[Aura] Support additonal event filters in DesktopEventFilter.

ui/aura_shell/desktop_event_filter.h

Index: ui/aura_shell/desktop_event_filter.h
diff --git a/ui/aura_shell/desktop_event_filter.h b/ui/aura_shell/desktop_event_filter.h
index 1ad0438e5ffe5c1a9c207affd17c8923c06436b6..a5cb2c85922840a45406e7c8b3eec583c6766114 100644
--- a/ui/aura_shell/desktop_event_filter.h
+++ b/ui/aura_shell/desktop_event_filter.h
@@ -7,17 +7,29 @@
 #pragma once
 
 #include "base/compiler_specific.h"
+#include "base/observer_list.h"
 #include "ui/aura/event_filter.h"
 #include "ui/aura_shell/aura_shell_export.h"
 
 namespace aura_shell {
 namespace internal {
 
+// DesktopEventFilter gets all desktop events first and can provide actions to
+// those events. It implements desktop features such as click to activate a
+// window and cursor change when moving mouse.
+// Additional event filters can be added to DesktopEventFilter. Events will
+// pass through those additional filters in their addition order and could be
+// consumed by any of those filters. If an event is consumed by a filter, the
+// rest of the filter(s) and DesktopEventFilter will not see the consumed event.
 class AURA_SHELL_EXPORT DesktopEventFilter : public aura::EventFilter {
  public:
   DesktopEventFilter();
   virtual ~DesktopEventFilter();
 
+  // Adds/removes additional event filters.
+  void AddFilter(aura::EventFilter* filter);
+  void RemoveFilter(aura::EventFilter* filter);
+
   // Overridden from EventFilter:
   virtual bool PreHandleKeyEvent(aura::Window* target,
                                  aura::KeyEvent* event) OVERRIDE;
@@ -34,6 +46,16 @@ class AURA_SHELL_EXPORT DesktopEventFilter : public aura::EventFilter {
   // default resize cursors for window edges.
   void HandleMouseMoved(aura::Window* target, aura::MouseEvent* event);
 
+  // Dispatches event to addtional filters. Returns false or
+  // ui::TOUCH_STATUS_UNKNOWN if event is consumed.
+  bool FilterKeyEvent(aura::Window* target, aura::KeyEvent* event);
+  bool FilterMouseEvent(aura::Window* target, aura::MouseEvent* event);
+  ui::TouchStatus FilterTouchEvent(aura::Window* target,
+                                   aura::TouchEvent* event);
+
+  // Additional event filters that pre-handles events.
+  ObserverList<aura::EventFilter, true> filters_;
+
   DISALLOW_COPY_AND_ASSIGN(DesktopEventFilter);
 };