Video Capture: Allow suspension of individual devices.

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 149 matching lines @@
   //      improve the video quality of unchanging content.
   //
   // The default implementation is a no-op. VideoCaptureDevice implementations
   // are not required to honor this request, especially if they do not
   // self-pause and/or if honoring the request would cause them to exceed their
   // configured maximum frame rate. Any VideoCaptureDevice that does self-pause,
   // however, should provide an implementation of this method that makes
   // reasonable attempts to honor these requests.
   virtual void RequestRefreshFrame() {}
 
+  // Optionally suspends frame delivery. The video capturer may or may not honor

[chfremer, 2016/09/27 00:54:59]

nit: "video capturer" -> "VideoCaptureDevice" or "implementation".

[miu, 2016/09/27 23:42:23]

Done.

+  // this request. The purpose of this is to allow minimizing resource usage
+  // while there are no frame consumers present.

[chfremer, 2016/09/27 00:54:59]

nit: The names "Suspend" and "Resume" already somewhat imply this, but we could
state more explicitly that suspending and resuming is supposed to be
significantly faster than stopping and starting. Or is it legal for
implementations to internally use stop and start as long as that is "fast
enough"?

[miu, 2016/09/27 23:42:23]

Done.
Generally, no, we always assume Start() and Stop() are not fast (i.e., could
take on the order of seconds to execute!).

+  virtual void MaybeSuspend() {}
+
+  // Resumes frame delivery, if it was suspended.
+  virtual void Resume() {}
+
   // Deallocates the video capturer, possibly asynchronously.
   //
   // This call requires the device to do the following things, eventually: put
   // hardware into a state where other applications could use it, free the
   // memory associated with capture, and delete the |client| pointer passed into
   // AllocateAndStart.
   //
   // If deallocation is done asynchronously, then the device implementation must
   // ensure that a subsequent AllocateAndStart() operation targeting the same ID
   // would be sequenced through the same task runner, so that deallocation
@@ skipping 24 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_