Alex Rivera | Logout

iOS: Using UIView's 'drawRect:' vs. its layer's delegate 'drawLayer:inContext:'

Asked 2011-02-12T16:39:56.390
75

I have a class which is a subclass of UIView. I am able to draw stuff inside the view either by implementing the drawRect method, or by implementing drawLayer:inContext: which is a delegate method of CALayer.

I have two questions:

  1. How to decide which approach to use? Is there a use case for each one?

  2. If I implement drawLayer:inContext:, it is called (and drawRect isn't, at least as far as putting a breakpoint can tell), even if I don't assign my view as the CALayer delegate by using:

    [[self layer] setDelegate:self];

    how come the delegate method is called if my instance is not defined to be the layer's delegate? and what mechanism prevents drawRect from being called if drawLayer:inContext: is called?

iphone ios delegates core-animation
Edit
Report

1 Answer

44

drawRect should only be implemented when absolutely needed. The default implementation of drawRect includes a number of smart optimizations, like intelligently caching the view's rendering. Overriding it circumvents all of those optimizations. That's bad. Using the layer drawing methods effectively will almost always outperform a custom drawRect. Apple uses a UIView as the delegate for a CALayer often - in fact, every UIView is the delegate of it's layer. You can see how to customize the layer drawing inside a UIView in several Apple samples including (at this time) ZoomingPDFViewer.

While the use of drawRect is common, it's a practice that has been discouraged since at least 2002/2003, IIRC. There aren't many good reasons left to go down that path.

Advanced Performance Optimization on iPhone OS (slide 15)

Core Animation Essentials

Understanding UIKit Rendering

Technical Q&A QA1708: Improving Image Drawing Performance on iOS

View Programming Guide: Optimizing View Drawing

answered 2012-02-08T05:18:15.517

Your Answer