Implement MediaCaptureDevices.

content/public/browser/media_capture_devices.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 CONTENT_PUBLIC_BROWSER_MEDIA_CAPTURE_DEVICES_H_
+#define CONTENT_PUBLIC_BROWSER_MEDIA_CAPTURE_DEVICES_H_
+
+#include "content/public/common/media_stream_request.h"
+
+namespace content {
+
+// This is a singleton class, used to get Audio/Video devices, it also provides
+// observer interface to notify the change of devices.
+CONTENT_EXPORT class MediaCaptureDevices {
+ public:
+  class Observer {

[jam, 2014/03/05 08:45:04]

i probably wasn't clear in the email thread, sorry. i had meant don't add this
observer here, and keep these callbacks on MediaObserver. The reason being is
that we don't need two similar media observer interfaces, plus their
accompanying registration methods

however these callbacks on MediaObserver should drop their parameters and just
act as notifiers only. the list can then be queried from here. this is to avoid
having the data being available in two different ways

[michaelbai, 2014/03/05 21:55:46]

Done.

+   public:
+    // Handle an information update consisting of a up-to-date audio capture
+    // device lists. This happens when a microphone is plugged in or unplugged.
+    virtual void OnUpdateAudioDevices(
+        const content::MediaStreamDevices& devices) {}
+
+    // Handle an information update consisting of a up-to-date video capture
+    // device lists. This happens when a camera is plugged in or unplugged.
+    virtual void OnUpdateVideoDevices(
+        const content::MediaStreamDevices& devices) {}
+
+    virtual ~Observer() {}
+  };
+
+  // Get signleton instance of MediaCaptureDevices.
+  static MediaCaptureDevices* GetInstance();
+
+  // Add/Remove Observer must be called on UI thread.
+  virtual void AddObserver(Observer* observer) = 0;
+  virtual void RemoveObserver(Observer* observer) = 0;
+
+  // Return all Audio/Video devices.
+  virtual const MediaStreamDevices& GetAudioCaptureDevices() = 0;
+  virtual const MediaStreamDevices& GetVideoCaptureDevices() = 0;
+
+  // Only for testing, should be called in IO thread.
+  virtual void SetAudioCaptureDevices(const MediaStreamDevices& devices) = 0;

[jam, 2014/03/05 08:45:04]

these test methods are called by chrome to fake the devices that content gives
it. why not keep these testing hooks contained in chrome? i.e. the chrome side
which calls content can have override methods.

[michaelbai, 2014/03/05 21:55:46]

Done.

+  virtual void SetVideoCaptureDevices(const MediaStreamDevices& devices) = 0;
+  // Unittests that do not require actual device enumeration should call this
+  // API on the singleton. It is safe to call this multiple times on the
+  // signleton.
+  virtual void DisableDeviceEnumerationForTesting() = 0;
+
+ private:
+  // This interface should only be implemented inside content.
+  friend class MediaCaptureDevicesImpl;
+  MediaCaptureDevices() {}
+  virtual ~MediaCaptureDevices() {}
+};
+
+}  // namespace content
+
+#endif  // CONTENT_PUBLIC_BROWSER_MEDIA_CAPTURE_DEVICES_H_