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diff --git a/dom/animation/EffectCompositor.h b/dom/animation/EffectCompositor.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_EffectCompositor_h
+#define mozilla_EffectCompositor_h
+
+#include "mozilla/AnimationPerformanceWarning.h"
+#include "mozilla/AnimationTarget.h"
+#include "mozilla/EnumeratedArray.h"
+#include "mozilla/HashTable.h"
+#include "mozilla/Maybe.h"
+#include "mozilla/OwningNonNull.h"
+#include "mozilla/PseudoElementHashEntry.h"
+#include "mozilla/RefPtr.h"
+#include "mozilla/ServoTypes.h"
+#include "nsCSSPropertyID.h"
+#include "nsCycleCollectionParticipant.h"
+#include "nsTHashMap.h"
+#include "nsTArray.h"
+
+class nsCSSPropertyIDSet;
+class nsAtom;
+class nsIFrame;
+class nsPresContext;
+enum class DisplayItemType : uint8_t;
+struct RawServoAnimationValueMap;
+
+namespace mozilla {
+
+class ComputedStyle;
+class EffectSet;
+class RestyleTracker;
+class StyleAnimationValue;
+struct AnimationProperty;
+struct NonOwningAnimationTarget;
+
+namespace dom {
+class Animation;
+class Element;
+class KeyframeEffect;
+}  // namespace dom
+
+class EffectCompositor {
+ public:
+  explicit EffectCompositor(nsPresContext* aPresContext)
+      : mPresContext(aPresContext) {}
+
+  NS_INLINE_DECL_CYCLE_COLLECTING_NATIVE_REFCOUNTING(EffectCompositor)
+  NS_DECL_CYCLE_COLLECTION_NATIVE_CLASS(EffectCompositor)
+
+  void Disconnect() { mPresContext = nullptr; }
+
+  // Animations can be applied at two different levels in the CSS cascade:
+  enum class CascadeLevel : uint32_t {
+    // The animations sheet (CSS animations, script-generated animations,
+    // and CSS transitions that are no longer tied to CSS markup)
+    Animations = 0,
+    // The transitions sheet (CSS transitions that are tied to CSS markup)
+    Transitions = 1
+  };
+  // We don't define this as part of CascadeLevel as then we'd have to add
+  // explicit checks for the Count enum value everywhere CascadeLevel is used.
+  static const size_t kCascadeLevelCount =
+      static_cast<size_t>(CascadeLevel::Transitions) + 1;
+
+  // NOTE: This can return null after Disconnect().
+  nsPresContext* PresContext() const { return mPresContext; }
+
+  enum class RestyleType {
+    // Animation style has changed but the compositor is applying the same
+    // change so we might be able to defer updating the main thread until it
+    // becomes necessary.
+    Throttled,
+    // Animation style has changed and needs to be updated on the main thread.
+    Standard,
+    // Animation style has changed and needs to be updated on the main thread
+    // as well as forcing animations on layers to be updated.
+    // This is needed in cases such as when an animation becomes paused or has
+    // its playback rate changed. In such cases, although the computed style
+    // and refresh driver time might not change, we still need to ensure the
+    // corresponding animations on layers are updated to reflect the new
+    // configuration of the animation.
+    Layer
+  };
+
+  // Notifies the compositor that the animation rule for the specified
+  // (pseudo-)element at the specified cascade level needs to be updated.
+  // The specified steps taken to update the animation rule depend on
+  // |aRestyleType| whose values are described above.
+  void RequestRestyle(dom::Element* aElement, PseudoStyleType aPseudoType,
+                      RestyleType aRestyleType, CascadeLevel aCascadeLevel);
+
+  // Schedule an animation restyle. This is called automatically by
+  // RequestRestyle when necessary. However, it is exposed here since we also
+  // need to perform this step when triggering transitions *without* also
+  // invalidating the animation style rule (which RequestRestyle would do).
+  void PostRestyleForAnimation(dom::Element* aElement,
+                               PseudoStyleType aPseudoType,
+                               CascadeLevel aCascadeLevel);
+
+  // Posts an animation restyle for any elements whose animation style rule
+  // is out of date but for which an animation restyle has not yet been
+  // posted because updates on the main thread are throttled.
+  void PostRestyleForThrottledAnimations();
+
+  // Clear all pending restyle requests for the given (pseudo-) element (and its
+  // ::before, ::after and ::marker elements if the given element is not
+  // pseudo).
+  void ClearRestyleRequestsFor(dom::Element* aElement);
+
+  // Called when computed style on the specified (pseudo-) element might
+  // have changed so that any context-sensitive values stored within
+  // animation effects (e.g. em-based endpoints used in keyframe effects)
+  // can be re-resolved to computed values.
+  void UpdateEffectProperties(const ComputedStyle* aStyle,
+                              dom::Element* aElement,
+                              PseudoStyleType aPseudoType);
+
+  // Get the animation rule for the appropriate level of the cascade for
+  // a (pseudo-)element. Called from the Servo side.
+  //
+  // The animation rule is stored in |RawServoAnimationValueMap|.
+  // We need to be careful while doing any modification because it may cause
+  // some thread-safe issues.
+  bool GetServoAnimationRule(const dom::Element* aElement,
+                             PseudoStyleType aPseudoType,
+                             CascadeLevel aCascadeLevel,
+                             RawServoAnimationValueMap* aAnimationValues);
+
+  // A variant on GetServoAnimationRule that composes all the effects for an
+  // element up to and including |aEffect|.
+  //
+  // Note that |aEffect| might not be in the EffectSet since we can use this for
+  // committing the computed style of a removed Animation.
