One Quickie

Responding to a scrollview scrolling (NSScrollView->General)
If you need to react to a scrollview being scrolled by the user, first tell the contentView (the NSClipView) to post notifications when its bounds changes, and then register to receive an NSViewBoundsDidChangeNotification:
    [[scrollView contentView] setPostsBoundsChangedNotifications: YES];

    NSNotificationCenter *center = [NSNotificationCenter defaultCenter] ;
    [center addObserver: self
            selector: @selector(boundsDidChangeNotification:)
            name: NSViewBoundsDidChangeNotification
            object: [scrollView contentView]];

- (void) boundsDidChangeNotification: (NSNotification *) notification
    [self setNeedsDisplay: YES];
    // or whatever work you need to do

} // boundsDidChangeNotification

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