Caching AudioOutputDevice instances in mixer manager

media/base/audio_renderer_mixer_input.h

Index: media/base/audio_renderer_mixer_input.h
diff --git a/media/base/audio_renderer_mixer_input.h b/media/base/audio_renderer_mixer_input.h
index a75320e78c681c73303c8d65bca0f9fb8a7cfd30..15795e10fdd63bb8032ba27e649902f4677cd275 100644
--- a/media/base/audio_renderer_mixer_input.h
+++ b/media/base/audio_renderer_mixer_input.h
@@ -43,8 +43,13 @@ class MEDIA_EXPORT AudioRendererMixerInput
                               const url::Origin& security_origin)>
       RemoveMixerCB;
 
+  typedef base::Callback<OutputDeviceInfo(const std::string& device_id,
+                                          const url::Origin& security_origin)>
+      GetDeviceInfoCB;
+
   AudioRendererMixerInput(const GetMixerCB& get_mixer_cb,

[miu, 2016/05/12 21:53:06]

Hmm...This is now the third callback being injected, which is starting to abuse
the main purpose of Callbacks (to invoke unknown external implementation after
an asynchronous operation completes). Consider replacing these with an interface
like:

  class MixerPool {
    virtual AudioRendererMixer* AcquireMixer(...) = 0;
    virtual void ReleaseMixer(...) = 0;
    virtual OutputDeviceInfo GetDeviceInfo(...) = 0;
  };

...and the constructor becomes:

  AudioRendererMixerInput(MixerPool* pool,
                          const std::string& device_id,
                          const url::Origin& security_origin);

The reason having all these callbacks is messy is because it becomes harder to
reason about object lifetimes and ownership/encapsulation of functionality.
You'd have to look for where each Callback object is being created and how
base::Bind() was used, and whether it calls via a raw, ref-counted, or weak
pointer. Also, there are subtle "surprises" in how copies of base::Callback
objects can invalidate the original (e.g., when bound with move types like
std::unique_ptr<>).

A teammate of mine recently did a big clean-up like this (reducing an
11-argument constructor down to 4 arguments!), and we actually resolved a few
bugs once the object graph became clearer:
https://codereview.chromium.org/1878883003/ and
https://codereview.chromium.org/1515023002/

[o1ka, 2016/05/17 17:17:24]

Done. (I actually was going to do that at some point, but then forgot about it
:) )

                           const RemoveMixerCB& remove_mixer_cb,
+                          const GetDeviceInfoCB& get_device_info_cb,
                           const std::string& device_id,
                           const url::Origin& security_origin);
 
@@ -87,6 +92,9 @@ class MEDIA_EXPORT AudioRendererMixerInput
   const GetMixerCB get_mixer_cb_;
   const RemoveMixerCB remove_mixer_cb_;
 
+  // Callback to get output device information in the absence of |mixer_|.
+  const GetDeviceInfoCB get_device_info_cb_;
+
   // AudioParameters received during Initialize().
   AudioParameters params_;