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diff --git a/dom/animation/EffectSet.h b/dom/animation/EffectSet.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_EffectSet_h
+#define mozilla_EffectSet_h
+
+#include "mozilla/DebugOnly.h"
+#include "mozilla/EffectCompositor.h"
+#include "mozilla/EnumeratedArray.h"
+#include "mozilla/TimeStamp.h"
+#include "mozilla/dom/KeyframeEffect.h"
+#include "nsHashKeys.h"  // For nsPtrHashKey
+#include "nsTHashSet.h"
+
+class nsPresContext;
+enum class DisplayItemType : uint8_t;
+
+namespace mozilla {
+
+namespace dom {
+class Element;
+}  // namespace dom
+
+enum class PseudoStyleType : uint8_t;
+
+// A wrapper around a hashset of AnimationEffect objects to handle
+// storing the set as a property of an element.
+class EffectSet {
+ public:
+  EffectSet()
+      : mCascadeNeedsUpdate(false),
+        mMayHaveOpacityAnim(false),
+        mMayHaveTransformAnim(false) {
+    MOZ_COUNT_CTOR(EffectSet);
+  }
+
+  ~EffectSet() {
+    MOZ_ASSERT(!IsBeingEnumerated(),
+               "Effect set should not be destroyed while it is being "
+               "enumerated");
+    MOZ_COUNT_DTOR(EffectSet);
+  }
+
+  // Methods for supporting cycle-collection
+  void Traverse(nsCycleCollectionTraversalCallback& aCallback);
+
+  static EffectSet* Get(const dom::Element* aElement,
+                        PseudoStyleType aPseudoType);
+  static EffectSet* GetOrCreate(dom::Element* aElement,
+                                PseudoStyleType aPseudoType);
+
+  static EffectSet* GetForFrame(const nsIFrame* aFrame,
+                                const nsCSSPropertyIDSet& aProperties);
+  static EffectSet* GetForFrame(const nsIFrame* aFrame,
+                                DisplayItemType aDisplayItemType);
+  // Gets the EffectSet associated with the specified frame's content.
+  //
+  // Typically the specified frame should be a "style frame".
+  //
+  // That is because display:table content:
+  //
+  // - makes a distinction between the primary frame and style frame,
+  // - associates the EffectSet with the style frame's content,
+  // - applies transform animations to the primary frame.
+  //
+  // In such a situation, passing in the primary frame here will return nullptr
+  // despite the fact that it has a transform animation applied to it.
+  //
+  // GetForFrame, above, handles this by automatically looking up the
+  // EffectSet on the corresponding style frame when querying transform
+  // properties. Unless you are sure you know what you are doing, you should
+  // try using GetForFrame first.
+  //
+  // If you decide to use this, consider documenting why you are sure it is ok
+  // to use this.
+  static EffectSet* GetForStyleFrame(const nsIFrame* aStyleFrame);
+
+  static EffectSet* GetForEffect(const dom::KeyframeEffect* aEffect);
+
+  static void DestroyEffectSet(dom::Element* aElement,
+                               PseudoStyleType aPseudoType);
+
+  void AddEffect(dom::KeyframeEffect& aEffect);
+  void RemoveEffect(dom::KeyframeEffect& aEffect);
+
+  void SetMayHaveOpacityAnimation() { mMayHaveOpacityAnim = true; }
+  bool MayHaveOpacityAnimation() const { return mMayHaveOpacityAnim; }
+  void SetMayHaveTransformAnimation() { mMayHaveTransformAnim = true; }
+  bool MayHaveTransformAnimation() const { return mMayHaveTransformAnim; }
+
+ private:
+  using OwningEffectSet = nsTHashSet<nsRefPtrHashKey<dom::KeyframeEffect>>;
+
+ public:
+  // A simple iterator to support iterating over the effects in this object in
+  // range-based for loops.
+  //
+  // This allows us to avoid exposing mEffects directly and saves the
+  // caller from having to dereference hashtable iterators using
+  // the rather complicated: iter.Get()->GetKey().
+  //
+  // XXX Except for the active iterator checks, this could be replaced by the
+  // STL-style iterators of nsTHashSet directly now.
