Bug 1180125 part 8 - Dispatch transition events from refresh driver; r=dbaron

This patch causes transition events to be dispatched as a separate step after
sampling the transitions. Eventually this will allow us to sample transitions
from their timeline (independently of where they came from and in potentially
any order) by separating the concepts of sampling and event dispatch.

layout/base/nsRefreshDriver.cpp

@@ -1490,25 +1490,44 @@ nsRefreshDriver::DispatchPendingEvents()
   }
 }
 
+namespace {
+  enum class AnimationEventType {
+    CSSAnimations,
+    CSSTransitions
+  };
+
+  struct DispatchAnimationEventParams {
+    AnimationEventType mEventType;
+    nsRefreshDriver* mRefreshDriver;
+  };
+}
+
 static bool
 DispatchAnimationEventsOnSubDocuments(nsIDocument* aDocument,
-                                      void* aRefreshDriver)
+                                      void* aParams)
 {
+  MOZ_ASSERT(aParams, "Animation event parameters should be set");
+  auto params = static_cast<DispatchAnimationEventParams*>(aParams);
+
   nsIPresShell* shell = aDocument->GetShell();
   if (!shell) {
     return true;
   }
 
   nsPresContext* context = shell->GetPresContext();
-  if (!context || context->RefreshDriver() != aRefreshDriver) {
+  if (!context || context->RefreshDriver() != params->mRefreshDriver) {
     return true;
   }
 
   nsCOMPtr<nsIDocument> kungFuDeathGrip(aDocument);
 
-  context->AnimationManager()->DispatchEvents();
+  if (params->mEventType == AnimationEventType::CSSAnimations) {
+    context->AnimationManager()->DispatchEvents();
+  } else {
+    context->TransitionManager()->DispatchEvents();
+  }
   aDocument->EnumerateSubDocuments(DispatchAnimationEventsOnSubDocuments,
-                                   nullptr);
+                                   aParams);
 
   return true;
 }
@@ -1521,7 +1540,18 @@ nsRefreshDriver::DispatchAnimationEvents()
   }
 
   nsIDocument* doc = mPresContext->Document();
-  DispatchAnimationEventsOnSubDocuments(doc, this);
+
+  // Dispatch transition events first since transitions conceptually sit
+  // below animations in terms of compositing order.
+  DispatchAnimationEventParams params { AnimationEventType::CSSTransitions,
+                                        this };
+  DispatchAnimationEventsOnSubDocuments(doc, &params);
+  if (!mPresContext) {
+    return;
+  }
+
+  params.mEventType = AnimationEventType::CSSAnimations;
+  DispatchAnimationEventsOnSubDocuments(doc, &params);
 }
 
 void

layout/style/nsTransitionManager.cpp

@@ -1003,6 +1003,4 @@ nsTransitionManager::FlushTransitions(FlushFlags aFlags)
   }
 
   MaybeStartOrStopObservingRefreshDriver();
-
-  mEventDispatcher.DispatchEvents(mPresContext);
 }

layout/style/nsTransitionManager.h

@@ -285,6 +285,7 @@ public:
       mozilla::Forward<mozilla::TransitionEventInfo>(aEventInfo));
   }
 
+  void DispatchEvents()  { mEventDispatcher.DispatchEvents(mPresContext); }
   void ClearEventQueue() { mEventDispatcher.ClearEventQueue(); }
 
 protected: