isChecked property

bool isChecked

If this node has Boolean state that can be controlled by the user, whether that state is checked or unchecked, corresponding to true and false, respectively.

Do not call the setter for this field if the owning RenderObject doesn't have checked/unchecked state that can be controlled by the user.

The getter returns null if the owning RenderObject does not have checked/unchecked state.

Implementation 

bool get isChecked => _hasFlag(SemanticsFlag.hasCheckedState) ? _hasFlag(SemanticsFlag.isChecked) : null;
void isChecked= (bool value)

Implementation 

set isChecked(bool value) {
  _setFlag(SemanticsFlag.hasCheckedState, true);
  _setFlag(SemanticsFlag.isChecked, value);
}