MediaStream audio: Refactor 3 separate "glue" implementations into one.

content/renderer/media/media_stream_audio_deliverer.h

Index: content/renderer/media/media_stream_audio_deliverer.h
diff --git a/content/renderer/media/media_stream_audio_deliverer.h b/content/renderer/media/media_stream_audio_deliverer.h
new file mode 100644
index 0000000000000000000000000000000000000000..5991a5f168533874ff2a0283aa4073a893f27dea
--- /dev/null
+++ b/content/renderer/media/media_stream_audio_deliverer.h
@@ -0,0 +1,156 @@
+// Copyright 2016 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 CONTENT_RENDERER_MEDIA_MEDIA_STREAM_AUDIO_DELIVERER_H_
+#define CONTENT_RENDERER_MEDIA_MEDIA_STREAM_AUDIO_DELIVERER_H_
+
+#include <algorithm>
+#include <vector>
+
+#include "base/synchronization/lock.h"
+#include "base/threading/thread_checker.h"
+#include "media/audio/audio_parameters.h"
+
+namespace content {
+
+// Template containing functionality common to both MediaStreamAudioSource and
+// MediaStreamAudioTrack. This is used for managing the connections between
+// objects through which audio data flows, and doing so in a thread-safe manner.
+//
+// The Consumer parameter of the template is the type of the objects to which
+// audio data is delivered: MediaStreamAudioTrack or MediaStreamAudioSink. It's
+// assumed the Consumer class defines methods named OnSetFormat() and OnData()
+// that have the same signature as the ones defined in this template.
+// MediaStreamAudioDeliverer will always guarantee the Consumer's OnSetFormat()
+// and OnData() methods are called sequentially.
+template <typename Consumer>
+class MediaStreamAudioDeliverer {
+ public:
+  MediaStreamAudioDeliverer() {}
+  ~MediaStreamAudioDeliverer() {}
+
+  // Returns the current audio parameters. These will be invalid before the
+  // first call to OnSetFormat(). This method is thread-safe.
+  media::AudioParameters GetAudioParameters() const {
+    base::AutoLock auto_lock(params_lock_);
+    return params_;
+  }
+
+  // Begin delivering audio to |consumer|. The caller must guarantee |consumer|
+  // is not destroyed until after calling RemoveConsumer(consumer). This method
+  // must be called on the main thread.
+  void AddConsumer(Consumer* consumer) {
+    DCHECK(thread_checker_.CalledOnValidThread());
+    DCHECK(consumer);
+    base::AutoLock auto_lock(consumers_lock_);
+    DCHECK(std::find(consumers_.begin(), consumers_.end(), consumer) ==
+           consumers_.end());
+    DCHECK(std::find(pending_consumers_.begin(), pending_consumers_.end(),
+                     consumer) == pending_consumers_.end());
+    pending_consumers_.push_back(consumer);
+  }
+
+  // Stop delivering audio to |consumer|. Returns true if |consumer| was the
+  // last consumer removed, false otherwise. When this method returns, no
+  // further calls will be made to OnSetFormat() or OnData() on any thread.
+  // This method must be called on the main thread.
+  bool RemoveConsumer(Consumer* consumer) {
+    DCHECK(thread_checker_.CalledOnValidThread());
+    base::AutoLock auto_lock(consumers_lock_);
+    const bool had_consumers =
+        !consumers_.empty() || !pending_consumers_.empty();
+    auto it = std::find(consumers_.begin(), consumers_.end(), consumer);
+    if (it != consumers_.end()) {
+      consumers_.erase(it);
+    } else {
+      it = std::find(pending_consumers_.begin(), pending_consumers_.end(),
+                     consumer);
+      if (it != pending_consumers_.end())
+        pending_consumers_.erase(it);
+    }
+    return had_consumers && consumers_.empty() && pending_consumers_.empty();
+  }
+
+  // Returns the current list of connected Consumers. This is normally used to
+  // send a notification to all consumers. This method must be called on the
+  // main thread.
+  void GetConsumerList(std::vector<Consumer*>* consumer_list) const {
+    DCHECK(thread_checker_.CalledOnValidThread());
+    base::AutoLock auto_lock(consumers_lock_);
+    *consumer_list = consumers_;
+    consumer_list->insert(consumer_list->end(),
+                          pending_consumers_.begin(), pending_consumers_.end());
+  }
+
+  // Change the format of the audio passed in the next call to OnData(). This
+  // method may be called on any thread but, logically, should only be called
+  // between calls to OnData().
+  void OnSetFormat(const media::AudioParameters& params) {
+    DCHECK(params.IsValid());
+    base::AutoLock auto_lock(consumers_lock_);
+    {

[o1ka, 2016/04/21 18:51:22]

nit: it would probably be more consistent to have a setter to pair with getter,
so that params_lock usage is visibly localized.

[miu, 2016/04/21 20:42:30]

This method *is* the setter.  ;)  But, I see that you're talking about this
inner block of code: I'd prefer not to split this out since |consumers_lock_|
should also be held while changing the audio params (to synchronize when we
notify consumers that the audio format has changed).  IMHO, it makes things more
readable to see that happening alongside with the params change.

+      base::AutoLock auto_params_lock(params_lock_);
+      if (params_.Equals(params))
+        return;
+      params_ = params;
+    }
+    pending_consumers_.insert(pending_consumers_.end(),
+                              consumers_.begin(), consumers_.end());
+    consumers_.clear();
+  }
+
+  // Deliver data to all consumers. This method may be called on any thread.
+  void OnData(const media::AudioBus& audio_bus,
+              base::TimeTicks reference_time) {
+    base::AutoLock auto_lock(consumers_lock_);
+
+    // Call OnSetFormat() for all pending consumers and move them to the
+    // active-delivery list.
+    if (!pending_consumers_.empty()) {
+      const media::AudioParameters params = GetAudioParameters();
+      DCHECK(params.IsValid());
+      for (Consumer* consumer : pending_consumers_)
+        consumer->OnSetFormat(params);
+      consumers_.insert(consumers_.end(),
+                        pending_consumers_.begin(), pending_consumers_.end());
+      pending_consumers_.clear();
+    }
+
+    // Deliver the audio data to each consumer.
+    for (Consumer* consumer : consumers_)
+      consumer->OnData(audio_bus, reference_time);
+  }
+
+ private:
+  // In debug builds, check that all methods that could cause object graph or
+  // data flow changes are being called on the main thread.
+  base::ThreadChecker thread_checker_;
+
+  // Protects concurrent access to |pending_consumers_| and |consumers_|.
+  mutable base::Lock consumers_lock_;
+
+  // Any consumers needing a call to OnSetFormat(), to be notified of the
+  // changed audio format, are placed in this list. This includes consumers
+  // added via AddConsumer() that need to have an initial OnSetFormat() call
+  // before audio data is first delivered. Consumers are moved from this list to
+  // |consumers_| on the audio thread.
+  std::vector<Consumer*> pending_consumers_;
+
+  // Consumers that are up-to-date on the current audio format and are receiving
+  // audio data are placed in this list.
+  std::vector<Consumer*> consumers_;
+
+  // Protects concurrent access to |params_|.
+  mutable base::Lock params_lock_;
+
+  // Specifies the current format of the audio passing through this
+  // MediaStreamAudioDeliverer.
+  media::AudioParameters params_;
+
+  DISALLOW_COPY_AND_ASSIGN(MediaStreamAudioDeliverer);
+};
+
+}  // namespace content
+
+#endif  // CONTENT_RENDERER_MEDIA_MEDIA_STREAM_AUDIO_DELIVERER_H_