NSTimer停止但无法转到下一页

Question

我正在使用NSTimer呼叫设备每24.5秒拍摄一次照片。

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
dispatch_async( dispatch_get_main_queue(), ^{
    _photoTimer = [NSTimer scheduledTimerWithTimeInterval:24.5 target:self selector:@selector(takePhotos) userInfo:nil repeats:YES];     
});});

在第11次，我切换到主队列并调用NSTimer停止，但它不会推送到下一页。

-(void)takePhotos{
  dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
   //taking photos....
  currenPageNumber +=1;

dispatch_async( dispatch_get_main_queue(), ^{
     //showing photos
 });
dispatch_async( dispatch_get_main_queue(), ^{
    if (currentCount == 11){
         [_photoTimer invalidate];
         _photoTimer = nil;
         UploadVC * uploadVC = [self.storyboard instantiateViewControllerWithIdentifier:@"UploadVC"];
         [self.navigationController presentViewController:uploadVC animated:YES completion:nil];
    }     
   });
   });
 }