Create a custom animatable property

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If you extend CALayer and implement your custom 

- (void) drawInContext:(CGContextRef) context

You can make an animatable property by overriding needsDisplayForKey (in your custom CALayer class) like this:

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

Of course, you also need to have a @property called percentage. From now on you can animate the percentage property using core animation. I did not check whether it works using the [UIView animateWithDuration...] call as well. It might work. But this worked for me:

CABasicAnimation *animation = [CABasicAnimation animationWithKeyPath:@"percentage"];
animation.duration = 1.0;
animation.fromValue = [NSNumber numberWithDouble:0];
animation.toValue = [NSNumber numberWithDouble:100];

[myCustomLayer addAnimation:animation forKey:@"animatePercentage"];

Oh and to use yourCustomLayer with myCircleView, do this:

[myCircleView.layer addSublayer:myCustomLayer];

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