How to observe NSScroller changes?

Question

I have an NSScrollView subclass and I would like to update another NSView based on the current scroll position. I tried KVC-observing the value of [self horizontalScroller] but that never gets called.

// In awakeFromNib
[[self horizontalScroller] addObserver:self
                            forKeyPath:@"value"
                               options:NSKeyValueObservingOptionNew
                               context:NULL];

// Later in the file
- (void)observeValueForKeyPath:(NSString *)keyPath 
                  ofObject:(id)object 
                    change:(NSDictionary *)change 
                   context:(void *)context {
    if (object == [self horizontalScroller] && [keyPath isEqualToString:@"value"]) {
        // This never gets called
    }
}

Do you see an error in my reasoning or know a better method of how to observe the scrolling of an NSScrollview?

Answer 1

Tell the scroll view's content view to post bounds changed notifications, then listen for NSViewBoundsDidChangeNotification.

[[aScrollView contentView] setPostsBoundsChangedNotifications:YES];
[[NSNotificationCenter defaultCenter] addObserver:self
                                         selector:@selector(boundsDidChangeNotification:)
                                             name:NSViewBoundsDidChangeNotification
                                           object:[scrollView contentView]];

As stated in the Scroll View Programming Guide, you can get the current scroll position this way:

NSPoint currentScrollPosition = [[theScrollView contentView] bounds].origin;