thumbHighlightImage is not useful in iOS 10, xib

[odyyy]

bulid a UIView in xib and change class to TTSwitch
change thumbHighlightImage in swift code
the switch color is not changed

    ttSwitch.thumbImage = #imageLiteral(resourceName: "switch_gray")
    ttSwitch.thumbHighlightImage = #imageLiteral(resourceName: "switch_yellow")
    ttSwitch.overlayImage = #imageLiteral(resourceName: "switch_background")
    ttSwitch.trackImage = #imageLiteral(resourceName: "switch_background")