GCD定时器

@interface ViewController ()

/** 定时器(这里不用带*，因为dispatch_source_t就是个类，内部已经包含了*) */
@property (nonatomic, strong) dispatch_source_t timer;

@end

     __block CGFloat count = 0.0;
    // 获得队列
    dispatch_queue_t queue = dispatch_get_global_queue(0, 0);

    self.timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, queue);
    //延时1.0秒开始执行,每隔2调一次
    dispatch_time_t start = dispatch_time(DISPATCH_TIME_NOW, (int64_t)(1.0 * NSEC_PER_SEC));
    uint64_t interval = (uint64_t)(2 * NSEC_PER_SEC);
    dispatch_source_set_timer(self.timer, start, interval, 0);
    
    // 设置回调
    dispatch_source_set_event_handler(self.timer, ^{
        
        NSLog(@"我这里要每隔2秒调用一次");
        
        count++;
        if (count == CGFLOAT_MAX) {
            // 取消定时器
            dispatch_cancel(self.timer);
            self.timer = nil;
        }
    });
    
    // 启动定时器
    dispatch_resume(self.timer);