- iOS 8.0+
- macOS 10.10+
- tvOS 9.0+
- watchOS 3.0+
The block takes a single parameter:
A fraction of the action’s The input value for the timing function, as determined by the
timingproperty and the action’s current progress.
Your block must return a floating-point value between
0 represents the starting state of the action’s animation and
1 represents the end state.