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POHow exactly to subclass CALayer and use a custom property?
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copied!<p>I am trying to create a subclass of <code>CALayer</code> with a custom <code>index</code> property that I can both animate and change directly in order to display a different picture based on the index.</p> <p>In the header, I declared:</p> <pre><code>@property NSUInteger index; </code></pre> <p>In the implementation, I overrode <code>needDisplayForKey</code>:</p> <pre><code>+ (BOOL)needsDisplayForKey:(NSString *)key { if ([key isEqualToString:@"index"]) return YES; else return [super needsDisplayForKey:key]; } </code></pre> <p>Now, in the <code>display</code> method, I want to display a picture based on the index. However, during an animation, the value of <code>self.index</code> never changes, so I have to query the value of the presentation layer, contrary to the example on <a href="https://developer.apple.com/library/mac/#documentation/cocoa/conceptual/coreanimation_guide/Articles/ProvidingCALayerContent.html#//apple_ref/doc/uid/TP40006642-SW1" rel="noreferrer">Providing CALayer Content by Subclassing</a>:</p> <pre><code>- (void)display { NSUInteger presentationIndex = [(CustomLayer *)[self presentationLayer] index]; self.contents = (id)[[images objectAtIndex:presentationIndex] CGImage]; } </code></pre> <p>The problem is, if I do that, I cannot set the <code>index</code> value directly outside of an animation, because it will only change the model layer, and the <code>display</code> method explicitly queries the presentation layer.</p> <p>If I add an initialization method that copies the value of <code>index</code>, it works:</p> <pre><code>- (id)initWithLayer:(id)layer { self = [super initWithLayer:layer]; if (self) { if ([layer isKindOfClass:[CustomLayer class]]) self.index = [(CustomLayer *)layer index]; } return self; } </code></pre> <p>However, after or before an animation, there is always a 1 image glitch because the presentation or the model value don't match (depending if I set <code>index</code> to the destination value or not).</p> <ul> <li>Surprisingly, it seems like the <code>drawInContext:</code> method always has the right value for <code>[self index]</code>, but it is not the method I want to use since I just set the <code>content</code> property with an image.</li> <li>I get different behaviors depending on the way I implement the <code>index</code> property. If I use <code>@dynamic index</code> (which works, even though the documentation doesn't say that custom property getters/setters would be dynamically implemented), <code>display</code> is called every time the value of <code>index</code> is changed. If I use <code>@synthesize</code> or implement a setter, <code>display</code> is not called, so I would need to change <code>content</code> in the setter too.</li> <li>Should I use an instance variable? Should I use the dynamic implementation? Or should I use <code>setValue:forKey:</code> instead?</li> </ul> <p>As you can see, I am a bit confused about how to get the result I want, and how to correctly implement a subclass of CALayer with a custom property. Any help, and explanations, would be appreciated!</p>

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