code: func (d *LoginDialog) OnCheck(fn func(string, string) bool) {
  	WindowManager().BeginUpdate()
  	defer WindowManager().EndUpdate()
  	d.onCheck = fn
  }
docs: // OnCheck sets the callback that is called when the
  // button "OK" is clicked. The dialog sends to the callback two arguments:
  // username and password. The callback validates the arguments and if
  // the credentials are valid it returns true. That means the dialog can be
  // closed. If the callback returns false then the dialog remains on the screen.