倒计时

   self.btn_yanzhengma.enabled = NO;
    __block int timeout=10; //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
                
            });
        }else{
            int minutes = timeout / 60;
            int seconds = timeout % 60;
            NSString *strTime = [NSString stringWithFormat:@"%.d秒后重新获取验证码",seconds];
            dispatch_async(dispatch_get_main_queue(), ^{
                //设置界面的按钮显示 根据自己需求设置
                [self.btn_yanzhengma setTitle:strTime forState:UIControlStateNormal];
                self.btn_yanzhengma.enabled = YES;
            });
            timeout--;
            
        }  
    });  
    dispatch_resume(_timer);

 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];
	}
}