If you want an NSSlider to snap on certain intervals, your option out-of-the-box is to give it tick marks (using -setNumberOfTickMarks:), and then enforce that it can only be set to values coinciding with a tick mark (using setAllowsTickMarkValuesOnly:). This is great, if you only want to accept values at certain intervals; but it’s not very helpful if you simply want to snap to values at key points (like quarters, or thirds).

I’ve seen a few solutions to this problem; the one that came closest ended up overriding

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-[NSSliderCell startTrackingAt:inView:],
-[NSSliderCell continueTracking:at:inView:]

and cleverly switching allowsTickMarkValuesOnly on and off at opportune moments. This worked visually, but the data that was streamed from the slider didn’t actually snap until after tracking ended. This is obviously a non-starter, if you’re showing continuous feedback for your slider.

My solution

So, I’ve come up with a simpler and slightly more clever solution, which behaves just as one would expect it to.

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@interface JSSnappingSliderCell : NSSliderCell
@end

@implementation JSSnappingSliderCell
static const CGFloat kSnappingZone = 10.0f;

- (BOOL)continueTracking:(NSPoint)lastPoint at:(NSPoint)currentPoint inView:(NSView *)controlView
{
  CGPoint snapToPoint = currentPoint;

  for (NSUInteger i = 0; i < self.numberOfTickMarks; i++)
  {
    NSRect tickMarkRect = [self rectOfTickMarkAtIndex:i];
    if (ABS(tickMarkRect.origin.x - currentPoint.x) <= kSnappingZone)
    {
      snapToPoint = [self rectOfTickMarkAtIndex:i].origin;
      break;
    }
  }

  return [super continueTracking:lastPoint at:snapToPoint inView:controlView];
}

@end

Enjoy, and do let me know if you have a better idea!