Replace changes_channel with an observer list.

chrome/browser/sync/util/channel.h

+// Copyright (c) 2010 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_SYNC_UTIL_CHANNEL_H_
+#define CHROME_BROWSER_SYNC_UTIL_CHANNEL_H_
+
+///////////////////////////////////////////////////////////////////////////////
+//
+// OVERVIEW:
+//
+//   A threadsafe container for a list of observers.  Observers are able to
+//   remove themselves during iteration, and can be added on any thread.  This
+//   allows observers to safely remove themselves during notifications.  It
+//   also provides a handler when an observer is added that will remove the
+//   observer on destruction.
+//
+//   It is expected that all observers are removed before destruction.
+//   The channel owner should notify all observers to disconnect on shutdown if
+//   needed to ensure this.
+//
+// TYPICAL USAGE:
+//
+//   class MyWidget {
+//    public:
+//     ...
+//
+//     class Observer : public ChannelEventHandler<FooEvent> {
+//      public:
+//       virtual void HandleChannelEvent(const FooEvent& w) = 0;
+//     };
+//
+//     ChannelHookup<MyEvent>* AddObserver(Observer* obs) {
+//       return channel_.AddObserver(obs);
+//     }
+//
+//     void RemoveObserver(Observer* obs) {
+//       channel_.RemoveObserver(obs);
+//     }
+//
+//     void NotifyFoo(FooEvent& event) {
+//       channel_.Notify(event);
+//     }
+//
+//    private:
+//     Channel<FooEvent> channel_;
+//   };
+//
+//
+///////////////////////////////////////////////////////////////////////////////
+
+#include "base/lock.h"
+#include "base/observer_list.h"
+
+namespace browser_sync {
+
+template <typename EventType>
+class Channel;
+
+class EventHandler {
+};
+
+template <typename EventType>
+class ChannelEventHandler : public EventHandler {
+ public:
+  virtual void HandleChannelEvent(const EventType& event) = 0;
+};
+
+// This class manages a connection to a channel.  When it is destroyed, it
+// will remove the listener from the channel observer list.
+template <typename EventType>
+class ChannelHookup {
+ public:
+  ChannelHookup(Channel<EventType>* channel,
+                browser_sync::ChannelEventHandler<EventType>* handler)
+      : channel_(channel),
+        handler_(handler) {}
+  ~ChannelHookup() {
+    channel_->RemoveObserver(handler_);
+  }
+
+ private:
+  Channel<EventType>* channel_;
+  browser_sync::ChannelEventHandler<EventType>* handler_;
+};
+
+template <typename EventType>
+class Channel {
+ public:
+  typedef ObserverListBase<EventHandler> ChannelObserverList;
+
+  Channel() : locking_thread_(0) {}
+
+  ChannelHookup<EventType>* AddObserver(
+      ChannelEventHandler<EventType>* observer) {
+    AutoLock scoped_lock(event_handlers_mutex_);
+    event_handlers_.AddObserver(observer);
+    return new ChannelHookup<EventType>(this, observer);
+  }
+
+  void RemoveObserver(ChannelEventHandler<EventType>* observer) {
+    // This can be called in response to a notification, so we may already have
+    // locked this channel on this thread.
+    bool need_lock = (locking_thread_ != PlatformThread::CurrentId());
+    if (need_lock)
+      event_handlers_mutex_.Acquire();
+
+    event_handlers_mutex_.AssertAcquired();
+    event_handlers_.RemoveObserver(observer);
+    if (need_lock)
+      event_handlers_mutex_.Release();
+  }
+
+  void Notify(const EventType& event) {
+    AutoLock scoped_lock(event_handlers_mutex_);
+
+    // This may result in an observer trying to remove itself, so keep track
+    // of the thread we're locked on.
+    locking_thread_ = PlatformThread::CurrentId();
+
+    ChannelObserverList::Iterator it(event_handlers_);
+    EventHandler* obs;
+    while ((obs = it.GetNext()) != NULL) {
+      static_cast<ChannelEventHandler<EventType>* >(obs)->
+          HandleChannelEvent(event);
+    }
+
+    // Set back to an invalid thread id.
+    locking_thread_ = 0;
+  }
+
+ private:
+  Lock event_handlers_mutex_;
+  PlatformThreadId locking_thread_;
+  ObserverList<EventHandler> event_handlers_;
+};
+
+}  // namespace browser_sync
+
+#endif  // CHROME_BROWSER_SYNC_UTIL_CHANNEL_H_