[WebRTC] Add ScreenCapturerDifferWrapper to share Differ across ScreenCapturers

webrtc/modules/desktop_capture/screen_capturer_differ_wrapper.h

+/*
+ *  Copyright (c) 2016 The WebRTC project authors. All Rights Reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#ifndef WEBRTC_MODULES_DESKTOP_CAPTURE_SCREEN_CAPTURER_DIFFER_WRAPPER_H_
+#define WEBRTC_MODULES_DESKTOP_CAPTURE_SCREEN_CAPTURER_DIFFER_WRAPPER_H_
+
+#include <memory>
+
+#include "webrtc/modules/desktop_capture/screen_capturer.h"
+#include "webrtc/modules/desktop_capture/shared_desktop_frame.h"
+
+namespace webrtc {
+
+// ScreenCapturer wrapper that calculates updated_region() by comparing frames
+// content.
+//
+// This class returns entire frame as updated if the frame size or frame stride
+// has been changed.
+class ScreenCapturerDifferWrapper : public ScreenCapturer,
+                                    public DesktopCapturer::Callback {
+ public:
+  // Creates a ScreenCapturerDifferWrapper with a ScreenCapturer implementation,
+  // and takes its ownership. If |diff_entire_frame| is true, this class will
+  // assume underlying capturer won't update DesktopFrame::updated_region() by
+  // discarding it, and always compare entire frame. Otherwise, this class will
+  // use DesktopFrame::updated_region() returned by underlying capturer as hints
+  // to improve the performance to calculate the dirty region.
+  explicit ScreenCapturerDifferWrapper(
+      std::unique_ptr<ScreenCapturer> base_capturer,
+      bool diff_entire_frame);

[Sergey Ulanov, 2016/08/18 05:07:58]

Couple ideas on how everything could work without this parameter:
 1. Capturers that don't know which parts of the screen have changed should set
updated_region() to full frame. (I think we already have this implemented for
all capturers).
 2. Capturers that don't set updated_region() can be detected very easily by
checking that udpated_region() is not empty in the first frame.

I think (1) is enough and we can avoid (2). But feel free to keep this parameter
if you think it's useful.

[Hzj_jie, 2016/08/23 00:54:59]

Thank you for raising this issue again, I have not considered it carefully in
last review iteration. The definition of updated_region() function requires a
capturer to mark a superset of all the updated regions it returns in the
DesktopFrame. So I agree (1) should be enough.

+  ~ScreenCapturerDifferWrapper() override;
+
+  // ScreenCapturer interface.
+  void Start(DesktopCapturer::Callback* callback) override;
+  void SetSharedMemoryFactory(
+      std::unique_ptr<SharedMemoryFactory> shared_memory_factory) override;
+  void Capture(const DesktopRegion& region) override;
+  bool GetScreenList(ScreenList* screens) override;
+  bool SelectScreen(ScreenId id) override;
+
+ private:
+  // DesktopCapturer::Callback interface.
+  void OnCaptureResult(Result result,
+                       std::unique_ptr<DesktopFrame> frame) override;
+
+  const std::unique_ptr<ScreenCapturer> base_capturer_;
+  const bool diff_entire_frame_;
+  DesktopCapturer::Callback* callback_;
+  std::unique_ptr<SharedDesktopFrame> last_frame_;
+};
+
+}  // namespace webrtc
+
+#endif  // WEBRTC_MODULES_DESKTOP_CAPTURE_SCREEN_CAPTURER_DIFFER_WRAPPER_H_