转为后台运行的程序照样可以获得系统事件、读写数据、访问系统服务、改变自己的界面，只不过比当前显示在主界面的程序而言，运行优先级低了一点。

Background applications can get events, read and write data, access services, repaint themselves and are generally not restricted other than to run at a lower priority than the foreground application.

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