iOS实现手机获取验证码倒计时效果

//创建一个全局并非队列  dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);   //创建一个定时器    _timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, queue);   //配置定时器，每秒执行   dispatch_source_set_timer(_timer, dispatch_walltime(NULL, 0), 1.0 * NSEC_PER_SEC, 0);   //创建事件处理器   dispatch_source_set_event_handler(_timer, ^{    timeout --;     if (timeout <= 0) {       //定时器结束，关闭        dispatch_source_cancel(_timer);       _timer = nil;       //回到主线程刷新UI       dispatch_async(dispatch_get_main_queue(), ^{       [_btn setTitle:@"获取验证码" forState:UIControlStateNormal];       });     }    else {  //回到主线程刷新UI   dispatch_async(dispatch_get_main_queue(), ^{     [_btn setTitle:[NSString stringWithFormat:@"%ds后重新获取",timeout] forState:UIControlStateNormal];     });     }    });  //启动定时器dispatch_resume(_timer);