Set up a handler for button state changes (clicks).

        Args:
            handler (func) : handler function to call when button is toggled.

        Returns:
            None

def on_click(self, handler):
        """ Set up a handler for button state changes (clicks).

        Args:
            handler (func) : handler function to call when button is toggled.

        Returns:
            None

        """
        self.on_change('active', lambda attr, old, new: handler(new))