diff -r 000000000000 -r 6474c204b198 xpcom/glue/Observer.h
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/xpcom/glue/Observer.h	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,77 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#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);
+  }
+
+  /**
+   * Remove the observer from the observer list.
+   * @return Whether the observer has been found in the list.
+   */
+  bool RemoveObserver(Observer<T>* aObserver) {
+    return mObservers.RemoveElement(aObserver);
+  }
+
+  uint32_t Length() {
+    return mObservers.Length();
+  }
+
+  void Broadcast(const T& aParam) {
+    uint32_t size = mObservers.Length();
+    for (uint32_t i=0; i<size; ++i) {
+      mObservers[i]->Notify(aParam);
+    }
+  }
+
+protected:
+  nsTArray<Observer<T>*> mObservers;
+};
+
+} // namespace mozilla
+
+#endif // mozilla_Observer_h