UIView layer's animationForKey: is always nil

I have a rather curious problem.

I have a view that I'm animating with a CAKeyframeAnimation. The animation works as expected however, once the delegate's animationDidStop:finished method is fired, the layer associated with the view always returns nil when checked with the layer's animationForKey: method.

Here's an illustration of the problem:

// This is in the view controller
- (void) doAnimation:(id)sender {
    CAKeyframeAnimation *positionAnimation = [CAKeyframeAnimation animationWithKeyPath:@"position"];

    [positionAnimation setValues:arrayOfPoint];
    [positionAnimation setDuration:0.5];
    // ViewController is the delegate and has the animationDidStop:finished: method
    positionAnimation.delegate = self;
    [positionAnimation setValue:@"I am Text" forKeyPath:@"positionAnimation"];

    [someView.layer addAnimation:positionAnimation forKey:@"positionAnimation"];
    someView.layer.position = newPosition;
}

Elsewhere in the controller, I have the animationDidStop:finished: method defined as follows:

- (void) animationDidStop:(CAAnimation *)anim finished:(BOOL)flag {

        NSArray *array = [someView.layer animationKeys];
        NSString *stringValue = [anim valueForKey:@"positionAnimation"];

        NSLog(@"valueForKey: %@", stringValue);
        // Prints 'I am Text'

        if(anim == [someView.layer animationForKey:@"positionAnimation"]) {
            //THIS NEVER GET CALLED
        }
}

Why does the if block never evaluate to True? Why is [someView.layer animationForKey:@"positionAnimation"]开发者_JAVA百科 always nil? Additionally, stringValue has the correct data, but array is also nil.

I can just guess, but maybe when method animationDidStop:finished: is called your animation has already been removed from the layer. So [someView.layer animationForKey:@"positionAnimation"] is nil.

Hope this'll help