getUserMeida: report device starting states

media/capture/video/video_capture_device.h

 // Copyright (c) 2012 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.
 //
 // VideoCaptureDevice is the abstract base class for realizing video capture
 // device support in Chromium. It provides the interface for OS dependent
 // implementations.
 // The class is created and functions are invoked on a thread owned by
 // VideoCaptureManager. Capturing is done on other threads, depending on the OS
 // specific implementation.
@@ skipping 196 matching lines @@
     // be StopAndDeAllocate()-ed. |reason| is a text description of the error.
     virtual void OnError(const tracked_objects::Location& from_here,
                          const std::string& reason) = 0;
 
     // VideoCaptureDevice requests the |message| to be logged.
     virtual void OnLog(const std::string& message) {}
 
     // Returns the current buffer pool utilization, in the range 0.0 (no buffers
     // are in use by producers or consumers) to 1.0 (all buffers are in use).
     virtual double GetBufferPoolUtilization() const = 0;
+
+    // VideoCaptureDevice reports it's successfully started.

[miu, 2017/02/09 22:21:49]

There are two things you'll want to consider here:

1. Any thread can call OnStarted() (at least, FWICT in this patch). Consider
whether this is acceptable, or that OnStarted() must be called on the same
thread that called AllocateAndStart().

2. OnStarted() may be called synchronously (i.e., within a call to
AllocateAndStart()), or asynchronously. Are your client impl's prepared for
that?

Regardless of whether you decide to change (or not change) anything here, please
document these behaviors in the method comments here.

[chfremer, 2017/02/09 23:34:52]

Regardless of which exact thread it comes from, we should guarantee two things:
1.) OnStarted() is not called concurrently with any of the other methods
(actually, none of the VideoCaptureDevice::Client methods should be called
concurrently)
2.) OnStarted() is guaranteed to get invoked before the first frame is
delivered.

[braveyao, 2017/02/14 01:05:49]

All the events will be posted to IO thread. See
https://cs.chromium.org/chromium/src/content/browser/renderer_host/media/vide....
Please let me know if there is any more concern.

[braveyao, 2017/02/14 01:05:49]

All the methods will be posted to IO thread
(https://cs.chromium.org/chromium/src/content/browser/renderer_host/media/vide...).
So there won't be concurrent calls.
The state machine will guarantee that the first frame won't be  processed until
OnStarted is received. (see
https://cs.chromium.org/chromium/src/content/renderer/media/video_capture_imp...)

Please let me know if you have more concerns.

[chfremer, 2017/02/14 19:21:22]

Please see my follow-up comment in Patch Set 2.

+    virtual void OnStarted() = 0;
   };
 
   ~VideoCaptureDevice() override;
 
   // Prepares the video capturer for use. StopAndDeAllocate() must be called
   // before the object is deleted.
   virtual void AllocateAndStart(const VideoCaptureParams& params,
                                 std::unique_ptr<Client> client) = 0;
 
   // In cases where the video capturer self-pauses (e.g., a screen capturer
@@ skipping 75 matching lines @@
 
  private:
   // Gets the power line frequency from the current system time zone if this is
   // defined, otherwise returns 0.
   PowerLineFrequency GetPowerLineFrequencyForLocation() const;
 };
 
 }  // namespace media
 
 #endif  // MEDIA_CAPTURE_VIDEO_VIDEO_CAPTURE_DEVICE_H_