How can I undo a TextBox's text changes caused by a binding?

Question

I have a TextBox to which i bound a string, if i now edit the text manually i will be able to undo those changes via TextBox.Undo(), if however i change the string and the TextBox's text is updated, i cannot undo those changes and the TextBox.CanUndo property will always be false.
I suppose this might have to do with the complete replacement of the text rather than a modification of it.

Any ideas on how i can get this to work?

Solution 1

OK, started to leave a comment and realized it was an answer :)

TextBox.Undo() is intended to undo a user's interaction with the text box not a value change in the property it's bound to. A change in the property the text box is bound to will just update the value of the TextBox, this is a different change than a user edit via focus/keyboard. If you need to Undo changes to your bound properties you probably need to investigate adding an Undo/Redo stack to your application.

Solution 2

I was facing the same issue (needed to accept input upon Enter and revert to original value upon Escape) and was able to handle it this way:

1. Set UpdateSourceTrigger of your TextBox.Text binding to Explicit.

2. Handle KeyDown event of your TextBox and put the following code in there:

   if (e.Key == Key.Enter || e.Key == Key.Escape)
   {
     BindingExpression be = ((TextBox)sender).GetBindingExpression(TextBox.TextProperty);
   
     if (e.Key == Key.Enter)
     {
       if (be != null) be.UpdateSource();
     }
     else if (e.Key == Key.Escape)
     {
       if (be != null) be.UpdateTarget(); //cancels newly supplied value and reverts to the original value
     }
   }
   

I found this solution to be very elegant because it can be used in DataTemplates too. For example in my case I used it to allow in-place editing of ListBox items.

Solution 3

Assign directly to the TextBox:

textBox.SelectAll();
textBox.SelectedText = newText;

Solution 4

The TextBox will apply the changes to the internal undo stack if they are applied in such a way that they appear to have come from the user, like so:

        Clipboard.SetText("NewTextHere");
        TextBox.Paste();

It's a terrible workaround, as it kills whatever the user has on the clipboard (the restoring of which is pessimistically discussed here: How do I backup and restore the system clipboard in C#?) but I thought it might be worth having posted nonetheless.

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Comments

[Paul Wheeler]

Have you tried Mode=TwoWay in your binding string?

[Paul Wheeler]

Never mind, TwoWay binding didn't work...

[H.B.]

I suspected that there would not be a simple solution, then again, it was not really a good idea to try and handle undoing by reverting a change that occurred in a control, rather than the model. Thanks.

[H.B.]

I think you misunderstood the question, this is about the undo functionality, i know how to overwrite existing text. There is no newText, i was trying to restore an old value that should have been stored in the TextBox's history.

[H.B.]

An interesting idea at the very least. Thanks for your contribution.

[Darkhydro]

@H.B. This works great! Doesn't overwrite the clipboard, and allows you to undo text assignments that weren't made by the user! Thanks Pixar! H.B. I would consider rewarding Pixar with the answer, since this does exactly what you asked for in the question.

[H.B.]

@Darkhydro: No, unfortunately the question is about text changes caused by bindings, i.e. the property that the TextBox.Text is bound to is changed, no interaction with the TextBox itself.

[Mike Marynowski]

This can be easily expanded to do what you want with an attached property, so you can do something like UserText.Value="{Binding Xyz}" which updates the text using the method in this answer when it changes instead of using the Text property directly. I know this answer is old, but might be of interest to someone coming by it now.

[H.B.]

This does not cover the case where immediate value propagation is desired. You probably would want to save on lost focus as well. Still, it might be a decent work around in some cases.