In my case, the drawRect: will not be called immediately after every single setNeedsDisplay is called. For example, I think the following code is same as my case.
for (int i = 0; i < 100; ++i)
{
[self setNeedsDisplay];
}
From the documentation:
When the actual content of your view changes, it is your responsibility to notify the system that your view needs to be redrawn. You do this by calling your view’s setNeedsDisplay or setNeedsDisplayInRect: method of the view. These methods let the system know that it should update the view during the next drawing cycle. Because it waits until the next drawing cycle to update the view, you can call these methods on multiple views to update them at the same time.
drawRect: will only be called in time for the next frame to be drawn, which means your entire loop will result in drawRect: only being called once at the next rendering iteration. This saves unnecessary computation as it avoids drawing frames that will never be displayed on the screen. It also lets you make multiple changes in separate places in your code, each time notifying the view that a refresh is needed, without losing performance, since calling setNeedsDisplay only tells the drawing system that a redraw is needed in the next frame; it doesn't force the immediate rendering of a frame that might never be displayed on the screen.