+  bool ComposeServoAnimationRuleForEffect(
+      dom::KeyframeEffect& aEffect, CascadeLevel aCascadeLevel,
+      RawServoAnimationValueMap* aAnimationValues);
+
+  bool HasPendingStyleUpdates() const;
+
+  static bool HasAnimationsForCompositor(const nsIFrame* aFrame,
+                                         DisplayItemType aType);
+
+  static nsTArray<RefPtr<dom::Animation>> GetAnimationsForCompositor(
+      const nsIFrame* aFrame, const nsCSSPropertyIDSet& aPropertySet);
+
+  static void ClearIsRunningOnCompositor(const nsIFrame* aFrame,
+                                         DisplayItemType aType);
+
+  // Update animation cascade results for the specified (pseudo-)element
+  // but only if we have marked the cascade as needing an update due a
+  // the change in the set of effects or a change in one of the effects'
+  // "in effect" state.
+  //
+  // This method does NOT detect if other styles that apply above the
+  // animation level of the cascade have changed.
+  static void MaybeUpdateCascadeResults(dom::Element* aElement,
+                                        PseudoStyleType aPseudoType);
+
+  // Update the mPropertiesWithImportantRules and
+  // mPropertiesForAnimationsLevel members of the given EffectSet, and also
+  // request any restyles required by changes to the cascade result.
+  //
+  // NOTE: This can be expensive so we should only call it if styles that apply
+  // above the animation level of the cascade might have changed. For all
+  // other cases we should call MaybeUpdateCascadeResults.
+  //
+  // This is typically reserved for internal callers but is public here since
+  // when we detect changes to the cascade on the Servo side we can't call
+  // MarkCascadeNeedsUpdate during the traversal so instead we call this as part
+  // of a follow-up sequential task.
+  static void UpdateCascadeResults(EffectSet& aEffectSet,
+                                   dom::Element* aElement,
+                                   PseudoStyleType aPseudoType);
+
+  // Helper to fetch the corresponding element and pseudo-type from a frame.
+  //
+  // For frames corresponding to pseudo-elements, the returned element is the
+  // element on which we store the animations (i.e. the EffectSet and/or
+  // AnimationCollection), *not* the generated content.
+  //
+  // For display:table content, which maintains a distinction between primary
+  // frame (table wrapper frame) and style frame (inner table frame), animations
+  // are stored on the content associated with the _style_ frame even though
+  // some (particularly transform-like animations) may be applied to the
+  // _primary_ frame. As a result, callers will typically want to pass the style
+  // frame to this function.
+  //
+  // Returns an empty result when a suitable element cannot be found including
+  // when the frame represents a pseudo-element on which we do not support
+  // animations.
+  static Maybe<NonOwningAnimationTarget> GetAnimationElementAndPseudoForFrame(
+      const nsIFrame* aFrame);
+
+  // Associates a performance warning with effects on |aFrame| that animate
+  // properties in |aPropertySet|.
+  static void SetPerformanceWarning(
+      const nsIFrame* aFrame, const nsCSSPropertyIDSet& aPropertySet,
+      const AnimationPerformanceWarning& aWarning);
+
+  // Do a bunch of stuff that we should avoid doing during the parallel
+  // traversal (e.g. changing member variables) for all elements that we expect
+  // to restyle on the next traversal.
+  //
+  // Returns true if there are elements needing a restyle for animation.
+  bool PreTraverse(ServoTraversalFlags aFlags);
+
+  // Similar to the above but for all elements in the subtree rooted
+  // at aElement.
+  bool PreTraverseInSubtree(ServoTraversalFlags aFlags, dom::Element* aRoot);
+
+  // Record a (pseudo-)element that may have animations that can be removed.
+  void NoteElementForReducing(const NonOwningAnimationTarget& aTarget);
+
+  bool NeedsReducing() const { return !mElementsToReduce.empty(); }
+  void ReduceAnimations();
+
+  // Returns the target element for restyling.
+  //
+  // If |aPseudoType| is ::after, ::before or ::marker, returns the generated
+  // content element of which |aElement| is the parent. If |aPseudoType| is any
+  // other pseudo type (other than PseudoStyleType::NotPseudo) returns nullptr.
+  // Otherwise, returns |aElement|.
+  static dom::Element* GetElementToRestyle(dom::Element* aElement,
+                                           PseudoStyleType aPseudoType);
+
+  // Returns true if any type of compositor animations on |aFrame| allow
+  // runnning on the compositor.
+  // Sets the reason in |aWarning| if the result is false.
+  static bool AllowCompositorAnimationsOnFrame(
+      const nsIFrame* aFrame,
+      AnimationPerformanceWarning::Type& aWarning /* out */);
+
+ private:
+  ~EffectCompositor() = default;
+
+  // Get the properties in |aEffectSet| that we are able to animate on the
+  // compositor but which are also specified at a higher level in the cascade
+  // than the animations level.
+  static nsCSSPropertyIDSet GetOverriddenProperties(
+      EffectSet& aEffectSet, dom::Element* aElement,
+      PseudoStyleType aPseudoType);
+
+  static nsPresContext* GetPresContext(dom::Element* aElement);
+
+  nsPresContext* mPresContext;
+
+  // Elements with a pending animation restyle. The associated bool value is
+  // true if a pending animation restyle has also been dispatched. For
+  // animations that can be throttled, we will add an entry to the hashtable to
+  // indicate that the style rule on the element is out of date but without
+  // posting a restyle to update it.
+  EnumeratedArray<CascadeLevel, CascadeLevel(kCascadeLevelCount),
+                  nsTHashMap<PseudoElementHashEntry, bool>>
+      mElementsToRestyle;
+
+  bool mIsInPreTraverse = false;
+
+  HashSet<OwningAnimationTarget> mElementsToReduce;
+};
+
+}  // namespace mozilla
+
+#endif  // mozilla_EffectCompositor_h