GCD倒计时写法

__block int timeout = 59; //倒计时时间
    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    dispatch_source_t _timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0,queue);
    dispatch_source_set_timer(_timer,dispatch_walltime(NULL, 0),1.0*NSEC_PER_SEC, 0); //每秒执行
    dispatch_source_set_event_handler(_timer, ^{
        if(timeout<=0){ //倒计时结束，关闭
            dispatch_source_cancel(_timer);
            dispatch_async(dispatch_get_main_queue(), ^{
                //设置界面的按钮显示 根据自己需求设置
                [fetchVerifyCodeButton_ setEnabled:YES];
                [fetchVerifyCodeButton_ setTitle:@"获取验证码" forState:UIControlStateNormal];
            });
        }else{
            int seconds = timeout % 60;
            NSString *strTime = [NSString stringWithFormat:@"%2d秒重新获取", seconds];
            dispatch_async(dispatch_get_main_queue(), ^{
                //设置界面的按钮显示 根据自己需求设置
                [fetchVerifyCodeButton_ setTitle:strTime forState:UIControlStateDisabled];
            });
            timeout--;
        }
    });
    dispatch_resume(_timer);

普通写法

 secondsCountDown = 60;//60秒倒计时
 countDownTimer = [NSTimer scheduledTimerWithTimeInterval:1 target:self selector:@selector(timeFireMethod) userInfo:nil repeats:YES];
-(void)timeFireMethod{
	secondsCountDown--;
	if(secondsCountDown==0){
	  [countDownTimer invalidate];
	}
}