Ensure that system animations take effect during view controller tran…

…sitions.

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

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

Tags: #material_motion

Differential Revision: http://codereview.cc/D3092

src/transitions/TransitionContext.swift

@@ -206,12 +206,28 @@ extension TransitionContext {
     self.runtime = MotionRuntime(containerView: containerView())
     self.replicator.containerView = containerView()
 
+    pokeSystemAnimations()
+
     let terminators = transition.willBeginTransition(withContext: self, runtime: self.runtime)
     runtime.whenAllAtRest(terminators) { [weak self] in
       self?.terminate()
     }
   }
 
+  // UIKit transitions will not animate any of the system animations (status bar changes, notably)
+  // unless we have at least one implicit UIView animation. Material Motion doesn't use implicit
+  // animations out of the box, so to ensure that system animations still occur we create an
+  // invisible throwaway view and apply an animation to it.
+  private func pokeSystemAnimations() {
+    let throwawayView = UIView()
+    containerView().addSubview(throwawayView)
+    UIView.animate(withDuration: transitionDuration(using: context), animations: {
+      throwawayView.frame = throwawayView.frame.offsetBy(dx: 1, dy: 0)
+    }, completion: { didComplete in
+      throwawayView.removeFromSuperview()
+    })
+  }
+
   private func terminate() {
     guard runtime != nil else { return }
     let completedInOriginalDirection = direction.value == initialDirection