Animating a custom property of CALayer subclass

Question

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?

Answer 1

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.

Answer 2

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

1. Make your property dynamic so that CA implements the accessors for you:

   @dynamic myInt;
   

2. Tell the layer that changes of the property require redrawing:

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

3. Use the value of myInt in your drawInContext: method. Now, when you animate myInt, Core Animation will interpolate the values for each step of the animation and repeatedly ask the layer to draw itself.

4. If you also want to enable implicit animations for this property, also override actionForKey:.