This project forked from firewolf-ljw/backgroundtask
swift demo：实现iOS应用在后台无限长时间运行
思路：
使用UIApplication对象的beginBackgroundTaskWithExpirationHandler申请后台执行任务，该任务只有大概3分钟的运行时间
应用申请到后台执行任务后，使用NSTimer开启一个定时任务，主要负责监控应用剩余的后台可执行时间，当可用的时间少于一个值时，播放一段默声音乐，然后调用UIApplication对象的endBackgroundTask方法将之前申请的后台执行任务结束掉，最后再重新申请一个后台执行任务，这样就可以实现后台不限时执行任务了
应用在后台播放音乐，需要开启Background Modes，然后勾选Audio and AirPlay即可
注：应用在后台运行的过程中重新申请后台执行任务是无效的，通过在网上查找资料，播放音乐可以让应用进入到一个假前台的状态，此时重新申请后台执行任务是有效的，如此循环n次，就可以获得大约3n的后台执行时间，从而达到后台无限时执行任务
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