How to check if CALayer exists

I have a CALayer which animates (moves) off-screen on the y axis. After it's off-screen i'm doing a [myLayer removeFromSuperLayer] so开发者_如何学Python its gone from the view and does not go back to start position.

While the layer is still in the view it can be paused and resumed by pushing a toggle button and this works all fine.

The only thing is that after the [myLayer removeFromSuperLayer] has run my app crashes. This is caused by the fact the button is trying to pause or resume the layer which doesn't exist anymore.

How can i check if the layer is removed or still in the view?

I thought something like this for the pause part of my toggle button:

if (self.myLayer == nil)
    {
        // here i want to add the layer again
        [self.view.layer addSublayer:myLayer];
        // immediately pause it
        [self pauseLayer:myLayer];

    }

else

    {
        // just pause no need to create the layer again because it's still there 
        [self pauseLayer:myLayer]; 
    }

As you might suspect the self.myLayer == nil is not the way to do it, but what is?

Thanks in advance.

Removing a layer from its superlayer will not cause the layer to become nil, which is why your self.myLayer == nil check is not working. However, you could easily set the field to nil when you remove it, like:

[self.myLayer removeFromSuperLayer];
self.myLayer = nil;

Of course, if you need to add it again after that then you'd need to either reassign the layer to self.myLayer (if you have a reference to it that you're keeping somewhere else) or create a new instance of the layer from scratch.