undo all changes made in the child view controller

问题

There are two entities: Author and Book. Author has an attribute authorName and a to-many relationships books. Book has several attributes and a relationship author. There is a view controller (VCAuthor) to edit an Author object. The child view controller(VCBook) is to edit books of the author. There is only one managedobjectcontext. In the VCBook class i group the undomanager as following

-(void)viewDidLoad
{    
    NSUndoManager *anUndoManager = [[NSUndoManager  alloc] init];
    [self.book.managedObjectContext setUndoManager:anUndoManager];
    [anUndoManager release];
    [self.book.managedObjectContext.undoManager beginUndoGrouping];
}

-(void)cancelAction:(id)sender
{
    NSLog(@"%@", self.author.authorName);
    [self.book.managedObjectContext.undoManager endUndoGrouping];
    [self.book.managedObjectContext.undoManager undoNestedGroup];
    self.book.managedObjectContext.undoManager = nil;
    NSLog(@"%@", self.author.authorName);    
    [self dismissModalViewControllerAnimated:YES];  
}

the cancelAction is linked to an cancel button on the VCBook which used to undo all the changes made in the VCBook.

Problems is here: First, in the VCAuthor, I edit the authorName in an UITextfiled authorNameTextField from Obama to Big Obama, and save it to the MOC by author.authorName = authorNameTextField.text in the - (void)viewWillDisappear:(BOOL)animated{} method. Then I went into the child view controller VCBook to edit books of the author and click the cancel button to get back to the VCAuthor. I find the authorName still be Obama, that means the expected change of the authorName has been undo. The change of the authorName is not in the undo group at all, and why could this happen? ps. of course i reloadData when i get back into VCAuthor. I just NSLog the authorName before and after the undo. Before undo the authorName is the changed one Big Obama, and after undo it become Obama

回答1:

Several things to consider. First, in a scenario like this, I would use a separate MOC instead of the undo manager. Namely, I'd do something like this (assuming ARC - you can do the mapping if necessary)...

You must have some other code providing the book to the VC through a setter, since you access it in viewDidLoad. I'd change viewDidLoad to something like this...

-(void)viewDidLoad
{
    self.managedObjectContext = [[NSManagedObjectContext alloc] init];
    self.managedObjectContext.parentContext = self.book.managedObjectContext;
    // We are in the main thread, so we can safely access the main MOC
    // Basically, move our object pointer to book into our local MOC.
    NSError * error = nil;
    Book *book = [self.managedObjectContext existingObjectWithID:self.book.objectID error:&error];
    // handle nil return and/or error
    self.book = book;
    // Now, your access to book will be in the local MOC, and any changes
    // you make to the book or book.author will all be confined to the local MOC.
}

Now, all you have to do is call

[self.managedObjectContext save:&error];

in your saveAndDismiss action. If you don't call save, none of the changes will be saved, they will all just automatically disappear.

EDIT

Note, that the above "save" just moves the object state into the parent context. So, the "main" MOC now has the changes from the "child" but none of the changes have been saved to disk yet.

来源：https://stackoverflow.com/questions/10443754/undo-all-changes-made-in-the-child-view-controller