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diff --git a/xpcom/ds/Observer.h b/xpcom/ds/Observer.h
new file mode 100644
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+++ b/xpcom/ds/Observer.h
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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 "nsTObserverArray.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() = default;
+  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.AppendElementUnlessExists(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(); }
+
+  /**
+   * Call Notify() on each item in the list.
+   */
+  void Broadcast(const T& aParam) {
+    for (Observer<T>* obs : mObservers.ForwardRange()) {
+      obs->Notify(aParam);
+    }
+  }
+
+ protected:
+  nsTObserverArray<Observer<T>*> mObservers;
+};
+
+}  // namespace mozilla
+
+#endif  // mozilla_Observer_h