Dispatches mouse movement messages in posted task to improve dragging smoothness

ui/views/widget/desktop_aura/x11_whole_screen_move_loop.cc

Index: ui/views/widget/desktop_aura/x11_whole_screen_move_loop.cc
diff --git a/ui/views/widget/desktop_aura/x11_whole_screen_move_loop.cc b/ui/views/widget/desktop_aura/x11_whole_screen_move_loop.cc
index 3f48b8993b7e83b6f4a9ddd878f6a85951fbb4bb..0b201d998e594abfea371c9be8a08ee3c22439d5 100644
--- a/ui/views/widget/desktop_aura/x11_whole_screen_move_loop.cc
+++ b/ui/views/widget/desktop_aura/x11_whole_screen_move_loop.cc
@@ -8,6 +8,7 @@
 // Get rid of a macro from Xlib.h that conflicts with Aura's RootWindow class.
 #undef RootWindow
 
+#include "base/bind.h"
 #include "base/message_loop/message_loop.h"
 #include "base/message_loop/message_pump_x11.h"
 #include "base/run_loop.h"
@@ -56,7 +57,9 @@ X11WholeScreenMoveLoop::X11WholeScreenMoveLoop(
     : delegate_(delegate),
       in_move_loop_(false),
       should_reset_mouse_flags_(false),
-      grab_input_window_(None) {
+      grab_input_window_(None),
+      motion_task_posted_(false) {
+  last_xmotion_.type = NoEventMask;
 }
 
 X11WholeScreenMoveLoop::~X11WholeScreenMoveLoop() {}
@@ -64,6 +67,14 @@ X11WholeScreenMoveLoop::~X11WholeScreenMoveLoop() {}
 ////////////////////////////////////////////////////////////////////////////////
 // DesktopWindowTreeHostLinux, MessagePumpDispatcher implementation:
 
+void X11WholeScreenMoveLoop::DispatchMouseMovement() {
+  if (!motion_task_posted_)
+    return;
+
+  motion_task_posted_ = false;
+  delegate_->OnMouseMovement(&last_xmotion_);
+}
+
 uint32_t X11WholeScreenMoveLoop::Dispatch(const base::NativeEvent& event) {
   XEvent* xev = event;
 
@@ -77,20 +88,31 @@ uint32_t X11WholeScreenMoveLoop::Dispatch(const base::NativeEvent& event) {
             screen->GetCursorScreenPoint() - drag_offset_);
         drag_widget_->SetBounds(gfx::Rect(location, drag_image_.size()));
       }
-      delegate_->OnMouseMovement(&xev->xmotion);
+      last_xmotion_ = xev->xmotion;

[varkha, 2014/03/27 05:41:39]

From sadrul@: Don't copy the whole event. Just keep track of the latest location
in a gfx::Point.

[varkha, 2014/03/27 05:46:33]

We need a NativeEvent& to pass to the delegate->OnMouseMovement. Do you think we
should assume that no other fields in the event are used (other than the time
and coordinates as used by DesktopDragDropClientAuraX11::OnMouseMovement)?

+      if (!motion_task_posted_) {
+        motion_task_posted_ = true;
+        base::MessageLoopForUI::current()->PostTask(
+            FROM_HERE,
+            base::Bind(&X11WholeScreenMoveLoop::DispatchMouseMovement,
+                       base::Unretained(this)));

[varkha, 2014/03/27 05:41:39]

From sadrul@: You should use a WeakPtr<> here, instead of using Unretained(),
and invalidate the weak-ptr in EndMoveLoop().
You should also get rid of |motion_task_posted_|.

[varkha, 2014/03/27 05:46:33]

Done.

+      }
       break;
     }
     case ButtonRelease: {
       if (xev->xbutton.button == Button1) {
         // Assume that drags are being done with the left mouse button. Only
         // break the drag if the left mouse button was released.
+        if (motion_task_posted_)
+          DispatchMouseMovement();
         delegate_->OnMouseReleased();
       }
       break;
     }
     case KeyPress: {
-      if (ui::KeyboardCodeFromXKeyEvent(xev) == ui::VKEY_ESCAPE)
+      if (ui::KeyboardCodeFromXKeyEvent(xev) == ui::VKEY_ESCAPE) {
+        motion_task_posted_ = false;
         EndMoveLoop();
+      }
       break;
     }
   }