iOS Programming: The Big Nerd Ranch Guide

For the More Curious: Layers, Bitmaps, and Contexts

A layer is simply a bitmap – a chunk of memory that holds the red, green, blue, and alpha values of each pixel. When you send the message setNeedsDisplay to a UIView instance, that method is forwarded to the view’s layer. After the run loop is done processing an event, every layer marked for re-display prepares a CGContextRef. Drawing routines called on this context generate pixels that end up in the layer’s bitmap.

How do drawing routines get called on the layer’s context? After a layer prepares its context, it sends the message drawLayer:inContext: to its delegate. The delegate of an implicit layer is its view, so in the implementation for drawLayer:inContext:, the view sends drawRect: to ...