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The General/AppKit comes with undo support in the text system, but unfortunatly this support is provided by the General/NSTextView class, not the General/NSTextStorage class. Adding undo support to General/NSTextStorage would make it easier to support text undo in a number of cases… problem is the implementation.

Here is what I have so far. Seems to work for me on paper, but I’m doing something wrong and corupting the undo stack. The basic idea here is to commit partial edits (ones that might be merged with later edits) to a separate General/HBTextStorageEdit object. This is done so that I have a “handle” on these edits and can remove them from the undo stack when they need to be merged with later edits. When a situation arrises where the edits cannot be merged they are commited to the undo stack. By this “commit” I mean that the edits are placed on the undo stack with the General/NSTextStorage as the target, they won’t be removed or merged with other edits after this commit.

So what do I need to do to get this thing working?

@interface General/HBTextStorage : General/NSTextStorage { General/NSMutableAttributedString *_textContents; General/HBTextStorageEdit *_editInProgress; }


@implementation General/HBTextStorage


@interface General/HBTextStorageEdit : General/NSObject { General/HBTextStorage *_target; General/NSRange _editRange; General/NSString *_replacedString; }


@implementation General/HBTextStorageEdit

@end (wrapped the code so that it won’t force hugely wide paragraphs on this page)

[[JesseGrosjean software(at)hogbay(dot)com

You want to be looking into

There look for General/UndoGroups and be happy. :-)

I would (rather that looking at the previous event on the stack) determine which operations should start a new General/UndoGroup and then appropriately end the current General/UndoGroup.

Just a thought,


That’s definitely the way to do it. – General/KritTer

If using the General/UndoGroup approach would I need to turn of setGroupsByEvent:? I would be a little worried about this approach (if i understand it correctly) since every char insert/delete would need to be placed on the undo stack. I’d like to have infinite undo and if you type for a few hours this could get expensive??? Hit me over the head with the don’t optimize it stupid until you know its slow if appropriate.

My current approach (I?ll post some code tonight, but it’s not quite working) is to have a new object called General/TextEditInProgress. For edits to the General/NSTextStorage that can be merged (contiguous single char insert/delete in my implementation) the undo target is set to be this General/TextEditInProgress object. For each new replaceCharactersInRange I

  1. Remove the current General/TextEditInProgress from the undo stack using removeTarget:
  2. See if the new change can be merged with the current General/TextEditInProgress
    • if yes merge changes and place General/TextEditInProgress back on undo stack. This means that the General/NSTextStorage is not the target of the undo, the General/TextEditInProgress is.
    • if no commit changes in General/TextEditInProgress to undo stack. This means put those changes on stack with General/NSTextStorage as target… so General/TextEditInProgress is no longer on the undo stack.
  3. if appropriate for the current change (if it is a single char insert/delete) then creat a new General/TextEditInProgress and put it on undo stack. Else commit the inverse change to the undo stack, General/NSTextStorage as target.

  4. Apply change to the General/NSTextStorages backing General/NSMutableAttributedString.

Ok, that was probably very unclear, but maybe it gives you an idea of my current approach. The big point is it should avoid putting an invocation on the undo stack for each character typed, but it’s not working yet, and who knows maybe it has some fundamental flaw that someone can point out to me.

Thanks for your comments, General/JesseGrosjean

Ignoring the undo issue for a moment, I would like to remind our readership that the custom text storage subclass listed above is an excellent starting point for more sophisticated auto-formatting / auto-completion projects that tend to come up at the same tim as syntax coloring. See also General/ImplementSyntaxHighlighting. – General/MikeTrent

I’ve added my current code to the top of this page.

This may be orthogonal to your needs, but I think one reason that people want undo in General/NSTextStorage is because they need to programmatically insert or change text and have it be able to be undone. I was struggling with the same problem, but then I found that the easiest solution was to keep the modification in General/NSTextView and just notify it to record the changes. Here’s my code – I do everything with attributed strings, so I added “-insertAttributedText:” and “-insertText:withAttributes:”. To change/insert text I simply set the selection and call one of these methods.

@implementation General/MyTextView

-(void)insertAttributedText:(General/NSAttributedString*)astring { General/NSRange range = [self selectedRange]; General/NSString *insertingText = [astring string]; General/NSString *selectedText = General/self string] substringWithRange:range]; [[NSString *stringForDelegate = insertingText;

// If only attributes are changing, pass nil.
if ([insertingText isEqualToString:selectedText])
    stringForDelegate = nil;

// Call delegate methods to force undo recording
if ([self shouldChangeTextInRange:range
    General/self textStorage] replaceCharactersInRange:range
    [self setSelectedRange:
        [[NSMakeRange(range.location+[astring length], 0)];
    [self didChangeText];
} }

-(void)insertText:(General/NSString)string withAttributes:(General/NSDictionary)attr { General/NSAttributedString *astring = General/[[NSAttributedString alloc] initWithString:string attributes:attr]; [self insertAttributedText:astring]; [astring release]; }


The calls to -shouldChangeTextInRange:replacementString: and -didChangeText are sufficient to get the General/NSTextView to record the undo. Like I said, this may not be what you actually wanted :-) but it solved my problem. –General/DrewThaler

Though very rare, there are cases where there is no way around a custom implementation. One example is some structure in the text that covers deeper semantics and can not be represented by a plain attributed string without loosing that information at the same time. Thanks for the headstart, Jesse. –General/MaxSeelemann