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diff --git a/dom/animation/AnimationEventDispatcher.h b/dom/animation/AnimationEventDispatcher.h
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+++ b/dom/animation/AnimationEventDispatcher.h
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_AnimationEventDispatcher_h
+#define mozilla_AnimationEventDispatcher_h
+
+#include "mozilla/AnimationComparator.h"
+#include "mozilla/Assertions.h"
+#include "mozilla/Attributes.h"
+#include "mozilla/ContentEvents.h"
+#include "mozilla/EventDispatcher.h"
+#include "mozilla/Variant.h"
+#include "mozilla/dom/AnimationPlaybackEvent.h"
+#include "mozilla/dom/KeyframeEffect.h"
+#include "mozilla/ProfilerMarkers.h"
+#include "nsCycleCollectionParticipant.h"
+#include "nsPresContext.h"
+
+class nsRefreshDriver;
+
+namespace mozilla {
+
+struct AnimationEventInfo {
+  struct CssAnimationOrTransitionData {
+    OwningAnimationTarget mTarget;
+    const EventMessage mMessage;
+    const double mElapsedTime;
+    // FIXME(emilio): is this needed? This preserves behavior from before
+    // bug 1847200, but it's unclear what the timeStamp of the event should be.
+    // See also https://github.com/w3c/csswg-drafts/issues/9167
+    const TimeStamp mEventEnqueueTimeStamp{TimeStamp::Now()};
+  };
+
+  struct CssAnimationData : public CssAnimationOrTransitionData {
+    const RefPtr<nsAtom> mAnimationName;
+  };
+
+  struct CssTransitionData : public CssAnimationOrTransitionData {
+    // For transition events only.
+    const AnimatedPropertyID mProperty;
+  };
+
+  struct WebAnimationData {
+    RefPtr<dom::AnimationPlaybackEvent> mEvent;
+  };
+
+  using Data = Variant<CssAnimationData, CssTransitionData, WebAnimationData>;
+
+  RefPtr<dom::Animation> mAnimation;
+  TimeStamp mScheduledEventTimeStamp;
+  Data mData;
+
+  OwningAnimationTarget* GetOwningAnimationTarget() {
+    if (mData.is<CssAnimationData>()) {
+      return &mData.as<CssAnimationData>().mTarget;
+    }
+    if (mData.is<CssTransitionData>()) {
+      return &mData.as<CssTransitionData>().mTarget;
+    }
+    return nullptr;
+  }
+
+  void MaybeAddMarker() const;
+
+  // For CSS animation events
+  AnimationEventInfo(RefPtr<nsAtom> aAnimationName,
+                     const NonOwningAnimationTarget& aTarget,
+                     EventMessage aMessage, double aElapsedTime,
+                     const TimeStamp& aScheduledEventTimeStamp,
+                     dom::Animation* aAnimation)
+      : mAnimation(aAnimation),
+        mScheduledEventTimeStamp(aScheduledEventTimeStamp),
+        mData(CssAnimationData{
+            {OwningAnimationTarget(aTarget.mElement, aTarget.mPseudoType),
+             aMessage, aElapsedTime},
+            std::move(aAnimationName)}) {
+    if (profiler_thread_is_being_profiled_for_markers()) {
+      MaybeAddMarker();
+    }
+  }
+
+  // For CSS transition events
+  AnimationEventInfo(const AnimatedPropertyID& aProperty,
+                     const NonOwningAnimationTarget& aTarget,
+                     EventMessage aMessage, double aElapsedTime,
+                     const TimeStamp& aScheduledEventTimeStamp,
+                     dom::Animation* aAnimation)
+      : mAnimation(aAnimation),
+        mScheduledEventTimeStamp(aScheduledEventTimeStamp),
+        mData(CssTransitionData{
+            {OwningAnimationTarget(aTarget.mElement, aTarget.mPseudoType),
+             aMessage, aElapsedTime},
+            aProperty}) {
+    if (profiler_thread_is_being_profiled_for_markers()) {
+      MaybeAddMarker();
+    }
+  }
+
+  // For web animation events
+  AnimationEventInfo(RefPtr<dom::AnimationPlaybackEvent>&& aEvent,
+                     TimeStamp&& aScheduledEventTimeStamp,
+                     dom::Animation* aAnimation)
+      : mAnimation(aAnimation),
+        mScheduledEventTimeStamp(std::move(aScheduledEventTimeStamp)),
+        mData(WebAnimationData{std::move(aEvent)}) {}
+
+  AnimationEventInfo(const AnimationEventInfo& aOther) = delete;
+  AnimationEventInfo& operator=(const AnimationEventInfo& aOther) = delete;
+
+  AnimationEventInfo(AnimationEventInfo&& aOther) = default;
+  AnimationEventInfo& operator=(AnimationEventInfo&& aOther) = default;
+
+  bool operator<(const AnimationEventInfo& aOther) const {
+    if (this->mScheduledEventTimeStamp != aOther.mScheduledEventTimeStamp) {
+      // Null timestamps sort first
+      if (this->mScheduledEventTimeStamp.IsNull() ||
+          aOther.mScheduledEventTimeStamp.IsNull()) {
+        return this->mScheduledEventTimeStamp.IsNull();
+      }
+      return this->mScheduledEventTimeStamp < aOther.mScheduledEventTimeStamp;
+    }
+
+    // Events in the Web Animations spec are prior to CSS events.
