I need to modify the contents of my text view programatically but also allow the user to undo the operation - how do you do this?

Before you modify the text view, manually push the opposite operation onto the General/NSUndoManager stack. Write a small method along the lines of this:

• (void)undoableReplaceTextviewCharactersInRange:(General/NSRange)range withString:(General/NSString *)string { General/NSRange inverseRange = General/NSMakeRange(range.location, [string length]); General/NSString *inverseString = [[textView string] substringWithRange:range]; [[undoManager prepareWithInvocationTarget:self] undoableReplaceTextviewCharactersInRange:inverseRange withString:inverseString]; [[textView textStorage] replaceCharactersInRange:range withString:string]; }