Make Observers into a templated class

core/fxcrt/include/cfx_observable.h

Index: core/fxcrt/include/cfx_observable.h
diff --git a/core/fxcrt/include/cfx_observable.h b/core/fxcrt/include/cfx_observable.h
new file mode 100644
index 0000000000000000000000000000000000000000..82514300b2236c7ac5b319e0388353b8d3e56850
--- /dev/null
+++ b/core/fxcrt/include/cfx_observable.h
@@ -0,0 +1,55 @@
+// Copyright 2016 PDFium 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 CORE_FXCRT_INCLUDE_CFX_OBSERVABLE_H_
+#define CORE_FXCRT_INCLUDE_CFX_OBSERVABLE_H_
+
+#include <set>
+
+#include "core/fxcrt/include/fx_system.h"
+
+template <class T>
+class CFX_Observable {
+ public:
+  class Observer {
+   public:
+    Observer() : m_pWatchedPtr(nullptr) {}
+    Observer(T** pWatchedPtr) : m_pWatchedPtr(pWatchedPtr) {}
+    Observer(const Observer& that) = delete;
+    ~Observer() {
+      if (m_pWatchedPtr)
+        (*m_pWatchedPtr)->RemoveObserver(this);
+    }
+    void SetWatchedPtr(T** pWatchedPtr) {
+      if (m_pWatchedPtr)
+        (*m_pWatchedPtr)->RemoveObserver(this);
+      m_pWatchedPtr = pWatchedPtr;
+      if (m_pWatchedPtr)
+        (*m_pWatchedPtr)->AddObserver(this);
+    }
+    void OnDestroy() {
+      if (m_pWatchedPtr) {
+        *m_pWatchedPtr = nullptr;
+        m_pWatchedPtr = nullptr;
+      }
+    }
+
+   private:
+    T** m_pWatchedPtr;
+  };
+
+  CFX_Observable() {}
+  CFX_Observable(const CFX_Observable& that) = delete;
+  ~CFX_Observable() {
+    for (auto* pObserver : m_Observers)
+      pObserver->OnDestroy();
+  }
+  void AddObserver(Observer* pObserver) { m_Observers.insert(pObserver); }
+  void RemoveObserver(Observer* pObserver) { m_Observers.erase(pObserver); }
+
+ private:
+  std::set<Observer*> m_Observers;
+};
+
+#endif  // CORE_FXCRT_INCLUDE_CFX_OBSERVABLE_H_