All views and subclasses are rendered using the
drawRect() method, but you should never call that method directly yourself. Instead, it gets called by the system whenever drawing is required, which allows it to to avoid multiple redraws if
drawRect() is called several times in a row.
Instead, if you want a view to redraw immediately, you should call its
setNeedsDisplay() method like this:
That will ask UIKit to redraw the button using
drawRect(), but only if a redraw is not already queued.
Available from iOS 2.0
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