ControlTextDidChange not working for setting string of NSTextField

I'm trying to find a method that monitors the text of NSTextField for changes. I tried the delegate method of -(void)controlTextDidChange:(NSNotification *)obj but it only works when the user types into the text field. If the text field string is programmatically set, such as with a button, the controlTextDidChange doesn't work.

Is there a method or another approach that I can use to monitor the contents of a NSTextField for changes?

My ButtonText class (set as delegate for the NSTextField):

#import "ButtonText.h"

@interface ButtonText ()

@property (weak) IBOutlet NSTextField *buttonField;

@end

@implementation ButtonText

- (IBAction)buttonTextA:(id)sender {
    [_buttonField setStringValue:@"text A here"];
}

- (IBAction)buttonTextB:(id)sender {
    [_buttonField setStringValue:@"and text B stuff"];
}

- (void)controlTextDidChange:(NSNotification *)obj {
    NSLog(@"controlTextDidChange: %@", _buttonField.stringValue);
}

@end

The XIB showing the buttons and text field:

One approach is to use KVO. In particular, add the ButtonText instance as an observer of buttonField's stringValue.

In more detail, in your file ButtonText, once the @property IBOutlet buttonField has been set (i.e. if ButtonText is an NSWindowController subclass, in -windowDidLoad, and if ButtonText is an NSViewController subclass in -loadView), call

[self.buttonField addObserver:self
                   forKeyPath:@"stringValue"
                      options:0
                      context:&ButtonTextKVOContext];

Define ButtonTextKVOContext previously in the file as follows:

static int ButtonTextKVOContext = 0;

Then override observeValueForKeyPath:ofObject:change:context: as follows:

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context
{
    if (context != &ButtonTextKVOContext) {
        [super observeValueForKeyPath:keyPath ofObject:object change:change context:context];
        return;
    }

    if (object == self.buttonField) {
        if ([keyPath isEqualToString:@"stringValue"]) {
            NSLog(@"controlTextDidChange: %@", _buttonField.stringValue);
        }
    }
}

Edit

Since ButtonText is not a subclass of NSWindowController or NSViewController, we'll use a slightly different approach. As before, we'll want to start observing "once the @property IBOutlet buttonField has been set". To do this, synthesize the property buttonField to be the member variable mButtonField writing

@synthesize buttonField = mButtonField;

and override buttonField's setter as follows:

- (void)setButtonField:(NSTextField *)buttonField
{
    [self stopObservingButtonField];
    mButtonField = buttonField;
    [self startObservingButtonField];
}

We need to make sure that ButtonText stops observing the button field when it deallocates as well, so override -dealloc as follows:

- (void)dealloc
{
    [self stopObservingButtonField];
}

It remains to define the methods -stopObservingButtonField and -startObservingButtonField:

- (void)stopObservingButtonField
{
    if (mButtonField) {
        [mButtonField removeObserver:self
                          forKeyPath:@"stringValue"
                             context:&ButtonTextKVOContext];
    }
}

- (void)startObservingButtonField
{
    if (mButtonField) {
        [self.buttonField addObserver:self
                           forKeyPath:@"stringValue"
                              options:0
                              context:&ButtonTextKVOContext];
    }
}

As a result of this arrangement, we must never set the mButtonField variable outside of the -setButtonField: method. (This isn't quite true, but if we do set mButtonField we must be sure to first of all stop observing its old value's @"stringValue" key path and start observing its new value's @"stringValue" key path. Doing this rather than simply calling -setButtonField: would very likely simply constitute code repetition and not be worthwhile.)

For reference, check out Apple's documentation on the NSKeyValueObserving protocol.