cc/scheduler/begin_frame_tracker.h

// Copyright 2014 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_SCHEDULER_BEGIN_FRAME_TRACKER_H_
#define CC_SCHEDULER_BEGIN_FRAME_TRACKER_H_

#include <set>
#include <string>

#include "base/location.h"
#include "base/trace_event/trace_event.h"
#include "base/trace_event/trace_event_argument.h"
#include "cc/output/begin_frame_args.h"

#define BEGINFRAMETRACKER_FROM_HERE FROM_HERE_WITH_EXPLICIT_FUNCTION("")

namespace cc {

// Microclass to trace and check properties for correct BeginFrameArgs (BFA)
// usage and provide a few helper methods.
//
// With DCHECKs enable, this class checks the following "invariants";
//  * BFA are monotonically increasing.
//  * BFA is valid.
//  * The BFA is only used inside a given period.
//  * A new BFA isn't used before the last BFA is finished with.
//
// With the tracing category "cc.debug.scheduler.frames" enabled the tracker
// will output the following trace information;
//  * Time period for which the BFA is in usage.
//  * The flow of BFA as they are passed between tracking objects.
//
// TODO(mithro): Record stats about the BeginFrameArgs
class CC_EXPORT BeginFrameTracker {
 public:
  explicit BeginFrameTracker(const tracked_objects::Location& location);
  ~BeginFrameTracker();

  // The Start and Finish methods manage the period that a BFA should be
  // accessed for. This allows tight control over the BFA and prevents
  // accidental usage in the wrong period when code is split across multiple
  // locations.

  // Start using a new BFA value and check invariant properties.
  // **Must** only be called after finishing with any previous BFA.
  void Start(BeginFrameArgs new_args);
  // Finish using the current BFA.
  // **Must** only be called while still using a BFA.
  void Finish();

  // The two accessors methods allow access to the BFA stored inside the
  // tracker. They are mutually exclusive, at any time it is only valid to call
  // one or the other. This makes sure you understand exactly which BFA you are
  // intending to use and verifies that is the case.

  // Get the current BFA object.
  // **Must** only be called between the start and finish methods calls.
  const BeginFrameArgs& Current() const;
  // Get the last used BFA.
  // **Must** only be called when **not** between the start and finish method
  // calls.
  const BeginFrameArgs& Last() const;

  // Helper method to try and return a valid interval property. Defaults to
  // BFA::DefaultInterval() is no other interval can be found. Can be called at
  // any time.
  base::TimeDelta Interval() const;

  void AsValueInto(base::TimeTicks now,
                   base::trace_event::TracedValue* dict) const;

  // The following methods violate principles of how BeginFrameArgs should be
  // used. These methods should only be used when there is no other choice.
  bool DangerousMethodHasStarted() const {
    return !current_updated_at_.is_null();
  }
  bool DangerousMethodHasFinished() const { return HasFinished(); }
  const BeginFrameArgs& DangerousMethodCurrentOrLast() const;

 private:
  // Return if currently not between the start/end period. This method should
  // be used extremely sparingly and normal indicates incorrect management of
  // the BFA object. Can be called at any time.
  bool HasFinished() const { return !current_finished_at_.is_null(); }

  const tracked_objects::Location location_;
  const std::string location_string_;

  base::TimeTicks current_updated_at_;
  BeginFrameArgs current_args_;
  base::TimeTicks current_finished_at_;
};

}  // namespace cc

#endif  // CC_SCHEDULER_BEGIN_FRAME_TRACKER_H_