1. 程序时，出现运行时异常，如上图所示。
2.异常原因分析。在ContentObserver的onChange方法中，调用了Toast.makeText方法。onChange方法应该在子线程运行，在android中的子线程中不能直接控制UI组件，否则就会报异常
3.Looper类别用来为一个线程开启一个消息循环。默认情况下Android中新诞生的线程是没有开启消息循环的。（主线程除外，主线程系统会自动为其创建Looper对象，开启消息循环）
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