cc/trees/swap_promise.h

// Copyright 2013 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#ifndef CC_TREES_SWAP_PROMISE_H_
#define CC_TREES_SWAP_PROMISE_H_

#include <stdint.h>

#include "cc/cc_export.h"
#include "components/viz/common/quads/compositor_frame_metadata.h"

namespace cc {

// When a change to the compositor's state/invalidation/whatever happens, a
// Swap Promise can be inserted into LayerTreeHost/LayerTreeImpl, to track
// whether the compositor's reply to the new state/invaliadtion/whatever is
// completed in the compositor, i.e. the compositor knows it has been sent
// to its output or not.
//
// If the commit results in a successful activation of the pending layer tree,
// SwapPromise::DidActivate() will be called.
//
// If the new compositor state is goint to be sent to the output,
// SwapPromise::WillSwap() will be called before the swap and
// SwapPromise::DidSwap() will be called once the swap is over.
//
// If the scheduler fails to activate the pending tree, or the compositor
// fails to send its new state to the output, SwapPromise::DidNotSwap() will
// be called. Note that it is possible to activate, and subsequently not swap.
//
// Promises complete afer either DidSwap() or DidNotSwap() is called, thus
// there are three possible call sequences:
//   DidNotSwap()
//   DidActivate() ; WillSwap(); DidSwap()
//   DidActivate() ; DidNotSwap()
//
// Clients that wish to use SwapPromise should have a subclass that defines
// the behavior of DidActivate(), WillSwap(), DidSwap() and DidNotSwap(). Notice
// that the promise can be broken at either main or impl thread, e.g. commit
// fails on main thread, new frame data has no actual damage so
// LayerTreeHostImpl::SwapBuffers() bails out early on impl thread, so don't
// assume that DidNotSwap() method is called at a particular thread. It is
// better to let the subclass carry thread-safe member data and operate on that
// member data in DidNotSwap().
class CC_EXPORT SwapPromise {
 public:
  enum DidNotSwapReason {
    SWAP_FAILS,
    COMMIT_FAILS,
    COMMIT_NO_UPDATE,
    ACTIVATION_FAILS,
  };

  SwapPromise() {}
  virtual ~SwapPromise() {}

  virtual void DidActivate() = 0;
  virtual void WillSwap(viz::CompositorFrameMetadata* metadata) = 0;
  virtual void DidSwap() = 0;
  virtual void DidNotSwap(DidNotSwapReason reason) = 0;
  // This is called when the main thread starts a (blocking) commit
  virtual void OnCommit() {}

  // A non-zero trace id identifies a trace flow object that is embedded in the
  // swap promise. This can be used for registering additional flow steps to
  // visualize the object's path through the system.
  virtual int64_t TraceId() const = 0;
};

}  // namespace cc

#endif  // CC_TREES_SWAP_PROMISE_H_