Hi Barış,
You can achieve the desired functionality you can use UndoableDelegateCommand and the diagram's UndoRedoService to execute it. The command allows you to execute an action and its undo logic. You can execute the command when you call the AutoFit() method and in the PreviewZoom event handler. Here is an example for this approach:
I hope this works for you.
Regards,
Martin
Telerik
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