Add support for on-demand transitions #39
Description
Generally-speaking, there are three categories of transition type:
- Those that make assumptions about the fore & back view controller.
- Those that make assumptions about the fore view controller.
- Those that make assumptions about the back view controller.
Our current transitionController.transition API doesn't necessarily make it easy for types 2 or 3 to associate Transition types during a transition. For example, during a contextual photo transition (type 1), our fore view controller may want to slide in its bottom toolbar (type 3). The contextual photo transition was likely configured by the back view controller when it created the presented view controller, but the back view controller shouldn't necessarily be forced to configure the toolbar transition as well.
Ideally, the fore view controller could react to a transition occurring and add extra transitions when it deems necessary. The back view controller could also provide similar facilities.
With this sort of approach we may find that it's actually beneficial to remove the transition = API and prefer a querying API instead.