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If you are an advanced user of drawRect, you will know that of course drawRect will not actually run until "all processing is finished."
setNeedsDisplay flags a view as invalidated and the OS, and basically waits until all processing is done. This can be infuriating in the common situation where you want to have:
- a view controller 1
- starts some function 2
- which incrementally 3
- creates a more and more complicated artwork and 4
- at each step, you setNeedsDisplay (wrong!) 5
- until all the work is done 6
Of course, when you do the above 1-6, all that happens is that drawRect is run once only after step 6.
Your goal is for the view to be refreshed at point 5. What to do?