+    if (this->IsWebAnimationEvent() != aOther.IsWebAnimationEvent()) {
+      return this->IsWebAnimationEvent();
+    }
+
+    AnimationPtrComparator<RefPtr<dom::Animation>> comparator;
+    return comparator.LessThan(this->mAnimation, aOther.mAnimation);
+  }
+
+  bool IsWebAnimationEvent() const { return mData.is<WebAnimationData>(); }
+
+  // TODO: Convert this to MOZ_CAN_RUN_SCRIPT (bug 1415230)
+  MOZ_CAN_RUN_SCRIPT_BOUNDARY void Dispatch(nsPresContext* aPresContext) {
+    if (mData.is<WebAnimationData>()) {
+      RefPtr playbackEvent = mData.as<WebAnimationData>().mEvent;
+      RefPtr target = mAnimation;
+      EventDispatcher::DispatchDOMEvent(target, nullptr /* WidgetEvent */,
+                                        playbackEvent, aPresContext,
+                                        nullptr /* nsEventStatus */);
+      return;
+    }
+
+    if (mData.is<CssTransitionData>()) {
+      const auto& data = mData.as<CssTransitionData>();
+      nsPIDOMWindowInner* win =
+          data.mTarget.mElement->OwnerDoc()->GetInnerWindow();
+      if (win && !win->HasTransitionEventListeners()) {
+        MOZ_ASSERT(data.mMessage == eTransitionStart ||
+                   data.mMessage == eTransitionRun ||
+                   data.mMessage == eTransitionEnd ||
+                   data.mMessage == eTransitionCancel);
+        return;
+      }
+
+      InternalTransitionEvent event(true, data.mMessage);
+      data.mProperty.ToString(event.mPropertyName);
+      event.mElapsedTime = data.mElapsedTime;
+      event.mPseudoElement =
+          nsCSSPseudoElements::PseudoTypeAsString(data.mTarget.mPseudoType);
+      event.AssignEventTime(WidgetEventTime(data.mEventEnqueueTimeStamp));
+      RefPtr target = data.mTarget.mElement;
+      EventDispatcher::Dispatch(target, aPresContext, &event);
+      return;
+    }
+
+    const auto& data = mData.as<CssAnimationData>();
+    InternalAnimationEvent event(true, data.mMessage);
+    data.mAnimationName->ToString(event.mAnimationName);
+    event.mElapsedTime = data.mElapsedTime;
+    event.mPseudoElement =
+        nsCSSPseudoElements::PseudoTypeAsString(data.mTarget.mPseudoType);
+    event.AssignEventTime(WidgetEventTime(data.mEventEnqueueTimeStamp));
+    RefPtr target = data.mTarget.mElement;
+    EventDispatcher::Dispatch(target, aPresContext, &event);
+  }
+};
+
+class AnimationEventDispatcher final {
+ public:
+  explicit AnimationEventDispatcher(nsPresContext* aPresContext)
+      : mPresContext(aPresContext), mIsSorted(true), mIsObserving(false) {}
+
+  NS_INLINE_DECL_CYCLE_COLLECTING_NATIVE_REFCOUNTING(AnimationEventDispatcher)
+  NS_DECL_CYCLE_COLLECTION_NATIVE_CLASS(AnimationEventDispatcher)
+
+  void Disconnect();
+
+  void QueueEvent(AnimationEventInfo&& aEvent);
+  void QueueEvents(nsTArray<AnimationEventInfo>&& aEvents);
+
+  // This will call SortEvents automatically if it has not already been
+  // called.
+  void DispatchEvents() {
+    mIsObserving = false;
+    if (!mPresContext || mPendingEvents.IsEmpty()) {
+      return;
+    }
+
+    SortEvents();
+
+    EventArray events = std::move(mPendingEvents);
+    // mIsSorted will be set to true by SortEvents above, and we leave it
+    // that way since mPendingEvents is now empty
+    for (AnimationEventInfo& info : events) {
+      info.Dispatch(mPresContext);
+
+      // Bail out if our mPresContext was nullified due to destroying the pres
+      // context.
+      if (!mPresContext) {
+        break;
+      }
+    }
+  }
+
+  void ClearEventQueue() {
+    mPendingEvents.Clear();
+    mIsSorted = true;
+  }
+  bool HasQueuedEvents() const { return !mPendingEvents.IsEmpty(); }
+
+ private:
+#ifndef DEBUG
+  ~AnimationEventDispatcher() = default;
+#else
+  ~AnimationEventDispatcher() {
+    MOZ_ASSERT(!mIsObserving,
+               "AnimationEventDispatcher should have disassociated from "
+               "nsRefreshDriver");
+  }
+#endif
+
+  // Sort all pending CSS animation/transition events by scheduled event time
+  // and composite order.
+  // https://drafts.csswg.org/web-animations/#update-animations-and-send-events
+  void SortEvents() {
+    if (mIsSorted) {
+      return;
+    }
+
+    for (auto& pending : mPendingEvents) {
+      pending.mAnimation->CachedChildIndexRef().reset();
+    }
+
+    mPendingEvents.StableSort();
+    mIsSorted = true;
+  }
+  void ScheduleDispatch();
+
+  nsPresContext* mPresContext;
+  using EventArray = nsTArray<AnimationEventInfo>;
+  EventArray mPendingEvents;
+  bool mIsSorted;
+  bool mIsObserving;
+};
+
+}  // namespace mozilla
+
+#endif  // mozilla_AnimationEventDispatcher_h