最后发现该通知实际上仅出现在android下拉菜单中，而不是由于我的设备运行的是Android 4而弹出. 5设备，并且通知会像本教程一样弹出.

Finally found that the notification were actually only appeared on the android drop down menu instead of popping up due to my device is running Android 4. I test again with Android 5 device and the notification pop up just like the tutorial.