Add a defaultModalPresentationStyle API for transitions with presenta…

…tion.

Summary: This allows transitions with presentation to choose the default modal presentation style for a view controller. This is necessary because the transition may choose not to use a presentation controller even though it conforms to the TransitionWithPresentation protocol (e.g. a modal transition that covers the whole screen may not need a presentation controller).

Reviewers: O2 Material Motion, O4 Material Apple platform reviewers, #material_motion, randcode-generator, markwei

Reviewed By: O2 Material Motion, O4 Material Apple platform reviewers, #material_motion, randcode-generator, markwei

Tags: #material_motion

Differential Revision: http://codereview.cc/D3230

src/transitions/Transition.swift

@@ -62,6 +62,17 @@ public protocol TransitionWithTermination: Transition {
  */
 public protocol TransitionWithPresentation: Transition {
 
+  /**
+   The modal presentation style this transition expects to use.
+
+   This value is queried when the transition is assigned to a view controller's
+   transitionController. The result, if any, is assigned to the view controller's
+   modalPresentationStyle property.
+
+   In order for a presentationController to be used, this method should return `.custom`.
+   */
+  func defaultModalPresentationStyle() -> UIModalPresentationStyle?
+
   /**
    Queried before the Transition object is instantiated and only once, when the fore view controller
    is initially presented.

src/transitions/TransitionController.swift

@@ -67,8 +67,9 @@ public final class TransitionController {
     set {
       _transitioningDelegate.transition = newValue
 
-      if newValue is TransitionWithPresentation {
-        _transitioningDelegate.associatedViewController?.modalPresentationStyle = .custom
+      if let presentationTransition = newValue as? TransitionWithPresentation,
+        let modalPresentationStyle = presentationTransition.defaultModalPresentationStyle() {
+        _transitioningDelegate.associatedViewController?.modalPresentationStyle = modalPresentationStyle
       }
     }
     get { return _transitioningDelegate.transition }