animation.m

- (id<CAAction>)actionForLayer:(CALayer *)layer forKey:(NSString *)event {
    if (layer == self.shapeLayer && [event isEqualToString:@"strokeEnd"]) {
        //Only perform animation action when in a UIView animation block
        CAAnimation *opacityAction = (id)[super actionForLayer:layer forKey:@"opacity"];
        if (opacityAction && ![opacityAction isKindOfClass:[NSNull class]] && [opacityAction isKindOfClass:[CAAnimation class]]) {
            CABasicAnimation *animation = [CABasicAnimation animationWithKeyPath:event];
            animation.fromValue = [layer.presentationLayer valueForKey:event];
            animation.duration = opacityAction.duration;
            return animation;
        } else {
            return [NSNull null];
        }
    }
    return [super actionForLayer:layer forKey:event];
}