A set of methods that provides information about an in-progress view controller transition.
Do not adopt this protocol in your own classes. UIKit creates an object that adopts this protocol and makes it available to your code when you animate changes using a transition coordinator object.
A transition coordinator context provides most of the same information as an object that adopts the UIViewControllerContextTransitioning protocol. You use this contextual information to determine the animation parameters, such as the view in which the animations take place, whether the transition is interactive, or whether the transition was the result of an interface orientation change. You then apply that information to the animations you create.
Most animations take place in the view returned by the containerView method. And at the time your animation blocks are executed, the view hierarchy already contains the view of the from view controller. You can use your animation blocks to animate additional content in that same container view or you can animate content in an entirely different view.