checkbox.go
func (c *CheckBox) OnChange(fn func(int)) {
	c.mtx.Lock()
	defer c.mtx.Unlock()

	c.onChange = fn
}
func (c *CheckBox) OnChange(fn func(int)) {
	c.mtx.Lock()
	defer c.mtx.Unlock()

	c.onChange = fn
}
// OnChange sets the callback that is called whenever the state
// of the CheckBox is changed. Argument of callback is the current
// CheckBox state: 0 - off, 1 - on, 2 - third state