UIButton Highlighted State not showing when clicking over a Selected UIButton

In swift this would be:

button.setBackgroundImage(shadowImage, forState: UIControlState.Selected.union(UIControlState.Highlighted))

In Swift v3 (Nov. 2016):

button.setBackgroundImage(shadowImage, for: UIControlState.selected.union(UIControlState.highlighted))

The various states: UIControlStateNormal, UIControlStateSelected, and (UIControlStateSelected | UIControlStateHighlighted) are all actually distinct. If you want your shadowImage to apply both in the (only) highlighted state and in the highlighted+selected state, you must also set:

[button setBackgroundImage:shadowImage forState:(UIControlStateHighlighted | UIControlStateSelected)]