iOS 验证码倒计时

__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
                        
                        /*
                          例如：
                          [button setTitle:titleName forState:0];
                          button.userInteractionEnabled = YES;
                        */
                    });
                }else{
                    int seconds = timeout % 60;
                    NSString *time = [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
                        
                        /*
                          例如：
                          [UIView beginAnimations:nil context:nil];
                        [UIView setAnimationDuration:1];
                        [button setTitle:[NSString stringWithFormat:@"%@s",time] forState:0];
                        [UIView commitAnimations];
                        button.userInteractionEnabled = NO;
                        */
                    });
                    timeout--;
                }
            });
            dispatch_resume(_timer);

把上面的代码段直接放到请求成功的回调就好