tristate property

bool tristate

If true, value can be true, false, or null, otherwise value must be true or false.

When tristate is true and value is null, then the control is considered to be in its third or "indeterminate" state.

Implementation 

bool get tristate => _tristate;
void tristate= (bool value)

Implementation 

set tristate(bool value) {
  assert(tristate != null);
  if (value == _tristate)
    return;
  _tristate = value;
  markNeedsSemanticsUpdate();
}