Flutter DropDownButton -下拉按钮禁用时如何显示所选值？

String selectedValue = 'Option 1';
bool isDropdownEnabled = false;

DropdownButton(
  value: selectedValue,
  onChanged: isDropdownEnabled ? (newValue) {
    setState(() {
      selectedValue = newValue;
    });
  } : null,
  items: [
    DropdownMenuItem(
      value: 'Option 1',
      child: Text('Option 1'),
      disabled: selectedValue == 'Option 1' && !isDropdownEnabled,
    ),
    DropdownMenuItem(
      value: 'Option 2',
      child: Text('Option 2'),
      disabled: selectedValue == 'Option 2' && !isDropdownEnabled,
    ),
    DropdownMenuItem(
      value: 'Option 3',
      child: Text('Option 3'),
      disabled: selectedValue == 'Option 3' && !isDropdownEnabled,
    ),
  ],
)