toggleable property
final
Set to true if this radio button is allowed to be returned to an indeterminate state by selecting it again when selected.
To indicate returning to an indeterminate state, RadioGroup.onChanged of the RadioGroup above the widget tree will be called with null.
If true, RadioGroup.onChanged is called with value when selected while RadioGroup.groupValue != value, and with null when selected again while RadioGroup.groupValue == value.
If false, RadioGroup.onChanged will be called with value when it is selected while RadioGroup.groupValue != value, and only by selecting another radio button in the group (i.e. changing the value of RadioGroup.groupValue) can this radio button be unselected.
The default is false.
Implementation
final bool toggleable;