VsyncDispatcher.h

/* -*- 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