I've not had time to check, but I don't think it's necessary to manually calculate the gradient positions. If nil is passed as the arg to positions in the gradient layer, the color positions along the gradient are automatically calculated. (unless things have changed recently?)

Maybe I had something else wrong here... however I got just a black background

Hmm... that's weird, sorry about that.

In the example project the gradient animation works without having to manually calculate the gradient stop positions. Are you doing something slightly different to the example project, which could potentially result in the black background instead of a gradient? Perhaps there's something unclear in the API that could be the cause?

I'll give it another try with a fresh checkout... I'll let u know tomorrow