mus: Implement interwindow drag and drop

services/ui/ws/window_server.cc

Index: services/ui/ws/window_server.cc
diff --git a/services/ui/ws/window_server.cc b/services/ui/ws/window_server.cc
index dd5e3cae6261063b7fe094896a1deb36b3d9f6c9..f4d9a4ec082a6ee855a047df80d7a017b7b147d6 100644
--- a/services/ui/ws/window_server.cc
+++ b/services/ui/ws/window_server.cc
@@ -38,6 +38,12 @@ struct WindowServer::CurrentMoveLoopState {
   gfx::Rect revert_bounds;
 };
 
+struct WindowServer::CurrentDragLoopState {
+  uint32_t change_id;
+  ServerWindow* window;
+  WindowTree* initiator;
+};
+
 WindowServer::WindowServer(WindowServerDelegate* delegate)
     : delegate_(delegate),
       surfaces_state_(new SurfacesState()),
@@ -484,6 +490,35 @@ gfx::Rect WindowServer::GetCurrentMoveLoopRevertBounds() {
   return gfx::Rect();
 }
 
+void WindowServer::StartDragLoop(uint32_t change_id,
+                                 ServerWindow* window,
+                                 WindowTree* initiator) {
+  current_drag_loop_.reset(
+      new CurrentDragLoopState{change_id, window, initiator});
+}
+
+void WindowServer::EndDragLoop() {
+  current_drag_loop_.reset();
+}
+
+uint32_t WindowServer::GetCurrentDragLoopChangeId() {

[sky, 2016/09/13 18:15:32]

Why does this information need to be held by the WindowServer? AFAICT it's only
needed by the WindowTree that initiates the drag. Shouldn't it be held there?

[Elliot Glaysher, 2016/09/14 22:19:16]

Parallelism with storing the move loop for the safety checks. If we have one
connection currently performing a drag, and a second tries to start a drag on a
different root in a different process, we should immediately return unsuccessful
the second one.

(Along with erroring on trying to start a move loop during a drag and vice
versa.)

[sky, 2016/09/14 22:52:02]

Isn't this information seldomly used? If so, can you iterate over all the
WindowTrees to see if one has a drag loop going on?

[Elliot Glaysher, 2016/09/15 17:12:40]

WindowServer doesn't publicly expose the WindowTreeMap. Adding a method on
WindowServer to expose it or write a method on WindowServer that searches for it
feels worse than this.

[sky, 2016/09/15 18:10:46]

Fair enough.

+  if (current_drag_loop_)
+    return current_drag_loop_->change_id;
+  return 0u;
+}
+
+ServerWindow* WindowServer::GetCurrentDragLoopWindow() {
+  if (current_drag_loop_)
+    return current_drag_loop_->window;
+  return nullptr;
+}
+
+WindowTree* WindowServer::GetCurrentDragLoopInitiator() {
+  if (current_drag_loop_)
+    return current_drag_loop_->initiator;
+  return nullptr;
+}
+
 void WindowServer::OnDisplayReady(Display* display, bool is_first) {
   if (gpu_channel_)
     display->platform_display()->OnGpuChannelEstablished(gpu_channel_);