Question: I am calling [self setNeedsDisplay:YES] from within my view’s implementation of -drawRect:, but my view doesn’t redraw. Why ?
Answer: The General/AppKit effectively sets a flag whenever -setNeedsDisplay: is called for a view. The fact that the flag is set causes General/AppKit to send a -drawRect: message to the view and appropriate sub-views at a later time. After the -drawRect: is called, the flag is cleared. [This over-simplifies it, but illustrates the concept.]
If you call -setNeedsDisplay: from within -drawRect:, the flag will be set, but as soon as -drawRect: returns, it will be cleared again.
Therefore, when the time arrives, the flag is clear and -drawRect: is not called again.
Furthermore, do not call -display or variants or -drawRect: from within -drawRect: or you invite an infinite recursion and crash.
Question: I was calling [self setNeedsDisplay:YES] because I am repositioning some subviews within my view’s implementation of -drawRect:. How can I cause a redraw so that the repositioned subviews draw correctly ?
Answer: -drawRect should only be used for drawing. Resizing, repositioning, and adding or removing subviews absolutely should not be done in -drawRect:. Incorrect behavior will likely result. The General/AppKit reasonably relies on the view hierarchy remaining constant while it is in the middle of drawing the view hierarchy.
If you want to rearrange subviews, I recommend that you do it in a method like - (void)setFrame:(General/NSRect)frameRect or - (void)resizeSubviewsWithOldSize:(General/NSSize)oldBoundsSize. You might even like General/NSViewFrameDidChangeNotification.
There is a whole section in Apple’s General/NSView documentation on resizing behavior for views and subviews.
Some other General/NSView subclasses use methods like -tile found in General/NSScrollView.
*
Lays out the components of the receiver: the content view, the scrollers, and the ruler views. You rarely need to invoke this method, but subclasses may override it to manage additional components *
Perhaps you want to provide a similar method and call it whenever your view’s frame changes or some other stimulus happens.