1.第一种 每一秒执行一次（重复性）
double delayInSeconds = 1.0;
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_TIME_NOW, delayInSeconds * NSEC_PER_SEC, 0.0);
dispatch_source_set_event_handler(timer, ^{
NSLog(@"timer date 1== %@",[NSDate date]);
});
dispatch_resume(timer);

2.第二种 二秒后执行 （一次性）
double delayInSeconds = 2.0;
dispatch_time_t popTime = dispatch_time(DISPATCH_TIME_NOW, (int64_t)(delayInSeconds * NSEC_PER_SEC));
dispatch_after(popTime, dispatch_get_main_queue(), ^(void){
NSLog(@"timer date 2== %@",[NSDate date]);
});

3.第三种 每一秒执行一次 （重复性）
[NSTimer scheduledTimerWithTimeInterval:1 target:self selector:@selector(testTimer) userInfo:nil repeats:YES];