一段倒计时的代码

  __block int timeout=60; //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                [_buttonResend setTitle:@"重发验证码" forState:UIControlStateNormal];                _buttonResend.userInteractionEnabled = YES;                _buttonResend.enabled = YES;            });        }else{            //            int minutes = timeout / 60;            int seconds = timeout % 60;            __block NSString *strTime = [NSString stringWithFormat:@"%.2d", seconds];            dispatch_async(dispatch_get_main_queue(), ^{                //设置界面的按钮显示 根据自己需求设置                DYYLog(@"__ForetPassWordViewController__%@",strTime);                if([strTime isEqualToString:@"00"])                {                    strTime = @"60";                }                [_buttonResend setTitle:[NSString stringWithFormat:@"%@秒后重新发送",strTime] forState:UIControlStateDisabled];                _buttonResend.enabled = NO;                _buttonResend.userInteractionEnabled = NO;            });            timeout--;        }    });    dispatch_resume(_timer);