+  class Iterator {
+   public:
+    explicit Iterator(EffectSet& aEffectSet)
+        : Iterator(aEffectSet, aEffectSet.mEffects.begin()) {}
+
+    Iterator() = delete;
+    Iterator(const Iterator&) = delete;
+    Iterator(Iterator&&) = delete;
+    Iterator& operator=(const Iterator&) = delete;
+    Iterator& operator=(Iterator&&) = delete;
+
+    static Iterator EndIterator(EffectSet& aEffectSet) {
+      return {aEffectSet, aEffectSet.mEffects.end()};
+    }
+
+#ifdef DEBUG
+    ~Iterator() {
+      MOZ_ASSERT(mEffectSet.mActiveIterators > 0);
+      mEffectSet.mActiveIterators--;
+    }
+#endif
+
+    bool operator!=(const Iterator& aOther) const {
+      return mHashIterator != aOther.mHashIterator;
+    }
+
+    Iterator& operator++() {
+      ++mHashIterator;
+      return *this;
+    }
+
+    dom::KeyframeEffect* operator*() { return *mHashIterator; }
+
+   private:
+    Iterator(EffectSet& aEffectSet,
+             OwningEffectSet::const_iterator aHashIterator)
+        :
+#ifdef DEBUG
+          mEffectSet(aEffectSet),
+#endif
+          mHashIterator(std::move(aHashIterator)) {
+#ifdef DEBUG
+      mEffectSet.mActiveIterators++;
+#endif
+    }
+
+#ifdef DEBUG
+    EffectSet& mEffectSet;
+#endif
+    OwningEffectSet::const_iterator mHashIterator;
+  };
+
+  friend class Iterator;
+
+  Iterator begin() { return Iterator(*this); }
+  Iterator end() { return Iterator::EndIterator(*this); }
+#ifdef DEBUG
+  bool IsBeingEnumerated() const { return mActiveIterators != 0; }
+#endif
+
+  bool IsEmpty() const { return mEffects.IsEmpty(); }
+
+  size_t Count() const { return mEffects.Count(); }
+
+  const TimeStamp& LastOverflowAnimationSyncTime() const {
+    return mLastOverflowAnimationSyncTime;
+  }
+  void UpdateLastOverflowAnimationSyncTime(const TimeStamp& aRefreshTime) {
+    mLastOverflowAnimationSyncTime = aRefreshTime;
+  }
+
+  bool CascadeNeedsUpdate() const { return mCascadeNeedsUpdate; }
+  void MarkCascadeNeedsUpdate() { mCascadeNeedsUpdate = true; }
+  void MarkCascadeUpdated() { mCascadeNeedsUpdate = false; }
+
+  void UpdateAnimationGeneration(nsPresContext* aPresContext);
+  uint64_t GetAnimationGeneration() const { return mAnimationGeneration; }
+
+  const nsCSSPropertyIDSet& PropertiesWithImportantRules() const {
+    return mPropertiesWithImportantRules;
+  }
+  nsCSSPropertyIDSet& PropertiesWithImportantRules() {
+    return mPropertiesWithImportantRules;
+  }
+  nsCSSPropertyIDSet PropertiesForAnimationsLevel() const {
+    return mPropertiesForAnimationsLevel;
+  }
+  nsCSSPropertyIDSet& PropertiesForAnimationsLevel() {
+    return mPropertiesForAnimationsLevel;
+  }
+
+ private:
+  OwningEffectSet mEffects;
+
+  // Refresh driver timestamp from the moment when the animations which produce
+  // overflow change hints in this effect set were last updated.
+
+  // This is used for animations whose main-thread restyling is throttled either
+  // because they are running on the compositor or because they are not visible.
+  // We still need to update them on the main thread periodically, however (e.g.
+  // so scrollbars can be updated), so this tracks the last time we did that.
+  TimeStamp mLastOverflowAnimationSyncTime;
+
+  // RestyleManager keeps track of the number of animation restyles.
+  // 'mini-flushes' (see nsTransitionManager::UpdateAllThrottledStyles()).
+  // mAnimationGeneration is the sequence number of the last flush where a
+  // transition/animation changed.  We keep a similar count on the
+  // corresponding layer so we can check that the layer is up to date with
+  // the animation manager.
+  uint64_t mAnimationGeneration = 0;
+
+  // Specifies the compositor-animatable properties that are overridden by
+  // !important rules.
+  nsCSSPropertyIDSet mPropertiesWithImportantRules;
+  // Specifies the properties for which the result will be added to the
+  // animations level of the cascade and hence should be skipped when we are
+  // composing the animation style for the transitions level of the cascede.
+  nsCSSPropertyIDSet mPropertiesForAnimationsLevel;
+
+#ifdef DEBUG
+  // Track how many iterators are referencing this effect set when we are
+  // destroyed, we can assert that nothing is still pointing to us.
+  uint64_t mActiveIterators = 0;
+#endif
+
+  // Dirty flag to represent when the mPropertiesWithImportantRules and
+  // mPropertiesForAnimationsLevel on effects in this set might need to be
+  // updated.
+  //
+  // Set to true any time the set of effects is changed or when
+  // one the effects goes in or out of the "in effect" state.
+  bool mCascadeNeedsUpdate = false;
+
+  bool mMayHaveOpacityAnim = false;
+  bool mMayHaveTransformAnim = false;
+};
+
+}  // namespace mozilla
+
+#endif  // mozilla_EffectSet_h