What is the role of key path in Core Animation?

问题:

I'm getting acquanted with Core Animation and trying to implement simple movement along a curve line. i've found really helpful piece of code here

There is a line there:

CAKeyframeAnimation *pathAnimation = [CAKeyframeAnimation animationWithKeyPath:@"position"];

Why do we need this property named keyPath? What is its role? The code works only if use the proposed NSString @"position" and nothing else. So is it a built-in constant string? It doesn't look so. Hope you see my confusion. Can you please explain in plain language how do i manage this keyPath property?

回答1:

• KeyPath says which animatable attribute is going to modified to create animation effect
• You can use any of the animatable property as keypath
• You can find them in the header file(say CALayer) on which you apply animation

 /* The position in the superlayer that the anchor point of the layer's
  bounds rect is aligned to. Defaults to the zero point. **Animatable**. */

   @property CGPoint position;

• Not a constant String , it is attribute name

回答2:

CABasicAnimation and CAKeyframeAnimation are inherited from CAPropertyAnimation. Both use its animationWithKeyPath: method.

KeyPath has to contain Animatable Properties. Find them here Link