put the morph representation in a world of its own. If that world crashes, you can still work on the morph via its object representation in the original world.

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