Implement callback subscription logic in HistoryMonitor

lib/tfw/components/history_monitor.py

@@ -17,17 +17,31 @@ class HistoryMonitor:
     def __init__(self, histfile):
         self.histfile = histfile
         self._history = []
+        self._last_length = len(self._history)
+        self._callbacks = []
         self.observer = Observer()
-        self.observer.schedule(CallbackEventHandler(self._fetch_history), dirname(self.histfile))
+        self.observer.schedule(CallbackEventHandler(self._fetch_history,
+                                                    self._invoke_callbacks),
+                               dirname(self.histfile))
 
     @property
     def history(self):
         return self._history
 
+    @property
+    def callbacks(self):
+        return self._callbacks
+
     def _fetch_history(self):
+        self._last_length = len(self._history)
         with open(self.histfile, 'r') as ifile:
             self._history = [line.rstrip() for line in ifile.readlines()]
 
+    def _invoke_callbacks(self):
+        if self._last_length < len(self._history):
+            for callback in self.callbacks:
+                callback(self.history)
+
     def watch(self):
         self.observer.start()