ios 倒计时

1、倒计时一般的三种方法

• NSTimer

[NSTimer scheduledTimerWithTimeInterval:1.0 target:self selector:@selector(handleTimer) userInfo:nil repeats:YES];

• Thread方式实现

#pragma mark 线程Thread方式实现
- (IBAction)firstBtnAction:(id)sender {
    //创建一个后台线程执行计时操作
    [self performSelectorInBackground:@selector(timerThread) withObject:nil];
}

- (void)timerThread {
    for (int i = TIMECOUNT; i >= 0 ; i--) {
        self.count--;
        //切换到主线程中更新UI
        [self performSelectorOnMainThread:@selector(updateFirstBtn) withObject:nil waitUntilDone:YES];
        sleep(1);
    }
}

• GCD

/**
 *  1、获取或者创建一个队列，一般情况下获取一个全局的队列
 *  2、创建一个定时器模式的事件源
 *  3、设置定时器的响应间隔
 *  4、设置定时器事件源的响应回调，当定时事件发生时，执行此回调
 *  5、启动定时器事件
 *  6、取消定时器dispatch源，【必须】
 *
 */
#pragma mark GCD实现
- (IBAction)thirdBtnAction:(id)sender {
    __block NSInteger second = TIMECOUNT;
    //(1)
    dispatch_queue_t quene = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    //(2)
    dispatch_source_t timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, quene);
    //(3)
    dispatch_source_set_timer(timer, DISPATCH_TIME_NOW, 1 * NSEC_PER_SEC, 0 * NSEC_PER_SEC);
    //(4)
    dispatch_source_set_event_handler(timer, ^{
        dispatch_async(dispatch_get_main_queue(), ^{
            if (second == 0) {
                self.thirdBtn.userInteractionEnabled = YES;
                [self.thirdBtn setTitle:[NSString stringWithFormat:@"点击获取验证码"] forState:UIControlStateNormal];
                second = TIMECOUNT;
                //(6)
                dispatch_cancel(timer);
            } else {
                self.thirdBtn.userInteractionEnabled = NO;
                [self.thirdBtn setTitle:[NSString stringWithFormat:@"%ld秒后重新获取",second] forState:UIControlStateNormal];
                second--;
            }
        });
    });
    //(5)
    dispatch_resume(timer);
}

2、cell中多个倒计时的情况

用NSTimer，一定有[[NSRunloop mainloop] addTimer:_timer forMode:NSRunloopCommonModes]这句，这样就避免了滑动tableView的时候倒计时停止的情况。

参考文章：
https://www.jianshu.com/p/af62a56ef7e2
http://www.cocoachina.com/ios/20161011/17722.html