Subclassing NSButtonEdit

1. Create NSButton subclass
2. Create NSButtonCell subclass

Note that overriding the cellClass class method of NSButton is not enough to force buttons created in Interface Builder (and told to use your custom class) to use your custom button cell class as well. When you drag an NSButton from the palette it is instantiated with an NSButtonCell cell. Telling Interface Builder that the button is of your custom class has no effect on the class of the cell in the button you just instantiated. You must either create an Interface Builder palette so that your instantiated button cell is of the correct class, or throw away the incorrect cell and replace it with a correct cell in your button's initWithCoder method.

How NSButtonCell draws

Typical redraw order of a standard NSButtonCell in Tiger (empirically determined):

1. highlight:withFrame:inView: calls drawWithFrame:inView:
2. drawWithFrame:inView: calls drawBezelWithFrame:inView:, but only if isBordered returns YES
3. drawWithFrame:inView: then calls drawInteriorWithFrame:inView:
4. drawInteriorWithFrame:inView: calls drawTitle:withFrame:inView:

At least with the button type I was using in my experiments, it appears that NSDrawWindowBackground is not called (at least, setting a symbolic breakpoint on it, it never got hit when playing with an empty NSButton/NSButtonCell subclass pair).