Implement notifiers.

The notifier thread is lazily started when the first notifier is added.
This avoids the extra thread/processing overhead when notifiers are not used.

src/Notifier.h

@@ -0,0 +1,88 @@
+/*----------------------------------------------------------------------------*/
+/* Copyright (c) FIRST 2015. All Rights Reserved.                             */
+/* Open Source Software - may be modified and shared by FRC teams. The code   */
+/* must be accompanied by the FIRST BSD license file in the root directory of */
+/* the project.                                                               */
+/*----------------------------------------------------------------------------*/
+
+#ifndef NT_NOTIFIER_H_
+#define NT_NOTIFIER_H_
+
+#include <atomic>
+#include <condition_variable>
+#include <mutex>
+#include <queue>
+#include <thread>
+#include <utility>
+#include <vector>
+
+#include "atomic_static.h"
+#include "ntcore_cpp.h"
+
+namespace nt {
+
+class Notifier {
+  friend class NotifierTest;
+ public:
+  static Notifier& GetInstance() {
+    ATOMIC_STATIC(Notifier, instance);
+    return instance;
+  }
+  ~Notifier();
+
+  void Start();
+  void Stop();
+
+  bool active() const { return m_active; }
+
+  unsigned int AddEntryListener(StringRef prefix,
+                                EntryListenerCallback callback);
+  void RemoveEntryListener(unsigned int entry_listener_uid);
+
+  void NotifyEntry(StringRef name, std::shared_ptr<Value> value, bool is_new);
+
+  unsigned int AddConnectionListener(ConnectionListenerCallback callback);
+  void RemoveConnectionListener(unsigned int conn_listener_uid);
+
+  void NotifyConnection(int connected, const ConnectionInfo& conn_info);
+
+ private:
+  Notifier();
+
+  void ThreadMain();
+
+  std::atomic_bool m_active;
+
+  std::mutex m_mutex;
+  std::condition_variable m_cond;
+  std::vector<std::pair<std::string, EntryListenerCallback>> m_entry_listeners;
+  std::vector<ConnectionListenerCallback> m_conn_listeners;
+
+  struct EntryNotification {
+    EntryNotification(StringRef name_, std::shared_ptr<Value> value_,
+                      bool is_new_)
+        : name(name_), value(value_), is_new(is_new_) {}
+
+    std::string name;
+    std::shared_ptr<Value> value;
+    bool is_new;
+  };
+  std::queue<EntryNotification> m_entry_notifications;
+
+  struct ConnectionNotification {
+    ConnectionNotification(int connected_, const ConnectionInfo& conn_info_)
+        : connected(connected_), conn_info(conn_info_) {}
+
+    int connected;
+    ConnectionInfo conn_info;
+  };
+  std::queue<ConnectionNotification> m_conn_notifications;
+
+  std::thread m_thread;
+
+  ATOMIC_STATIC_DECL(Notifier)
+};
+
+}  // namespace nt
+
+#endif  // NT_NOTIFIER_H_