Create a thread-safe observer list. Will be used

by SystemMonitor.

Right now the class requires that Observers be RefCounted<>.  This is because we invoke tasks via NewRunnableMethod for them.  However, because we manually track lifecycle via AddObserver/RemoveObserver, we could override the RunnableMethodTraits to not require RefCounted<>.   This would have the advantage that callers do not need to make all Observers be RefCounted, but makes it more critical that observers not forget to call RemoveObserver().
Review URL: http://codereview.chromium.org/7353

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@3787 0039d316-1c4b-4281-b951-d872f2087c98

1 files changed, 190 insertions, 0 deletions

diff --git a/base/observer_list_threadsafe.h b/base/observer_list_threadsafe.h
new file mode 100644
index 0000000..fcfa0ed
--- /dev/null
+++ b/base/observer_list_threadsafe.h
@@ -0,0 +1,190 @@
+// Copyright (c) 2006-2008 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_OBSERVER_LIST_THREADSAFE_H_
+#define BASE_OBSERVER_LIST_THREADSAFE_H_
+
+#include <vector>
+#include <algorithm>
+
+#include "base/basictypes.h"
+#include "base/logging.h"
+#include "base/message_loop.h"
+#include "base/observer_list.h"
+#include "base/ref_counted.h"
+#include "base/task.h"
+
+///////////////////////////////////////////////////////////////////////////////
+//
+// OVERVIEW:
+//
+//   A thread-safe container for a list of observers.
+//   This is similar to the observer_list (see observer_list.h), but it 
+//   is more robust for multi-threaded situations.
+//   
+//   The following use cases are supported:
+//    * Observers can register for notifications from any thread.
+//      Callbacks to the observer will occur on the same thread where
+//      the observer initially called AddObserver() from.
+//    * Any thread may trigger a notification via NOTIFY_OBSERVERS.
+//    * Observers can remove themselves from the observer list inside
+//      of a callback.
+//    * If one thread is notifying observers concurrently with an observer
+//      removing itself from the observer list, the notifications will
+//      be silently dropped.
+//
+//   The drawback of the threadsafe observer list is that notifications
+//   are not as real-time as the non-threadsafe version of this class.
+//   Notifications will always be done via PostTask() to another thread,
+//   whereas with the non-thread-safe observer_list, notifications happen
+//   synchronously and immediately.
+//
+//   IMPLEMENTATION NOTES
+//   The ObserverListThreadSafe maintains an ObserverList for each thread
+//   which uses the ThreadSafeObserver.  When Notifying the observers, 
+//   we simply call PostTask to each registered thread, and then each thread
+//   will notify its regular ObserverList.
+//
+///////////////////////////////////////////////////////////////////////////////
+template <class ObserverType>
+class ObserverListThreadSafe : 
+    public base::RefCountedThreadSafe<ObserverListThreadSafe<ObserverType> > {
+ public:
+  ObserverListThreadSafe() {}
+
+  ~ObserverListThreadSafe() {
+    typename ObserversListMap::const_iterator it;
+    for (it = observer_lists_.begin(); it != observer_lists_.end(); ++it)
+      delete (*it).second;
+    observer_lists_.clear();
+  }
+
+  // Add an observer to the list.
+  void AddObserver(ObserverType* obs) {
+    ObserverList<ObserverType>* list = NULL;
+    MessageLoop* loop = MessageLoop::current();
+    {
+      AutoLock lock(list_lock_);
+      if (observer_lists_.find(loop) == observer_lists_.end())
+        observer_lists_[loop] = new ObserverList<ObserverType>();
+      list = observer_lists_[loop];
+    }
+    list->AddObserver(obs);
+  }
+
+  // Remove an observer from the list.
+  // If there are pending notifications in-transit to the observer, they will
+  // be aborted.
+  // RemoveObserver MUST be called from the same thread which called
+  // AddObserver.
+  void RemoveObserver(ObserverType* obs) {
