Bug 678694 - (5/7) Add a non-XPCOM observer system. sr=cjones

This adding an Observer<T> class and an ObserverList<T> class. The former can
get notifications. The later have an array if Observer<T>s and can Broadcast
notifications to them.

xpcom/glue/Makefile.in

@@ -130,6 +130,7 @@ EXPORTS_mozilla = \
   SSE.h \
   arm.h \
   unused.h \
+  Observer.h \
   $(NULL)
 
 EXPORTS_mozilla/threads = \

xpcom/glue/Observer.h

@@ -0,0 +1,105 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* ***** BEGIN LICENSE BLOCK *****
+ * Version: MPL 1.1/GPL 2.0/LGPL 2.1
+ *
+ * The contents of this file are subject to the Mozilla Public License Version
+ * 1.1 (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ * http://www.mozilla.org/MPL/
+ *
+ * Software distributed under the License is distributed on an "AS IS" basis,
+ * WITHOUT WARRANTY OF ANY KIND, either express or implied. See the License
+ * for the specific language governing rights and limitations under the
+ * License.
+ *
+ * The Original Code is mozilla.org code.
+ *
+ * The Initial Developer of the Original Code is Mozilla Foundation
+ * Portions created by the Initial Developer are Copyright (C) 2011
+ * the Initial Developer. All Rights Reserved.
+ *
+ * Contributor(s):
+ *   Mounir Lamouri <mounir.lamouri@mozilla.com> (Original Author)
+ *
+ * Alternatively, the contents of this file may be used under the terms of
+ * either of the GNU General Public License Version 2 or later (the "GPL"),
+ * or the GNU Lesser General Public License Version 2.1 or later (the "LGPL"),
+ * in which case the provisions of the GPL or the LGPL are applicable instead
+ * of those above. If you wish to allow use of your version of this file only
+ * under the terms of either the GPL or the LGPL, and not to allow others to
+ * use your version of this file under the terms of the MPL, indicate your
+ * decision by deleting the provisions above and replace them with the notice
+ * and other provisions required by the GPL or the LGPL. If you do not delete
+ * the provisions above, a recipient may use your version of this file under
+ * the terms of any one of the MPL, the GPL or the LGPL.
+ *
+ * ***** END LICENSE BLOCK ***** */
+
+#ifndef mozilla_Observer_h
+#define mozilla_Observer_h
+
+#include "nsTArray.h"
+
+namespace mozilla {
+
+/**
+ * Observer<T> provides a way for a class to observe something.
+ * When an event has to be broadcasted to all Observer<T>, Notify() method
+ * is called.
+ * T represents the type of the object passed in argument to Notify().
+ *
+ * @see ObserverList.
+ */
+template <class T>
+class Observer
+{
+public:
+  virtual ~Observer() { }
+  virtual void Notify(const T& aParam) = 0;
+};
+
+/**
+ * ObserverList<T> tracks Observer<T> and can notify them when Broadcast() is
+ * called.
+ * T represents the type of the object passed in argument to Broadcast() and
+ * sent to Observer<T> objects through Notify().
+ *
+ * @see Observer.
+ */
+template <class T>
+class ObserverList
+{
+public:
+  /**
+   * Note: When calling AddObserver, it's up to the caller to make sure the
+   * object isn't going to be release as long as RemoveObserver hasn't been
+   * called.
+   *
+   * @see RemoveObserver()
+   */
+  void AddObserver(Observer<T>* aObserver) {
+    mObservers.AppendElement(aObserver);
+  }
+
+  void RemoveObserver(Observer<T>* aObserver) {
+    mObservers.RemoveElement(aObserver);
+  }
+
+  PRUint32 Length() {
+    return mObservers.Length();
+  }
+
+  void Broadcast(const T& aParam) {
+    PRUint32 size = mObservers.Length();
+    for (PRUint32 i=0; i<size; ++i) {
+      mObservers[i]->Notify(aParam);
+    }
+  }
+
+protected:
+  nsTArray<Observer<T>*> mObservers;
+};
+
+} // namespace mozilla
+
+#endif // mozilla_Observer_h