[WebRTC] Implement Windows ScreenDrawer to test ScreenCapturer*

webrtc/modules/desktop_capture/screen_drawer.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 MODULES_DESKTOP_CAPTURE_SCREEN_DRAWER_H_
+#define MODULES_DESKTOP_CAPTURE_SCREEN_DRAWER_H_
+
+#include <stdint.h>
+
+#include <memory>
+
+#include "webrtc/modules/desktop_capture/desktop_geometry.h"
+
+namespace webrtc {
+
+// A set of platform independent functions to draw various of shapes on the
+// screen. This class is for testing ScreenCapturer* implementations only, and
+// should not be used in production logic.
+class ScreenDrawer {
+ public:
+  // Creates a ScreenDrawer for a certain platform.

[Sergey Ulanov, 2016/08/09 17:43:01]

s/a certain/the current/

[Hzj_jie, 2016/08/11 18:42:03]

Done.

+  static std::unique_ptr<ScreenDrawer> Create();
+
+  ScreenDrawer();
+  virtual ~ScreenDrawer();
+
+  // Returns a rect, on which this instance can draw.
+  virtual DesktopRect DrawableRegion() = 0;
+
+  // Draws a rectangle to cover |rect| with color |rgba|. Note, rect.bottom()
+  // and rect.right() two lines are not included.
+  virtual void DrawRectangle(DesktopRect rect, uint32_t rgba) = 0;
+
+  // Clears all content on the screen. In another word, draw a black rectangle
+  // to cover entire DrawableRegion().
+  void Clear() { DrawRectangle(DrawableRegion(), 0); }

[Sergey Ulanov, 2016/08/09 17:43:01]

Thus us an interface, so it shouldn't have any non-virtual methods.
https://google.github.io/styleguide/cppguide.html#Interfaces

[Hzj_jie, 2016/08/11 18:42:03]

Is there any benefit we treat this class as a pure interface?

[Sergey Ulanov, 2016/08/12 05:49:22]

The issue here is not whether we decide to call it interface or not, but whether
the reader of this code know that it's not interface. Everything above this line
looks like  a pure interface and is consistent with other platform-specific code
in this directory. So it will be confusing for the reader if you make an
exception for this one method. For example someone may be looking for x11
implementation of this method in screen_drawer_x11.cc and would be surprised not
to find it there. Also potentially Clear() may be implemented differently on
some platforms, so there is a benefit to making it virtual anyway.

Another issue here is that this is non-trivial method, and I don't think we want
it inlined in a header, even if it's not virtual.

+};
+
+}  // namespace webrtc
+
+#endif  // MODULES_DESKTOP_CAPTURE_SCREEN_DRAWER_H_