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