baseimage-tutorial-framework/lib/tfw/components/history_monitor.py

from watchdog.observers import Observer
from watchdog.events import PatternMatchingEventHandler
from os.path import dirname


class CallbackEventHandler(PatternMatchingEventHandler):
    def __init__(self, files, *callbacks):
        super().__init__(files)
        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 = []
        self._last_length = len(self._history)
        self._callbacks = []
        self.observer = Observer()
        self.observer.schedule(CallbackEventHandler([self.histfile],
                                                    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()

    def stop(self):
        self.observer.stop()
        self.observer.join()