使用GCD实现简单循环

//设置时间间隔（每五秒）
dispatch_source_t _timer;
NSTimeInterval period = 5.f;
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
_timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, queue);
dispatch_source_set_timer(_timer, dispatch_walltime(NULL, 0), period * NSEC_PER_SEC, 0);

// 事件回调
dispatch_source_set_event_handler(_timer, ^{
dispatch_async(dispatch_get_main_queue(),^{

[pjm mosCalculation];  //每五秒计算一次mos值

});
});

// 开启定时器
dispatch_resume(_timer);
// 关闭定时器
// dispatch_source_cancel(_timer);