mus: Implement interwindow drag and drop

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..20053bdef9e23cf04fa767b28b87745f91d7f75b
--- /dev/null
+++ b/ui/views/mus/drop_target_mus.h
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+// 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 <memory>
+
+#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 {
+
+class DropTargetMus : public ui::WindowDropTarget, public aura::WindowObserver {
+ public:
+  DropTargetMus(aura::Window* root_window);
+  ~DropTargetMus() override;
+
+ private:
+  // Common functionality for the ui::DropTargetWin methods to translate from
+  // mus data types to Aura ones.
+  void Translate(uint32_t key_state,
+                 const gfx::Point& position,
+                 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:
+  uint32_t OnDragEnter(std::map<std::string, std::vector<uint8_t>> mime_types,
+                       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 OnDragDrop(uint32_t key_state,
+                      const gfx::Point& position,
+                      uint32_t effect_bitmask) 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_