- (IBAction)sendMes:(UIButton *)sender {  __block int timeout = 10 ; //倒計時時間  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); //每秒執行  dispatch_source_set_event_handler(_timer, ^{ //每秒鐘回調的方法 跟NSTimer 一樣的的執行方法   if(timeout<=0){ //倒計時結束，關閉    dispatch_source_cancel(_timer);        dispatch_async(dispatch_get_main_queue(), ^{     //設置界面的按鈕顯示     [sender setTitle:@"發送驗證碼" forState:UIControlStateNormal];     sender.enabled = YES;    });       }else{    int seconds = timeout % 60;    NSString *strTime = [NSString stringWithFormat:@"%d", seconds];    dispatch_async(dispatch_get_main_queue(), ^{     NSLog(@"----%@", [NSThread currentThread]);     //設置界面的按鈕顯示     [sender setTitle:[NSString stringWithFormat:@"(%@)重新獲取",strTime] forState:UIControlStateDisabled];     sender.enabled = NO;    });    timeout--;   }  });  //一定要調用resume 開啟定時器的功能 }