python中可以使用watchdog来监控目录文件的变化，但是如何停止监控呢？

下边这段代码可以点击按钮可以开始监控D:\HtmlTemp目录下文件的变化，但是为啥点程序就无法操作了？如何才能实现随时可以停止监控的功能呢？

# _*_ coding:UTF-8 _*_
from PySide2.QtWidgets import QApplication,QMessageBox
from PySide2.QtUiTools import QUiLoader
from pynput.mouse import Button, Controller
from watchdog.observers import Observer
from watchdog.events import *
import time
import os
import shutil

class Zsfz:
    def __init__(self):
        self.ui=QUiLoader().load('ui/testfilemonitor.ui')
        self.ui.btnstart.clicked.connect(self.start)

    def start(self):
        class MyHandler(FileSystemEventHandler):
            def on_modified(self, event):
                print("文件被修改了 %s" % event.src_path)

            def on_created(self, event):
                print("文件被创建了 %s" % event.src_path)

        a = r"D:\HtmlTemp"
        path = a
        event_handler = MyHandler()
        observer = Observer()
        observer.schedule(event_handler, path, recursive=True)
        observer.start()

        try:
            while True:
                time.sleep(1)

        except KeyboardInterrupt:
            observer.stop()
        observer.join()
app=QApplication([])
zsfz=Zsfz()
zsfz.ui.show()
app.exec_()