CATiledLayer and UIKit graphics

Q:  I'm using a CATiledLayer in my application and I occasionally see incorrect rendering or crashes, especially when I have multiple views on screen. What can I do to avoid this?

A: I'm using a CATiledLayer in my application and I occasionally see incorrect rendering or crashes, especially when I have multiple views on screen. What can I do to avoid this?

Therefore, on iOS 4.0 and later, you can override -drawRect: and do all your drawings as normal, but if you plan to support iOS prior to 4.0, you should still be aware of the issues raised in this document.

The CATiledLayer implements its drawing by using a background thread to fetch the contents of each tile, however the drawing functions provided by UIKit rely on a global context stack, and as such when a CATiledLayer starts rendering, using any of UIKit's drawing functions can result in a race condition.

Practically what this means is that you cannot use any UIKit's drawing methods when drawing to a CATiledLayer, and you cannot do your drawing inside of a UIView's -drawRect: method. Instead you must implement -drawLayer:inContext: and draw only using Core Graphics and other thread-safe functions that accept a CGContextRef to target drawing to.

If your -drawRect: method uses UIImage's draw methods, UIColor's set methods, any string drawing method from NSString(UIStringDrawing) or any of the UIRect or UIGraphics functions provided by UIKit, then your -drawLayer:inContext: method must be rewritten to avoid using those methods, and instead use Core Graphics directly. Not all operations from NSString(UIStringDrawing) are trivially converted to Core Graphics, so you may wish to contact Developer Technical Support.

As UIKit uses the existence of an override of -drawRect: to determine some functionality of a UIView, you should still implement the -drawRect: method, but it does not need to do anything (and will not be called due to your implementation of -drawLayer:inContext:). An example of a correct implementation of a UIView that uses a CATiledLayer is in Listing 1.

Listing 1  An implementation of a UIView subclass that uses a CATiledLayer for the layerClass.

#import <QuartzCore/QuartzCore.h>
#import <UIKit/UIKit.h>
#import "MyTiledView.h"

+(Class)layerClass
{
    return [CATiledLayer class];
}

-(void)drawRect:(CGRect)r
{
    // UIView uses the existence of -drawRect: to determine if should allow its CALayer
    // to be invalidated, which would then lead to the layer creating a backing store and
    // -drawLayer:inContext: being called.
    // By implementing an empty -drawRect: method, we allow UIKit to continue to implement
    // this logic, while doing our real drawing work inside of -drawLayer:inContext:
}

-(void)drawLayer:(CALayer*)layer inContext:(CGContextRef)context
{
    // Do all your drawing here. Do not use UIGraphics to do any drawing, use Core Graphics instead.
}

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