[WebRTC] Add ScreenCapturerDifferWrapper to share Differ across ScreenCapturers

webrtc/modules/desktop_capture/differ.h

Index: webrtc/modules/desktop_capture/differ.h
diff --git a/webrtc/modules/desktop_capture/differ.h b/webrtc/modules/desktop_capture/differ.h
index 9ab059bcaa24cdb8491b86791e954423dcd95cc6..dca38676fe831ba5a64445d685ef76ebbb8a9a9a 100644
--- a/webrtc/modules/desktop_capture/differ.h
+++ b/webrtc/modules/desktop_capture/differ.h
@@ -15,10 +15,13 @@
 #include <vector>
 
 #include "webrtc/base/constructormagic.h"
-#include "webrtc/modules/desktop_capture/desktop_region.h"
+#include "webrtc/typedefs.h"
 
 namespace webrtc {
 
+class DesktopRect;
+class DesktopRegion;
+
 // TODO(sergeyu): Simplify differ now that we are working with DesktopRegion.
 // diff_info_ should no longer be needed, as we can put our data directly into
 // the region that we are calculating.
@@ -37,22 +40,33 @@ class Differ {
   int bytes_per_pixel() { return bytes_per_pixel_; }
   int bytes_per_row() { return bytes_per_row_; }
 
-  // Given the previous and current screen buffer, calculate the dirty region
+  // Given the previous and current screen buffer, calculates the dirty region

[Jamie, 2016/08/03 23:52:25]

FWIW, either calculate or calculates would be appropriate here. Since the other
comments here are written as instructions rather than descriptions (eg,
"Identify all the blocks", not "Identifies all the blocks"), I'd argue that the
original was more consistent.

[Hzj_jie, 2016/08/04 02:28:07]

Done.

   // that encloses all of the changed pixels in the new screen.
-  void CalcDirtyRegion(const uint8_t* prev_buffer, const uint8_t* curr_buffer,
+  void CalcDirtyRegion(const uint8_t* prev_buffer,
+                       const uint8_t* curr_buffer,
                        DesktopRegion* region);
 
+  // Given the previous and current screen buffer, and a superset of dirty
+  // region as hint, calculates the dirty region that encloses all the changed
+  // pixels in the new screen.
+  void CalcDirtyRegionWithHints(const uint8_t* prev_buffer,
+                                const uint8_t* curr_buffer,
+                                const DesktopRegion& hints,
+                                DesktopRegion* region);
+
  private:
   // Allow tests to access our private parts.
   friend class DifferTest;
 
-  // Identify all of the blocks that contain changed pixels.
-  void MarkDirtyBlocks(const uint8_t* prev_buffer, const uint8_t* curr_buffer);
+  // Identify all of the blocks that contain changed pixels in the |rect|.
+  void MarkDirtyBlocks(const uint8_t* prev_buffer,
+                       const uint8_t* curr_buffer,
+                       const DesktopRect& rect);
 
   // After the dirty blocks have been identified, this routine merges adjacent
   // blocks into a region.
   // The goal is to minimize the region that covers the dirty blocks.
-  void MergeBlocks(DesktopRegion* region);
+  void MergeBlocks(DesktopRegion* region) const;

[Jamie, 2016/08/03 23:52:25]

Optional: I'm not sure it's worth adding these consts. I don't have a strong
opinion about it, but I don't think it's something we commonly do, so it leaves
the reader wondering what's special about this class that needs it.

[Hzj_jie, 2016/08/04 02:28:07]

IMO, a const indeed tells the reader, this function does not need to impact the
class internal states. i.e. It impacts only the parameters.

And in Google C++ style guide, using const for functions is strongly
recommended. (https://goo.gl/sbqlAI) So does Chromium have a different
suggestion?

 
   // Checks whether the upper-left portions of the buffers are equal. The size
   // of the portion to check is specified by the |width| and |height| values.
@@ -64,23 +78,23 @@ class Differ {
                           int width, int height);
 
   // Dimensions of screen.
-  int width_;
-  int height_;
+  const int width_;
+  const int height_;
 
   // Number of bytes for each pixel in source and dest bitmap.
   // (Yes, they must match.)
-  int bytes_per_pixel_;
+  const int bytes_per_pixel_;
 
   // Number of bytes in each row of the image (AKA: stride).
-  int bytes_per_row_;
-
-  // Diff information for each block in the image.
-  std::unique_ptr<bool[]> diff_info_;
+  const int bytes_per_row_;
 
   // Dimensions and total size of diff info array.
-  int diff_info_width_;
-  int diff_info_height_;
-  int diff_info_size_;
+  const int diff_info_width_;
+  const int diff_info_height_;
+  const int diff_info_size_;
+
+  // Diff information for each block in the image.
+  const std::unique_ptr<bool[]> diff_info_;
 
   RTC_DISALLOW_COPY_AND_ASSIGN(Differ);
 };