bool value

Whether this control is current "active" (checked, on, selected) or "inactive" (unchecked, off, not selected).

When the value changes, this object starts the positionController and position animations to animate the visual appearance of the control to the new value.

Source 

bool get value => _value;
void value=(bool value)

Source 

set value(bool value) {
  assert(value != null);
  if (value == _value)
    return;
  _value = value;
  markNeedsSemanticsUpdate(onlyChanges: true, noGeometry: true);
  _position
    ..curve = Curves.easeIn
    ..reverseCurve = Curves.easeOut;
  if (value)
    _positionController.forward();
  else
    _positionController.reverse();
}