iOS GCD 定时器

dispatch_queue_t queue = dispatch_queue_create("serial", DISPATCH_QUEUE_SERIAL);
    
    self.timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, queue);
    
    dispatch_time_t start = dispatch_time(DISPATCH_TIME_NOW, 0);
    
    uint64_t interval = (uint64_t)(1.0 * NSEC_PER_SEC);
    
    dispatch_source_set_timer(self.timer, start, interval, 0);
    
    dispatch_source_set_event_handler(self.timer, ^{
       
        static int i = 0;
        i++;
        NSLog(@"currentThread is %@", [NSThread currentThread]);
        NSLog(@"i is %d", i);
        
        if (i == 4) {
            dispatch_cancel(self.timer);
            self.timer = nil;
        }
        
    });
    
    dispatch_resume(self.timer);