Animating a custom property of CALayer subclass

I have a CALayer subclass, MyLayer, that has a NSInteger property called myInt. I'd really like to animate this property via CABasicAnimation, but it seems CABasicAnimation only works on so-called "animatable" properties (bounds, position, etc). Is there something I can override to make my custom myInt property animatable?

Yes, it's possible (only in the latest Core Animation releases though, I believe, ie iPhone 3.0+ and OS X 10.6+).

@dynamic myInt;

+ (BOOL)needsDisplayForKey:(NSString*)key {
    if ([key isEqualToString:@"myInt"]) {
        return YES;
    } else {
        return [super needsDisplayForKey:key];
    }
}

There is a way to retain the iVars of your custom CALayer subclasses. You override initWithLayer:, the method which is called to create a copy of custom layers. For example, if you have a layer in which you want to create a custom property called 'angle', you might use the following code:

@implementation AngledLayer
@synthesize angle = _angle

// Tell Core Animation that this key should be animated
+ (BOOL) needsDisplayForKey:(NSString *)key
{
    if ([key isEqualToString:@"angle"]) return YES;
    return [super needsDisplayForKey:key];
}


// Make sure that, when the layer is copied, so is the custom ivar
- (id) initWithLayer:(id)layer
{
    self = [super initWithLayer:layer];
    if (self) {
        AngledLayer *angledVersion = (AngledLayer *)layer;
        self.angle = angledVersion.angle;
    }
    return self;
}

And bob's your uncle! Note that you can't use this object with implicit animation, for which you'd also have to overide the actionForKey: method.

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