Start work on HistoryMonitor to read command history

lib/tfw/components/history_monitor.py

@@ -0,0 +1,37 @@
+from watchdog.observers import Observer
+from watchdog.events import FileSystemEventHandler
+from collections import deque
+from os.path import dirname
+
+
+class CallbackEventHandler(FileSystemEventHandler):
+    def __init__(self, *callbacks):
+        super().__init__()
+        self.callbacks = callbacks
+
+    def on_modified(self, event):
+        for callback in self.callbacks:
+            callback()
+
+
+class HistoryMonitor:
+    def __init__(self, histfile):
+        self.histfile = histfile
+        self._history = deque()
+        self.observer = Observer()
+        self.observer.schedule(CallbackEventHandler(self._fetch_history), dirname(self.histfile))
+
+    @property
+    def history(self):
+        return self._history
+
+    def _fetch_history(self):
+        with open(self.histfile, 'r') as ifile:
+            self._history = deque(ifile.readlines())
+
+    def watch(self):
+        self.observer.start()
+
+    def stop(self):
+        self.observer.stop()
+        self.observer.join()