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diff --git a/components/prefs/pref_notifier_impl.h b/components/prefs/pref_notifier_impl.h
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+++ b/components/prefs/pref_notifier_impl.h
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+// 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 BASE_PREFS_PREF_NOTIFIER_IMPL_H_
+#define BASE_PREFS_PREF_NOTIFIER_IMPL_H_
+
+#include <list>
+#include <string>
+
+#include "base/callback.h"
+#include "base/compiler_specific.h"
+#include "base/containers/hash_tables.h"
+#include "base/macros.h"
+#include "base/observer_list.h"
+#include "base/prefs/base_prefs_export.h"
+#include "base/prefs/pref_notifier.h"
+#include "base/prefs/pref_observer.h"
+#include "base/threading/thread_checker.h"
+
+class PrefService;
+
+// The PrefNotifier implementation used by the PrefService.
+class BASE_PREFS_EXPORT PrefNotifierImpl
+    : public NON_EXPORTED_BASE(PrefNotifier) {
+ public:
+  PrefNotifierImpl();
+  explicit PrefNotifierImpl(PrefService* pref_service);
+  ~PrefNotifierImpl() override;
+
+  // If the pref at the given path changes, we call the observer's
+  // OnPreferenceChanged method.
+  void AddPrefObserver(const std::string& path, PrefObserver* observer);
+  void RemovePrefObserver(const std::string& path, PrefObserver* observer);
+
+  // We run the callback once, when initialization completes. The bool
+  // parameter will be set to true for successful initialization,
+  // false for unsuccessful.
+  void AddInitObserver(base::Callback<void(bool)> observer);
+
+  void SetPrefService(PrefService* pref_service);
+
+ protected:
+  // PrefNotifier overrides.
+  void OnPreferenceChanged(const std::string& pref_name) override;
+  void OnInitializationCompleted(bool succeeded) override;
+
+  // A map from pref names to a list of observers. Observers get fired in the
+  // order they are added. These should only be accessed externally for unit
+  // testing.
+  typedef base::ObserverList<PrefObserver> PrefObserverList;
+  typedef base::hash_map<std::string, PrefObserverList*> PrefObserverMap;
+
+  typedef std::list<base::Callback<void(bool)>> PrefInitObserverList;
+
+  const PrefObserverMap* pref_observers() const { return &pref_observers_; }
+
+ private:
+  // For the given pref_name, fire any observer of the pref. Virtual so it can
+  // be mocked for unit testing.
+  virtual void FireObservers(const std::string& path);
+
+  // Weak reference; the notifier is owned by the PrefService.
+  PrefService* pref_service_;
+
+  PrefObserverMap pref_observers_;
+  PrefInitObserverList init_observers_;
+
+  base::ThreadChecker thread_checker_;
+
+  DISALLOW_COPY_AND_ASSIGN(PrefNotifierImpl);
+};
+
+#endif  // BASE_PREFS_PREF_NOTIFIER_IMPL_H_