NSTextField with NSFormatter results in broken continuous binding - macos

Question

I have a textfield which has to be unique so I added my custom NSFormatter (see below)
The formatter works, as you can see on the screenshot, but the continuous binding, which I am using is broken, so for example the bound text does no longer get updated in real-time.
I found a possible cause here, but I don't know how to work around this problem and re-enable the continuous binding:
...
12. If the view has an NSFormatter attached to it, the value is
formatted by the NSFormatter instance. Proceed to Step 17.
...
17. The updated value is displayed in the user interface.
So it looks like it's intentionally skipping the steps we want. This
is very annoying. I tried NSValueTransformer, but adding that to an
editable NSTextField makes it non-editable.
My formatter
- (BOOL)getObjectValue:(out id *)obj forString:(NSString *)string errorDescription:(out NSString **)error {
if([string isNotEqualTo:#"todo-invalid-value"]){
*obj = string;
NSLog(#"YES");
return YES;
} else {
if(error){
*error = #"ERROR: not allowed";
}
return NO;
}
}
- (NSString *)stringForObjectValue:(id)obj {
return (NSString *)obj;
}
Working validation
Please note that the title of the list item should be updated with the text, that I entered in the textfield.

Answer 1

I ran into the same problem over the weekend, and eventually discovered a post from 2008 by Yann Disser on the cocoa-dev mailing list which shed some light on my problem.
I had an existing NSFormatter that was working fine and when I broke down the components, so I spent a little more time on it this morning and located Yann's post.
The key is that you need to return a different object than the one that is passed in. It's subtle, but the docs say: If conversion is successful, upon return contains the object created from string.
The problem I was having stemmed from the fact that the NSString that was coming in was actually an NSMutableString and was getting modified later.
Here's the code modified to return [NSString stringWithString: string], which should fix your problem:
- (BOOL)getObjectValue:(out id *)obj forString:(NSString *)string errorDescription:(out NSString **)error {
if([string isNotEqualTo:#"todo-invalid-value"]){
*obj = [NSString stringWithString: string];
NSLog(#"YES");
return YES;
} else {
if(error){
*error = #"ERROR: not allowed";
}
return NO;
}
}