c++ / mojo changes for 'external window mode'

services/ui/public/interfaces/window_tree_host.mojom

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 module ui.mojom;
 
 import "services/ui/public/interfaces/window_tree.mojom";
 import "ui/gfx/geometry/mojo/geometry.mojom";
 
 // WindowTreeHost encapsulates a unique underlying platform window, with a tree
 // of windows.
 interface WindowTreeHost {
   // Sets the size of the platform window.
   SetSize(gfx.mojom.Size size);
 
   // Sets a title string to be displayed on the platform window.
   SetTitle(string title);
 };
 
+// WindowTreeHostFactoryRegistrar is the entry point to obtain a
+// WindowTreeHostFactory instance in 'external window mode'.
+// Callers also obtain a mojo handle to the unique ws::WindowTree instance,
+// on the server-side.
+//
+// NOTE: WindowTreeHostFactoryRegistrar::Register and
+// WindowTreeHostFactory::CreatePlatformWindow are put on separate interfaces,
+// so that the interface containing ::CreatePlatformWindow is obtained by
+// calling ::Register. That eliminates the possibility of ::CreatePlatformWindow
+// being called before ::Register.
+interface WindowTreeHostFactoryRegistrar {
+   Register(WindowTreeHostFactory& window_tree_host_factory,
+            WindowTree& tree_request,
+            WindowTreeClient client);
+};
+
+// WindowTreeHostFactory triggers the creation of WindowTreeHost instances.
 interface WindowTreeHostFactory {
   // Creates a new WindowTreeHost. |tree_client| is queried for the
   // WindowManager.
   CreateWindowTreeHost(WindowTreeHost& window_tree_host,
                        WindowTreeClient tree_client);

[fwang, 2017/03/09 08:14:07]

Do you think you can remove the tree_client parameter and used the one
registered by WindowTreeHostFactoryRegistrar?

+
+  // Creates a new WindowTreeHost in 'external window mode'.
+  // One WindowTree/WindowTreeClient pair can serve one or more WindowTreeHost
+  // instances.
+  CreatePlatformWindow(WindowTreeHost& window_tree_host, uint32 client_id);
 };