Fix screen flickering for Windows platform…

webrtc/modules/desktop_capture/window_capturer_win.cc

Index: webrtc/modules/desktop_capture/window_capturer_win.cc
diff --git a/webrtc/modules/desktop_capture/window_capturer_win.cc b/webrtc/modules/desktop_capture/window_capturer_win.cc
index 54b2768aa803fe265d90a6b1f40884aafc795f88..2b38863fc2a0b1eafc76e5f5668e7cb6cd23288b 100644
--- a/webrtc/modules/desktop_capture/window_capturer_win.cc
+++ b/webrtc/modules/desktop_capture/window_capturer_win.cc
@@ -11,6 +11,7 @@
 #include "webrtc/modules/desktop_capture/window_capturer.h"
 
 #include <assert.h>
+#include <set>
 
 #include "webrtc/base/scoped_ptr.h"
 #include "webrtc/base/checks.h"
@@ -103,6 +104,13 @@ class WindowCapturerWin : public WindowCapturer {
 
   AeroChecker aero_checker_;
 
+  // JavaScript fucntion ChooseDesktopMedia() leads SelectWindow() and Capture()

[Sergey Ulanov, 2016/02/20 18:47:49]

nit: no need mention JavaScript here. Just say that we want to avoid flickering
for the case when SelectWindow() calls are interleaved with Capture(). 

[GeorgeZ, 2016/02/22 17:08:45]

Done.

+  // to be called in interleaved sequence. The size of each window need to be
+  // tracked so that when there is no size change, capture will use BitBlt()
+  // instead of PrintWindow() when aero is enabled. keep using PrintWindow() for
+  // each frame capture will lead to screen flickering.
+  std::map<HWND, DesktopSize> window_size_map_;
+
   RTC_DISALLOW_COPY_AND_ASSIGN(WindowCapturerWin);
 };
 
@@ -120,6 +128,21 @@ bool WindowCapturerWin::GetWindowList(WindowList* windows) {
   if (!EnumWindows(&WindowsEnumerationHandler, param))
     return false;
   windows->swap(result);
+
+  // Remove windows not in the list.
+  std::set<HWND> ids;

[Sergey Ulanov, 2016/02/20 18:47:49]

I think it would be better to just build a new map here instead of building a
set and then filtering entries from the existing map. 
I.e.:
std::map<HWND, DesktopSize> new_map;
for (const auto& hwnd : *windows) {
  new_map[hwnd] = window_size_map_[hwnd];
}
window_size_map_.swap(new_map);

This makes the code simpler, and it's likely to be faster.

[GeorgeZ, 2016/02/22 17:08:45]

Good idea.

+  for (const auto& item : *windows) {
+    ids.insert(reinterpret_cast<HWND>(item.id));
+  }
+  auto itr = window_size_map_.begin();
+  while (itr != window_size_map_.end()) {
+    if (!ids.count(itr->first)) {
+      itr = window_size_map_.erase(itr);
+    } else {
+      ++itr;
+    }
+  }
+
   return true;
 }
 
@@ -128,7 +151,11 @@ bool WindowCapturerWin::SelectWindow(WindowId id) {
   if (!IsWindow(window) || !IsWindowVisible(window) || IsIconic(window))
     return false;
   window_ = window;
-  previous_size_.set(0, 0);
+  if (window_size_map_.count(window)) {
+    previous_size_ = window_size_map_[window];
+  } else {
+    previous_size_.set(0, 0);
+  }
   return true;
 }
 
@@ -170,6 +197,7 @@ void WindowCapturerWin::Capture(const DesktopRegion& region) {
     memset(frame->data(), 0, frame->stride() * frame->size().height());
 
     previous_size_ = frame->size();
+    window_size_map_[window_] = previous_size_;
     callback_->OnCaptureCompleted(frame);
     return;
   }
@@ -236,6 +264,7 @@ void WindowCapturerWin::Capture(const DesktopRegion& region) {
   ReleaseDC(window_, window_dc);
 
   previous_size_ = frame->size();
+  window_size_map_[window_] = previous_size_;
 
   frame->mutable_updated_region()->SetRect(
       DesktopRect::MakeSize(frame->size()));