Add KeyedServiceShutdownNotifier for classes that depend on keyed services but can't be keyed servi…

components/keyed_service/core/keyed_service_shutdown_notifier.h

+// Copyright 2015 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 COMPONENTS_KEYED_SERVICE_CORE_KEYED_SERVICE_SHUTDOWN_NOTIFIER_H_
+#define COMPONENTS_KEYED_SERVICE_CORE_KEYED_SERVICE_SHUTDOWN_NOTIFIER_H_
+
+#include "base/callback_list.h"
+#include "components/keyed_service/core/keyed_service.h"
+
+// This is a helper class for objects that depend on one or more keyed services,
+// but which cannot be keyed services themselves, for example because they don't
+// correspond 1:1 to a context, or because they have a different lifetime.
+//
+// To use this class, add a factory class and declare the dependencies there.
+// This class (being a KeyedService itself) will be shut down before its
+// dependencies and notify its observers.
+class KEYED_SERVICE_EXPORT KeyedServiceShutdownNotifier : public KeyedService {
+ public:
+  using Subscription = base::CallbackList<void()>::Subscription;
+
+  KeyedServiceShutdownNotifier();
+  ~KeyedServiceShutdownNotifier() override;
+
+  // Subscribe for a notification when the keyed services this object depends on
+  // (as defined by its factory) are shut down. The subscription object can be
+  // destroyed to unsubscribe.
+  scoped_ptr<Subscription> Subscribe(const base::Closure& callback);
+
+ private:
+  // KeyedService implementation:
+  void Shutdown() override;
+
+  base::CallbackList<void()> callback_list_;
+
+  DISALLOW_COPY_AND_ASSIGN(KeyedServiceShutdownNotifier);
+};
+
+#endif  // COMPONENTS_KEYED_SERVICE_CORE_KEYED_SERVICE_SHUTDOWN_NOTIFIER_H_