mash: Use the new mus drag and drop API to get drag working in mash.

ui/views/mus/drop_target_mus.h

Index: ui/views/mus/drop_target_mus.h
diff --git a/ui/views/mus/drop_target_mus.h b/ui/views/mus/drop_target_mus.h
new file mode 100644
index 0000000000000000000000000000000000000000..0a2bffb6df1cb43d25150d3f92361cd6a4582875
--- /dev/null
+++ b/ui/views/mus/drop_target_mus.h
@@ -0,0 +1,91 @@
+// Copyright 2016 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.
+
+#ifndef UI_VIEWS_MUS_DROP_TARGET_MUS_H_
+#define UI_VIEWS_MUS_DROP_TARGET_MUS_H_
+
+#include <map>
+#include <memory>
+#include <string>
+#include <vector>
+
+#include "base/macros.h"
+#include "services/ui/public/cpp/window_drop_target.h"
+#include "ui/aura/window_observer.h"
+
+namespace aura {
+class RootWindow;
+class Window;
+
+namespace client {
+class DragDropDelegate;
+}
+}
+
+namespace ui {
+class DropTargetEvent;
+class OSExchangeData;
+}
+
+namespace views {
+
+// An adapter class which takes signals from mus' WindowDropTarget, performs
+// targeting on the underlying aura::Window tree, and dispatches them to the
+// aura DragDropDelegate of the targeted aura::Window.
+class DropTargetMus : public ui::WindowDropTarget, public aura::WindowObserver {
+ public:
+  explicit DropTargetMus(aura::Window* root_window);
+  ~DropTargetMus() override;
+
+ private:
+  // Common functionality for the ui::DropTargetWin methods to translate from

[sky, 2016/09/23 16:14:09]

DropTargetWin?

+  // mus data types to Aura ones.
+  void Translate(uint32_t key_state,
+                 const gfx::Point& position,

[sky, 2016/09/23 16:14:08]

If position is a screen location, please name it as such.

+                 uint32_t effect_bitmask,
+                 std::unique_ptr<ui::OSExchangeData>* data,
+                 std::unique_ptr<ui::DropTargetEvent>* event,
+                 aura::client::DragDropDelegate** delegate);
+
+  void NotifyDragLeave();
+
+  // Overridden from ui::WindowDropTarget:
+  void OnDragDropStart(
+      std::map<std::string, std::vector<uint8_t>> mime_data) override;

[sky, 2016/09/23 16:14:08]

Just noticed this. How come this doesn't take a const map&?

[Elliot Glaysher, 2016/09/23 21:40:48]

To minimize the number of copies of an arbitrarily large map.

- Incoming mojo map in WindowTreeClient gets move()d to WTC storage.
- The first time the cursor enters a window, its drop target receives an
OnDragDropStart with a std::map. This creates a copy of the map.
- If that map was passed in const&, when it reaches its final storage
assignment, it  needs to be copied a second time due to the constness.
- But if that map is consistently move()d, we won't be building a second copy.

(OK, but then why not make something like linked_ptr<mojo::Map<...>>? Because
the OSExchangeData object is mutable.)

+  uint32_t OnDragEnter(uint32_t key_state,
+                       const gfx::Point& position,
+                       uint32_t effect_bitmask) override;
+  uint32_t OnDragOver(uint32_t key_state,
+                      const gfx::Point& position,
+                      uint32_t effect_bitmask) override;
+  void OnDragLeave() override;
+  uint32_t OnCompleteDrop(uint32_t key_state,
+                          const gfx::Point& position,
+                          uint32_t effect_bitmask) override;
+  void OnDragDropDone() override;
+
+  // Overridden from aura::WindowObserver:
+  void OnWindowDestroyed(aura::Window* window) override;
+
+  // The root window associated with this drop target.
+  aura::Window* root_window_;
+
+  // The Aura window that is currently under the cursor. We need to manually
+  // keep track of this because mus will only call our drag enter method once
+  // when the user enters the associated mus::Window. But inside mus there
+  // could be multiple aura windows, so we need to generate drag enter events
+  // for them.
+  aura::Window* target_window_;
+
+  // The entire drag data payload. We receive this during the drag enter event
+  // and cache it so we don't send this multiple times. We reset this value on
+  // leave or drop.
+  std::map<std::string, std::vector<uint8_t>> mime_data_;
+
+  DISALLOW_COPY_AND_ASSIGN(DropTargetMus);
+};
+
+}  // namespace views
+
+#endif  // UI_VIEWS_MUS_DROP_TARGET_MUS_H_