binding NSTextField to NSNumber

Question

I'm trying to use an NSTextField for integer user input. The text field is bound to an NSNumber property and in the setter method I cleanup the input value (make sure it's an int) and set the property if necessary. I send the willChangeValueForKey: and didChangeValueForKey:, but the UI doesn't update to the new values while that text field is still active.

So for example I can type "12abc" in the text field that the setter method cleans up to "12", but the text field still shows "12abc".

I have "Continuously Update Value" checked in the interface builder.

(I've also noticed that the setter method receives an NSString, not an NSNumber. Is that normal?)

What's the correct way of hooking up an NSTextField to an NSNumber? What does the setter method look like for the property? How to prevent non-numeric values from showing up in the text field?

Answer 1

I send the willChangeValueForKey: and didChangeValueForKey:, but the UI doesn't update to the new values while that text field is still active.

There are very few reasons to send those messages. Usually, you can do the same job better and more cleanly by implementing and using accessors (or, better yet, properties). KVO will send the notifications for you when you do that.

In your case, you want to either reject or filter bogus inputs (like “12abc”). The correct tool for this task is Key-Value Validation.

To enable this, check the “Validates Immediately” box on the binding in IB, and implement a validation method.

Filtering:

- (BOOL) validateMyValue:(inout NSString **)newValue error:(out NSError **)outError {
    NSString *salvagedNumericPart;
    //Determine whether you can salvage a numeric part from the string; in your example, that would be “12”, chopping off the “abc”.
    *newValue = salvagedNumericPart; //@"12"
    return (salvagedNumericPart != nil);
}

Rejecting:

- (BOOL) validateMyValue:(inout NSString **)newValue error:(out NSError **)outError {
    BOOL isEntirelyNumeric;
    //Determine whether the whole string (perhaps after stripping whitespace) is a number. If not, reject it outright.
    if (isEntirelyNumeric) {
        //The input was @"12", or it was @" 12 " or something and you stripped the whitespace from it, so *newValue is @"12".
        return YES;
    } else {
        if (outError) {
            *outError = [NSError errorWithDomain:NSCocoaErrorDomain code: NSKeyValueValidationError userInfo:nil];
        }
        //Note: No need to set *newValue here.
        return NO;
    }
}

(I've also noticed that the setter method receives an NSString, not an NSNumber. Is that normal?)

Yes, unless you use a value transformer that transforms strings into numbers, connect a number formatter to the formatter outlet, or substitute an NSNumber for the NSString in your validation method.

Answer 2

There’s an important comment on Peter Hosey’s excellent answer that I’d like to bring up to the top level (because I missed it on my first pass).

If you want to validate / modify the NSTextField each time a character is entered, rather than only when the user submits the field, then you can’t get what you want from bindings alone. You need to assign a delegate to the text field, and then implement - (void)controlTextDidChange:(NSNotification *)aNotification in the delegate. This will get called every time the text changes. If you want, you can call the value validator in controlTextDidChange.

For example:

- (void)controlTextDidChange:(NSNotification *)aNotification
{
    NSError *outError;
    NSControl *textField = [aNotification object];
    NSString *myText = [textField stringValue];

    // myObject is the model object that owns the text in question
    // the validator can modify myText by reference
    [myObject validateValue:&myText error:&outError]];

    // update the NSNextField with the validated text
    [postingObject setStringValue:myText];
}