Simple checkbox with grouping.
valueFromFormData( $name, $data) : mixed
A widget can split itself in multiple input form. For example you can have a datetime value in your model and you use 2 inputs one for the date and one for the time to input the value. So the widget must know how to get back the values from the submitted form.
Value or null if not defined.
Returns the HTML ID attribute of this Widget for use by a <label>, given the ID of the field. Returns None if no ID is available.
This hook is necessary because some widgets have multiple HTML elements and, thus, multiple IDs. In that case, this method should return an ID value that corresponds to the first ID in the widget's tags.