+    ObserverList<ObserverType>* list = NULL;
+    MessageLoop* loop = MessageLoop::current();
+    {
+      AutoLock lock(list_lock_);
+      DCHECK(observer_lists_.find(loop) != observer_lists_.end()) <<
+          "RemoveObserver called on for unknown thread";
+      list = observer_lists_[loop];
+
+      // If we're about to remove the last observer from the list,
+      // then we can remove this observer_list entirely.
+      if (list->size() == 1)
+        observer_lists_.erase(loop);
+    }
+    list->RemoveObserver(obs);
+
+    // If RemoveObserver is called from a notification, the size will be
+    // nonzero.  Instead of deleting here, the NotifyWrapper will delete
+    // when it finishes iterating.
+    if (list->size() == 0)
+      delete list;
+  }
+
+  // Notify methods.
+  // Make a thread-safe callback to each Observer in the list.
+  // Note, these calls are effectively asynchronous.  You cannot assume
+  // that at the completion of the Notify call that all Observers have
+  // been Notified.  The notification may still be pending delivery.
+  template <class Method>
+  void Notify(Method m) {
+    UnboundMethod<ObserverType, Method, Tuple0> method(m, MakeTuple());
+    Notify<Method, Tuple0>(method);
+  }
+
+  template <class Method, class A>
+  void Notify(Method m, const A &a) {
+    UnboundMethod<ObserverType, Method, Tuple1<A> > method(m, MakeTuple(a));
+    Notify<Method, Tuple1<A> >(method);
+  }
+
+  // TODO(mbelshe):  Add more wrappers for Notify() with more arguments.
+
+ private:
+  template <class Method, class Params>
+  void Notify(const UnboundMethod<ObserverType, Method, Params>& method) {
+    AutoLock lock(list_lock_);
+    typename ObserversListMap::iterator it;
+    for (it = observer_lists_.begin(); it != observer_lists_.end(); ++it) {
+      MessageLoop* loop = (*it).first;
+      ObserverList<ObserverType>* list = (*it).second;
+      loop->PostTask(FROM_HERE,
+          NewRunnableMethod(this, 
+              &ObserverListThreadSafe<ObserverType>::
+                 template NotifyWrapper<Method, Params>, list, method));
+    }
+  }
+
+  // Wrapper which is called to fire the notifications for each thread's
+  // ObserverList.  This function MUST be called on the thread which owns
+  // the unsafe ObserverList.
+  template <class Method, class Params>
+  void NotifyWrapper(ObserverList<ObserverType>* list, 
+      const UnboundMethod<ObserverType, Method, Params>& method) {
+
+    // Check that this list still needs notifications.
+    {
+      AutoLock lock(list_lock_);
+      typename ObserversListMap::iterator it =
+          observer_lists_.find(MessageLoop::current());
+
+      // The ObserverList could have been removed already.  In fact, it could
+      // have been removed and then re-added!  If the master list's loop
+      // does not match this one, then we do not need to finish this
+      // notification.
+      if (it == observer_lists_.end() || it->second != list)
+        return;
+    }
+
+    {
+      typename ObserverList<ObserverType>::Iterator it(*list);
+      ObserverType* obs;
+      while ((obs = it.GetNext()) != NULL)
+        method.Run(obs);
+    }
+
+    // If there are no more observers on the list, we can now delete it.
+    if (list->size() == 0) {
+#ifndef NDEBUG
+      {
+        AutoLock lock(list_lock_);
+        // Verify this list is no longer registered.
+        typename ObserversListMap::iterator it =
+            observer_lists_.find(MessageLoop::current());
+        DCHECK(it == observer_lists_.end() || it->second != list);
+      }
+#endif
+      delete list;
+    }
+  }
+
+  typedef std::map<MessageLoop*, ObserverList<ObserverType>*> ObserversListMap;
+
+  // These are marked mutable to facilitate having NotifyAll be const.
+  Lock list_lock_;  // Protects the observer_lists_.
+  ObserversListMap observer_lists_;
+
+  DISALLOW_EVIL_CONSTRUCTORS(ObserverListThreadSafe);
+};
+
+#endif  // BASE_OBSERVER_LIST_THREADSAFE_H_