Support detaching/attaching panels via inter-strip drags.

chrome/browser/ui/panels/panel_drag_controller.h

 // Copyright (c) 2012 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 CHROME_BROWSER_UI_PANELS_PANEL_DRAG_CONTROLLER_H_
 #define CHROME_BROWSER_UI_PANELS_PANEL_DRAG_CONTROLLER_H_
 #pragma once
 
 #include <set>
 #include "base/basictypes.h"
 #include "ui/gfx/point.h"
 
 class Panel;
+class PanelManager;
+class PanelStrip;
 
 // Responsible for handling drags initiated for all panels, including both
 // intra-strip and inter-strip drags.
 class PanelDragController {
  public:
-  PanelDragController();
+  explicit PanelDragController(PanelManager* panel_manager);
   ~PanelDragController();
 
   // Drags the given panel.
   // |mouse_location| is in screen coordinate system.
   void StartDragging(Panel* panel, const gfx::Point& mouse_location);
   void Drag(const gfx::Point& mouse_location);
   void EndDragging(bool cancelled);
 
   // Asynchronous confirmation of panel having been closed.
   void OnPanelClosed(Panel* panel);
 
   bool IsDragging() const { return dragging_panel_ != NULL; }
 
   Panel* dragging_panel() const { return dragging_panel_; }
-  gfx::Point dragging_panel_original_position() const {
-    return dragging_panel_original_position_;
+
+#ifdef UNIT_TEST
+  static int GetDetachDockedPanelThreshold() {
+    return kDetachDockedPanelThreshold;
   }
 
+  static int GetDockDetachedPanelThreshold() {
+    return kDockDetachedPanelThreshold;
+  }
+#endif
+
  private:
+  // Used to figure out if the panel can be dragged to other strip.
+  PanelStrip* ComputeDragTagetStrip(
+      const gfx::Point& mouse_location, gfx::Point* new_panel_position) const;
+  bool CanDragToDockedStrip(
+      const gfx::Point& mouse_location, gfx::Point* new_panel_position) const;
+  bool CanDragToDetachedStrip(
+      const gfx::Point& mouse_location, gfx::Point* new_panel_position) const;
+
+  PanelManager* panel_manager_;  // Weak, owns us.
+
   // Panel currently being dragged.
   Panel* dragging_panel_;
 
-  // Original position, in screen coordinate system, of the panel being dragged.
-  // This is used to get back to the original position when we cancel the
-  // dragging.
-  gfx::Point dragging_panel_original_position_;
+  // The original panel strip when the drag is started.
+  PanelStrip* dragging_panel_original_strip_;
 
   // The mouse location, in screen coordinates, when StartDragging or Drag was
   // pveviously called.
   gfx::Point last_mouse_location_;
 
+  // The mouse location, in screen coordinates, when the drag starts at the
+  // docked strip.
+  gfx::Point mouse_location_on_docked_;
+
+  // The minimum distance that the docked panel gets dragged up in order to
+  // make it free-floating.

[Andrei, 2012/03/07 22:55:13]

Could we formulate the condition for detaching in terms of how  high the bottom
of the panel is as well? 

We could keep track of the panel's "imaginary" position (where it would be if it
followed the mouse exactly). To do that you only need to remember the offset
from the mouse position to the panel's origin when you start dragging.

The panel's "imaginary" bottom is current mouse position + offset from mouse to
origin at the start of the drag in the docked strip + panel height. 

Now detach == panel's "imaginary" bottom is  50 pixels from the bottom of the
docked area, and attach == it's less than 20 pixels away.

The advantage is that we don't need to keep track of "mouse last position when
docked", which for me was hard to follow.  

[jianli, 2012/03/08 01:18:26]

Done.

+  static const int kDetachDockedPanelThreshold;
+
+  // Indicates how close the bottom of the detached panel is to the bottom of
+  // the docked area such that the detached panel becomes docked.
+  static const int kDockDetachedPanelThreshold;

[Andrei, 2012/03/07 22:55:13]

Why do they start with a "k"? I thought "k" was reserved for enum constants.

[jianli, 2012/03/08 01:18:26]

Per coding style guide, we tend to prefix any constants with k.

+
   DISALLOW_COPY_AND_ASSIGN(PanelDragController);
 };
 
 #endif  // CHROME_BROWSER_UI_PANELS_PANEL_DRAG_CONTROLLER_H_