Adds ability to suspend notifying observers when display list is changed

ui/display/display_list.h

 // 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_DISPLAY_DISPLAY_LIST_H_
 #define UI_DISPLAY_DISPLAY_LIST_H_
 
 #include <stdint.h>
 
+#include <memory>
 #include <vector>
 
 #include "base/observer_list.h"
 #include "ui/display/display.h"
 #include "ui/display/display_export.h"
 
 namespace display {
 
 class Display;
+class DisplayList;
 class DisplayObserver;
 
+// See description in DisplayLock::SuspendObserverUpdates.
+class DISPLAY_EXPORT DisplayListObserverLock {
+ public:
+  ~DisplayListObserverLock();
+
+ private:
+  friend class DisplayList;
+
+  explicit DisplayListObserverLock(DisplayList* display_list);
+
+  DisplayList* display_list_;
+
+  DISALLOW_COPY_AND_ASSIGN(DisplayListObserverLock);
+};
+
 // Maintains an ordered list of display::Displays as well as operations to add,
 // remove and update said list. Additionally maintains display::DisplayObservers
 // and updates them as appropriate.
 class DISPLAY_EXPORT DisplayList {
  public:
   using Displays = std::vector<display::Display>;
 
   enum class Type {
     PRIMARY,
     NOT_PRIMARY,
   };
 
   DisplayList();
   ~DisplayList();
 
   void AddObserver(display::DisplayObserver* observer);
   void RemoveObserver(display::DisplayObserver* observer);
 
   const Displays& displays() const { return displays_; }
 
   Displays::const_iterator FindDisplayById(int64_t id) const;
   Displays::iterator FindDisplayById(int64_t id);
 
   Displays::const_iterator GetPrimaryDisplayIterator() const;
 
+  // Internally increments a counter that while non-zero results in observers
+  // not being called for any changes to the displays. It is assumed once
+  // callers release the last lock they call the observers appropriately.
+  std::unique_ptr<DisplayListObserverLock> SuspendObserverUpdates();
+
   // Updates the cached id based on display.id() as well as whether the Display
   // is the primary display.
   void UpdateDisplay(const display::Display& display, Type type);
 
   // Adds a new Display.
   void AddDisplay(const display::Display& display, Type type);
 
   // Removes the Display with the specified id.
   void RemoveDisplay(int64_t id);
 
+  base::ObserverList<display::DisplayObserver>* observers() {
+    return &observers_;
+  }
+
  private:
+  friend class DisplayListObserverLock;
+
+  bool should_notify_observers() const {
+    return observer_suspend_lock_count_ == 0;
+  }
+  void IncrementObserverSuspendLockCount();
+  void DecrementObserverSuspendLockCount();
+
   std::vector<display::Display> displays_;
   int primary_display_index_ = -1;
   base::ObserverList<display::DisplayObserver> observers_;
 
+  int observer_suspend_lock_count_ = 0;
+
   DISALLOW_COPY_AND_ASSIGN(DisplayList);
 };
 
 }  // namespace display
 
 #endif  // UI_DISPLAY_DISPLAY_LIST_H_