iPhone应用程序中的例外情况：模态转换已在进行中

基于拼凑在一起的一些教程,我认为目前这是一个相当简单的应用程序.我在OSX 10.6.4中使用XCode 3.2.3.它起初是一个标准的iPhone“基于窗口的应用程序”.使用界面构建器我在这里使用O’Reilly视频教程添加了一个Tab Bar Controller：

http://broadcast.oreilly.com/2009/06/tab-bars-and-navigation-bars-t.html

在第一个标签中,我有一个带有两个按钮的标准UIView.两者都调用相同的函数来显示UIImagePickerController：

-(IBAction) btnPhotoClicked:(id)sender {
UIImagePickerController *imagePicker = [[UIImagePickerController alloc] init];
imagePicker.delegate = self;
if((UIButton *)sender == btnChoosePhoto)
{
    imagePicker.allowsEditing = YES;
    imagePicker.sourceType = UIImagePickerControllerSourceTypeSavedPhotosAlbum;
} else {
    imagePicker.sourceType = UIImagePickerControllerSourceTypeCamera;
}

[self presentModalViewController:imagePicker animated:YES];
[imagePicker release];
}

我在模拟器中运行代码,所以只需单击“选择照片”按钮.当对话框与所选照片一起发布时,此功能运行：

-(void)imagePickerController:(UIImagePickerController *)picker didFinishPickingMediaWithInfo:(NSDictionary  *)info {
NSURL *mediaUrl;

mediaUrl = (NSURL *)[info valueForKey:UIImagePickerControllerMediaURL];

if (mediaUrl == nil)
{
    imagePuzzle = (UIImage *) [info valueForKey:UIImagePickerControllerEditedImage];
    if(imagePuzzle == nil)
    {
        //--- Original Image was selected ---
        imagePuzzle = (UIImage *) [info valueForKey:UIImagePickerControllerOriginalImage];
    }
    else {
        //--- Get the edited image ---
        //--- If it was successful the above valueForKey:UIImagePickerControllerEditedImage
        //--- would have assigned it already.
    }
}
else {
    //--- Muppet selected a video
}

// Animate the picker window going away
[picker dismissModalViewControllerAnimated:YES];
ImageViewController *imageViewController = [[ImageViewController alloc] init];
imageViewController.delegate = self;    
[self presentModalViewController:imageViewController animated:YES];
[imageViewController release];
}

这就是我的问题所在.我尝试了很多不同的黑客和迭代,但上面的代码是最简单的问题.当imageViewController显示为模式对话时,抛出以下异常：

2010-07-09 15:29:29.667 Golovomka[15183:207] *** Terminating app due to uncaught
exception 'NSInternalInconsistencyException',reason: 'Attempting to begin a modal
transition from <NewViewController: 0x5915f80> to <ImageViewController: 0x594a350>     
while a transition is already in progress. Wait for viewDidAppear/viewDidDisappear
to know the current transition has completed'

我该如何解决这个问题？我尝试过延迟和其他技巧,但是我真的不明白我应该如何使用viewDidAppear或viewDidDisappear来帮助我.另外值得注意的是,一个非常基本的应用程序,其中一个视图加载选择器,然后显示另一个带有图像的视图,不会产生错误.任何帮助感激不尽.

-(void)viewDidAppear:(BOOL)animated
{
    if (/*just visited ImagePicker*/)
    {
        ImageViewController *imageViewController = [[ImageViewController alloc] init];
        imageViewController.delegate = self;    
        [self presentModalViewController:imageViewController animated:YES];
        [imageViewController release];
    }
}

[picker dismissModalViewControllerAnimated:YES];