objective-c : three steps to create a custom transition in iOS

snippet.m

//three steps to create a custom transition in iOS:

//1. Create the animation controller. The first step is to create a class that implements the UIViewControllerAnimatedTransitioning protocol. This class contains the code to perform the actual animation, so this class is referred to as the animation controller.

//2. Before presenting a view controller, set its transitioning delegate. Before you present a view controller, you should set a class as its transitioning delegate (usually the presenting view controller). By doing this the delegate will get a callback asking for the animation controller to use when presenting the view controller.

//3. Return the animation controller in the callback. Finally, implement the callback method to return an instance of the animation controller you created in step 1.