如何从两个不同的视图层次结构“链接”两个图层？

Question

I would like to know whether it's possible - and if so, how - to "link" or chain together two layers from two different views, so that when one of the layers moves (because it's being animated) it 'drags' the other layer in the same direction and with the same timing.

The reason I want to do this is because I have two translucent views, both of the same size and initially one on top of the other, but at some point the topmost view is animated and I need to mask off the contents of the bottom view with the 'shape' of the topmost one. I already created a layer mask for the bottom view which is the same size as the top view, but I need that layer mask to be dragged along with it when it starts moving.

Best,

Answer 1

There is no way to listen, so to speak, for an animation to occur, however, you could setup a display link and just check/poll the layers to see if anything is animating. I think it would create a lot of overhead, but it could potentially give you what you're looking for.

I would probably prefer to just animate both layers at the same time. I don't know what event triggers a layer to start animating in your scenario, but it's there where I would look to try to animate the second layer along with it.

If you want to try to go the display link route you can do this:

CADisplayLink *displayLink = [CADisplayLink displayLinkWithTarget:self 
                                                         selector:
                                                               @selector(
                                                         displayLinkDidUpdate:)];

// Add to the run loop
[displayLink  addToRunLoop:[NSRunLoop mainRunLoop] 
                   forMode:NSDefaultRunLoopMode];

Then, implement the display update method:

- (void)displayLinkDidUpdate:(CADisplayLink *)sender
{
  CALayer *presentationLayer = (CALayer*)[_animatingLayer presentationLayer];

  // Do something with the presentation layer

}

If the layer is animating, the presentation layer will have the current mid-flight animation values.