Thumb image does not move to the edge of UISlider

Most upvoted answer didn't work for me without changes. Here's what did -- it's a drop in replacement for UISlider:

class Slider: UISlider {

    override func thumbRect(forBounds bounds: CGRect, trackRect rect: CGRect, value: Float)->CGRect {

            var thumbRect = super.thumbRect(forBounds: bounds, trackRect: rect, value: value)

            //determine value dependent offset
            var offsetForValue: CGFloat = 0
            if value != 0 {offsetForValue = thumbRect.size.width * CGFloat(value / (self.maximumValue - self.minimumValue)) - ((value > 0) ? 2 : -2)}

            //apply offset to thumb rect
            thumbRect.origin.x += offsetForValue

            return thumbRect
    }
}

What you see is the UISlider working as designed, apparently.

By default, at the extreme values, the thumb's edge is aligned with the edges of the track, rather than the thumb's center being positioned on the edge of the track. So this is a problem if you provide a thumb image whose left and right edges are not opaque, since then the track underneath becomes visible.

One partial fix is to override thumbRectForBounds(_:trackRect:value:), so that the thumbs' centers range across the width of the slider. The code below shows this:

class CenteredThumbSlider : UISlider {
  override func thumbRectForBounds(bounds: CGRect, trackRect rect: CGRect, value: Float) -> CGRect
  {
    let unadjustedThumbrect = super.thumbRectForBounds(bounds, trackRect: rect, value: value)
    let thumbOffsetToApplyOnEachSide:CGFloat = unadjustedThumbrect.size.width / 2.0
    let minOffsetToAdd = -thumbOffsetToApplyOnEachSide
    let maxOffsetToAdd = thumbOffsetToApplyOnEachSide
    let offsetForValue = minOffsetToAdd + (maxOffsetToAdd - minOffsetToAdd) * CGFloat(value / (self.maximumValue - self.minimumValue))
    var origin = unadjustedThumbrect.origin
    origin.x += offsetForValue
    return CGRect(origin: origin, size: unadjustedThumbrect.size)
  }
}

However, I would say this is only a partial fix since I do not believe this also modifies the position of the thumb's tap region, so this fix above has the unfortunate side-effect of making the thumbs even less easy to touch than they are by default, since the slider still listens for touches more toward the center of the slider.

I recently had to deal with customization of the slider (subclassed), including adding tickmarks on the slider. Thumb image is not centered on the value and since you replaced image it is positioned where it is supposed to be according to implementation. In order to do what you are trying to do I had to adjust position of the thumb image using

- (CGRect)thumbRectForBounds:(CGRect)bounds trackRect:(CGRect)rect value:(float)value