[chrome.displaySource] Add a base class for audio encoders.

extensions/renderer/api/display_source/wifi_display/wifi_display_audio_encoder.h

+// 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 EXTENSIONS_RENDERER_API_DISPLAY_SOURCE_WIFI_DISPLAY_WIFI_DISPLAY_AUDIO_ENCODER_H_
+#define EXTENSIONS_RENDERER_API_DISPLAY_SOURCE_WIFI_DISPLAY_WIFI_DISPLAY_AUDIO_ENCODER_H_
+
+#include "content/public/renderer/media_stream_audio_sink.h"
+#include "extensions/renderer/api/display_source/wifi_display/wifi_display_media_encoder.h"
+#include "third_party/wds/src/libwds/public/audio_codec.h"
+
+namespace extensions {
+
+// This interface is a base class for audio used by the Wi-Fi Display media
+// pipeline.
+// Threading: the client code should belong to a single thread.

[asargent_no_longer_on_chrome, 2016/04/26 19:06:05]

optional: if you think clients of this class might get this wrong, you can
enforce this using base::ThreadChecker and DCHECKS using ThreadChecker's
CalledOnValidThread method

[e_hakkinen, 2016/04/27 09:08:23]

I think it is more likely for clients to get this wrong for video encoders than
for audio encoders as audio encoders have smaller client interface compared to
video encoders. As such, I am inclined to add a protected thread checker to the
media encoder base class and reuse that in all encoders. I will do that in a
separate CL.

+class WiFiDisplayAudioEncoder : public WiFiDisplayMediaEncoder,
+                                public content::MediaStreamAudioSink {
+ public:
+  using AudioEncoderCallback =
+      base::Callback<void(scoped_refptr<WiFiDisplayAudioEncoder>)>;
+
+  static void Create(const wds::AudioCodec& audio_codec,
+                     const AudioEncoderCallback& encoder_callback);
+
+ protected:
+  static const size_t kInvalidCodecModeValue = ~static_cast<size_t>(0u);
+
+  explicit WiFiDisplayAudioEncoder(const wds::AudioCodec& audio_codec);
+  ~WiFiDisplayAudioEncoder() override;
+
+  size_t GetAudioCodecMode() const;
+
+  const wds::AudioCodec audio_codec_;
+};
+
+}  // namespace extensions
+
+#endif  // EXTENSIONS_RENDERER_API_DISPLAY_SOURCE_WIFI_DISPLAY_WIFI_DISPLAY_AUDIO_ENCODER_H_