使用GCD实现轮询

static dispatch_source_t _timer;
_timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0)); 
dispatch_source_set_timer(_timer, dispatch_walltime(NULL, 0), 2 * NSEC_PER_SEC, 0); //每秒执行

dispatch_source_set_event_handler(_timer, ^{
        dispatch_async(dispatch_get_main_queue(), ^{
            //需要轮询的内容
        });
});

// 开启定时器
dispatch_resume(_timer);
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(60 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
        // 轮询超时
        dispatch_cancel(_timer);
});