Crash fix: Remove MessageLoop from AudioPowerMonitor and instead use MessageLoopProxy in AudioOutpu…

media/audio/audio_power_monitor.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 MEDIA_AUDIO_AUDIO_POWER_MONITOR_H_
 #define MEDIA_AUDIO_AUDIO_POWER_MONITOR_H_
 
 #include <limits>
+#include <utility>
 
 #include "base/callback.h"
+#include "base/synchronization/lock.h"
 #include "media/base/media_export.h"
 
 // An audio signal power monitor.  It is periodically provided an AudioBus by
 // the native audio thread, and the audio samples in each channel are analyzed
 // to determine the average power of the signal over a time period.  Here
 // "average power" is a running average calculated by using a first-order
 // low-pass filter over the square of the samples scanned.  Whenever reporting
 // the power level, this running average is converted to dBFS (decibels relative
 // to full-scale) units.
 //
 // Note that extreme care has been taken to make the AudioPowerMonitor::Scan()
 // method safe to be called on the native audio thread.  The code acquires no
 // locks, nor engages in any operation that could result in an
 // undetermined/unbounded amount of run-time.
 
 namespace base {
-class MessageLoop;
 class TimeDelta;
 }
 
 namespace media {
 
 class AudioBus;
 
 class MEDIA_EXPORT AudioPowerMonitor {
  public:
-  // Reports power level in terms of dBFS (see zero_power() and max_power()
-  // below).  |clipped| is true if any *one* sample exceeded maximum amplitude
-  // since the last invocation.
-  typedef base::Callback<void(float power_dbfs, bool clipped)>
-      PowerMeasurementCallback;
-
   // |sample_rate| is the audio signal sample rate (Hz).  |time_constant|
   // characterizes how samples are averaged over time to determine the power
   // level; and is the amount of time it takes a zero power level to increase to
-  // ~63.2% of maximum given a step input signal.  |measurement_period| is the
-  // time length of signal to analyze before invoking the callback to report the
-  // current power level.  |message_loop| is where the |callback| task will be
-  // posted.
-  AudioPowerMonitor(int sample_rate,
-                    const base::TimeDelta& time_constant,
-                    const base::TimeDelta& measurement_period,
-                    base::MessageLoop* message_loop,
-                    const PowerMeasurementCallback& callback);
+  // ~63.2% of maximum given a step input signal.
+  AudioPowerMonitor(int sample_rate, const base::TimeDelta& time_constant);
 
   ~AudioPowerMonitor();
 
+  // Reset power monitor to initial state (zero power level).  This should not
+  // be called while another thread is scanning.
+  void Reset();
+
   // Scan more |frames| of audio data from |buffer|.  It is safe to call this
   // from a real-time priority thread.
   void Scan(const AudioBus& buffer, int frames);
 
+  // Returns the current power level in dBFS and clip status.  Clip status is
+  // true whenever any *one* sample scanned exceeded maximum amplitude since
+  // this method's last invocation.  It is safe to call this method from any
+  // thread.
+  std::pair<float, bool> ReadCurrentPowerAndClip();
+
   // dBFS value corresponding to zero power in the audio signal.
   static float zero_power() { return -std::numeric_limits<float>::infinity(); }
 
   // dBFS value corresponding to maximum power in the audio signal.
   static float max_power() { return 0.0f; }
 
  private:
   // The weight applied when averaging-in each sample.  Computed from the
   // |sample_rate| and |time_constant|.
   const float sample_weight_;
 
-  // Number of audio frames to be scanned before reporting the current power
-  // level via callback, as computed from |sample_rate| and
-  // |measurement_period|.
-  const int num_frames_per_callback_;
+  // Accumulated results over one or more calls to Scan().  These should only be
+  // touched by the thread invoking Scan().
+  float average_power_;
+  bool has_clipped_;
 
-  // MessageLoop and callback used to notify of the current power level.
-  base::MessageLoop* const message_loop_;
-  const PowerMeasurementCallback power_level_callback_;
-
-  // Accumulated results over one or more calls to Scan().
-  float average_power_;
-  bool clipped_since_last_notification_;
-  int frames_since_last_notification_;
-
-  // Keep track of last reported results to forgo making redundant callbacks.
-  float last_reported_power_;
-  bool last_reported_clipped_;
+  // Copies of power and clip status, used to deliver results synchronously
+  // across threads.
+  base::Lock reading_lock_;
+  float power_reading_;
+  bool clipped_reading_;
 
   DISALLOW_COPY_AND_ASSIGN(AudioPowerMonitor);
 };
 
 }  // namespace media
 
 #endif  // MEDIA_AUDIO_AUDIO_POWER_MONITOR_H_