The ui() component (formerly hover) takes care of hovering, clicks, enabled state and focus.

The layout() component takes care of the position of objects.

The button() component makes use of ui() for hover, press and focus. It mainly does the drawing. The checkbox() component does the same, but additionally has isChecked and check/uncheck logic. The radio() component has additionally a group setting and logic for exclusive selection.

TODO: should value and label go in ui?