Support auto-hide taskbar for panels on Windows.

chrome/browser/ui/panels/auto_hide_bottom_bar.h

+// Copyright (c) 2011 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_AUTO_HIDE_BOTTOM_BAR_H_
+#define CHROME_BROWSER_UI_PANELS_AUTO_HIDE_BOTTOM_BAR_H_
+#pragma once
+
+#include "base/memory/ref_counted.h"
+
+namespace gfx {
+class Rect;
+}
+
+// Encapsulates the logic to deal with always-on-top system/app bar that is
+// located at the bottom of the screen and set to auto-hide, like Windows
+// taskbar or MacOSX dock.
+class AutoHideBottomBar : public base::RefCountedThreadSafe<AutoHideBottomBar> {

[Dmitry Titov, 2011/08/22 22:35:53]

why this class is refcounted? It seems the singleton PanelManager keeps the only
pointer to it and can completely control its lifetime via scoped_ptr?

[jianli, 2011/08/22 23:28:10]

The test also keep a ref count to the mock because ref count is needed for
PostTask. Added comment for it.

+ public:
+  enum Visibility {

[Dmitry Titov, 2011/08/22 22:35:53]

What do you think about this: Do we also need AUTOHIDE_DISABLED here? 
Then we can fire OnAutoHideBottomBarVisibilityChanged with this as well - for
example, when user moves the bar to the side or clears 'autohide' checkbox in
the settings. The panels might need to rearrange then - for example, VISIBLE
panel may slide to the bottom if taskbar was moved to the left. Otherwise, how
would panels know when to move?

[jianli, 2011/08/22 23:28:10]

I think this state is not needed because this class in only intended to deal
with auto-hide bottom bar. If the bottom bar is not in auto-hide mode, we do not
need to know about its visibility and height.

When the auto-hide mode is changed, the work area is also changed. Thus
PanelManager::OnDisplayChanged is called, which also calls
AutoHideBottomBar::UpdateWorkArea.

+    VISIBLE,
+    ANIMATING,
+    HIDDEN
+  };
+
+  // Observer can listen to various events regarding the bottom bar changes.
+  // StartMonitoring should be called first if you want observer to receive
+  // the notifications.
+  class Observer {
+   public:
+    virtual void OnAutoHideBottomBarVisibilityChanged(
+        Visibility visibility) = 0;
+    virtual void OnAutoHideBottomBarHeightChanged(int height) = 0;
+  };
+
+  static AutoHideBottomBar* Create(Observer* observer);
+
+  virtual ~AutoHideBottomBar() { }
+
+  // This should be called each time when the work area is changed. We only
+  // care about the bottom bar that sits on the screen that hosts the specified
+  // work area.
+  virtual void UpdateWorkArea(const gfx::Rect& work_area) = 0;
+
+  // Returns true if the auto-hide bottom bar exists.
+  virtual bool Exists() = 0;

[Dmitry Titov, 2011/08/22 22:35:53]

See comment above about AUTOHIDE_DISABLED. If we had that, we would not need
this method.

[jianli, 2011/08/22 23:28:10]

See my reply above.

+
+  // Returns the fixed height of the bottom bar. Note that the height remains

[Dmitry Titov, 2011/08/22 22:35:53]

Suggestion on comment (not sure it's clearer though):
Returns the height of the bottom bar that corresponds to Visibility VISIBLE.
This value does not depend on current bar's Visibility. The height of the bar
when it's HIDDEN is usually between 0 and few pixels.

[jianli, 2011/08/22 23:28:10]

Done.

+  // unchanged no matter whether the bottom bar is brought up or down.
+  virtual int GetHeight() = 0;
+
+  virtual Visibility GetVisibility() = 0;
+};
+
+#endif  // CHROME_BROWSER_UI_PANELS_AUTO_HIDE_BOTTOM_BAR_H_