Using CALayer Delegate

Question

I have a UIView whose layers will have sublayers. I\'d like to assign delegates for each of those sublayers, so the delegate method can tell the layer what to draw. My que

Answer 1

I prefer the following solution. I would like to use the drawLayer:inContext: method of the UIView to render a subview that I might add without adding extra classes all over the place. My solution is as follows:

Add the following files to your project:

UIView+UIView_LayerAdditions.h with contents:

@interface UIView (UIView_LayerAdditions)

- (CALayer *)createSublayer;

@end

UIView+UIView_LayerAdditions.m with contents

#import "UIView+UIView_LayerAdditions.h"

static int LayerDelegateDirectorKey;

@interface LayerDelegateDirector: NSObject{ @public UIView *view; } @end
@implementation LayerDelegateDirector

- (void)drawLayer:(CALayer *)layer inContext:(CGContextRef)ctx
{
    [view drawLayer:layer inContext:ctx];
}

@end

@implementation UIView (UIView_LayerAdditions)

- (LayerDelegateDirector *)director
{
    LayerDelegateDirector *director = objc_getAssociatedObject(self, &LayerDelegateDirectorKey);
    if (director == nil) {
        director = [LayerDelegateDirector new];
        director->view = self;
        objc_setAssociatedObject(self, &LayerDelegateDirectorKey, director, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
    }
    return director;
}

- (CALayer *)createSublayer
{
    CALayer *layer = [CALayer new];
    layer.contentsScale = [UIScreen mainScreen].scale;
    layer.delegate = [self director];
    [self.layer addSublayer:layer];
    [layer setNeedsDisplay];
    return layer;
}

@end

Now add the header to your .pch file. If you add a layer using the createSublayer method, it will automagically show up without bad allocs in the override to drawLayer:inContext:. As far as I know the overhead of this solution is minimal.

Answer 2

A note regarding "helper" classes for use as a layer's delegate (with ARC at least):

Make sure you store a "strong" reference to your alloc/init'd helper class (such as in a property). Simply assigning the alloc/init'd helper class to the delegate seems to cause crashes for me, presumably because mylayer.delegate is a weak reference to your helper class (as most delegates are), so the helper class gets freed up before the layer can use it.

If I assign the helper class to a property, then assign it to the delegate, my weird crashes go away, and things behave as expected.