Evaluate whether the presentation controller should be able to conform to Transition

[jverkoey]

Presentation controllers as of 1.0.0 can conform to Transition in order to receive their own start invocation. This was done so that presentation controller-specific motion could be implemented in a way that is consistent with the actual transition implementations.

In building out some more transitions I feel that it's possible to drop this requirement in favor of encouraging the Transition object to manage all motion instead. The presentation controller can then simply focus on the willBegin/didEnd events in order to manage the lifecycle and visibility of views.

The concrete proposal of this issue is to:

• Remove support for presentation controllers conforming to Transition.

[jverkoey]

From my explorations I've been able to find the following benefits to allowing presentation controllers to add animations alongside the transition:

• It allows you to customize the animation timing, notably the duration and delay.

I've also found the following drawbacks:

• If we implement fallback support, it's possible for our presentation controller to be presented alongside a transition but then to be dismissed alongside another or none at all (if a system transition is used). In such cases the presentation controller's start implementation won't be invoked. I'm currently exploring solutions to this.