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diff --git a/widget/VsyncDispatcher.h b/widget/VsyncDispatcher.h
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+++ b/widget/VsyncDispatcher.h
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+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* 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_widget_VsyncDispatcher_h
+#define mozilla_widget_VsyncDispatcher_h
+
+#include "mozilla/DataMutex.h"
+#include "mozilla/TimeStamp.h"
+#include "nsISupportsImpl.h"
+#include "nsTArray.h"
+#include "mozilla/RefPtr.h"
+#include "VsyncSource.h"
+
+namespace mozilla {
+
+class VsyncObserver {
+  NS_INLINE_DECL_PURE_VIRTUAL_REFCOUNTING
+
+ public:
+  // The method is called when a vsync occurs.
+  // In general, this vsync notification will occur on the hardware vsync
+  // thread from VsyncSource. But it might also be called on PVsync ipc thread
+  // if this notification is cross process. Thus all observer should check the
+  // thread model before handling the real task.
+  virtual void NotifyVsync(const VsyncEvent& aVsync) = 0;
+
+ protected:
+  VsyncObserver() = default;
+  virtual ~VsyncObserver() = default;
+};  // VsyncObserver
+
+class VsyncDispatcher;
+
+// Used to dispatch vsync events in the parent process to compositors.
+//
+// When the compositor is in-process, CompositorWidgets own a
+// CompositorVsyncDispatcher, and directly attach the compositor's observer
+// to it.
+//
+// When the compositor is out-of-process, the CompositorWidgetDelegate owns
+// the vsync dispatcher instead. The widget receives vsync observer/unobserve
+// commands via IPDL, and uses this to attach a CompositorWidgetVsyncObserver.
+// This observer forwards vsync notifications (on the vsync thread) to a
+// dedicated vsync I/O thread, which then forwards the notification to the
+// compositor thread in the compositor process.
+class CompositorVsyncDispatcher final : public VsyncObserver {
+  NS_INLINE_DECL_THREADSAFE_REFCOUNTING(CompositorVsyncDispatcher, override)
+
+ public:
+  explicit CompositorVsyncDispatcher(RefPtr<VsyncDispatcher> aVsyncDispatcher);
+
+  // Called on the vsync thread when a hardware vsync occurs
+  void NotifyVsync(const VsyncEvent& aVsync) override;
+
+  // Compositor vsync observers must be added/removed on the compositor thread
+  void SetCompositorVsyncObserver(VsyncObserver* aVsyncObserver);
+  void Shutdown();
+
+ private:
+  virtual ~CompositorVsyncDispatcher();
+  void ObserveVsync(bool aEnable);
+
+  RefPtr<VsyncDispatcher> mVsyncDispatcher;
+  Mutex mCompositorObserverLock MOZ_UNANNOTATED;
+  RefPtr<VsyncObserver> mCompositorVsyncObserver;
+  bool mDidShutdown;
+};
+
+// Dispatch vsync events to various observers. This is used by:
+//  - CompositorVsyncDispatcher
+//  - Parent process refresh driver timers
+//  - IPC for content process refresh driver timers (VsyncParent <->
+//  VsyncMainChild)
+//  - IPC for content process worker requestAnimationFrame (VsyncParent <->
+//  VsyncWorkerChild)
+//
+// This class is only used in the parent process.
+// There is one global vsync dispatcher which is managed by gfxPlatform.
+// On Linux Wayland, there is also one vsync source and vsync dispatcher per
+// widget.
+// A vsync dispatcher can swap out its underlying VsyncSource. This happens, for
+// example, when the layout.frame_rate pref is modified, causing us to switch
+// between hardware vsync and software vsync.
+class VsyncDispatcher final {
+  NS_INLINE_DECL_THREADSAFE_REFCOUNTING(VsyncDispatcher)
+
+ public:
+  explicit VsyncDispatcher(gfx::VsyncSource* aVsyncSource);
+
+  // Please check CompositorVsyncDispatcher::NotifyVsync().
+  void NotifyVsync(const VsyncEvent& aVsync);
+
+  // Swap out the underlying vsync source. Can be called on any thread.
+  // aVsyncSource must be non-null.
+  void SetVsyncSource(gfx::VsyncSource* aVsyncSource);
+
+  // Always non-null.
+  RefPtr<gfx::VsyncSource> GetCurrentVsyncSource();
+
+  TimeDuration GetVsyncRate();
+
+  // Add a vsync observer to this dispatcher. This is a no-op if the observer is
+  // already registered. Can be called from any thread.
+  void AddVsyncObserver(VsyncObserver* aVsyncObserver);
+
+  // Remove a vsync observer from this dispatcher. This is a no-op if the
+  // observer is not registered. Can be called from any thread.
+  void RemoveVsyncObserver(VsyncObserver* aVsyncObserver);
+
+  // Add and remove an observer for vsync which can only be notified on the
+  // main thread. Note that keeping an observer registered means vsync will keep
+  // firing, which may impact power usage. So this is intended only for "short
+  // term" vsync observers.
+  // These methods must be called on the parent process main thread, and the
+  // observer will likewise be notified on the parent process main thread.
+  void AddMainThreadObserver(VsyncObserver* aObserver);
+  void RemoveMainThreadObserver(VsyncObserver* aObserver);
+
+ private:
+  virtual ~VsyncDispatcher();
+
+  // Can be called on any thread.
+  void UpdateVsyncStatus();
+
+  // Can only be called on the main thread.
+  void NotifyMainThreadObservers(VsyncEvent aEvent);
+
+  struct State {
+    explicit State(gfx::VsyncSource* aVsyncSource)
+        : mCurrentVsyncSource(aVsyncSource) {}
+    State(State&&) = default;
+    ~State() = default;
+
+    nsTArray<RefPtr<VsyncObserver>> mObservers;
+    nsTArray<RefPtr<VsyncObserver>> mMainThreadObservers;
+    VsyncId mLastVsyncIdSentToMainThread;
+    VsyncId mLastMainThreadProcessedVsyncId;
+
+    // Always non-null.
+    RefPtr<gfx::VsyncSource> mCurrentVsyncSource;
+
+    // The number of skipped vsyncs since the last non-skipped vsync.
+    // Reset to zero every n vsyncs, where n is given by the pref
+    // gfx.display.frame-rate-divisor.
+    int32_t mVsyncSkipCounter = 0;
+
+    bool mIsObservingVsync = false;
+  };
+
+  DataMutex<State> mState;
+};
+
+}  // namespace mozilla
+
+#endif  // mozilla_widget_VsyncDispatcher_h