Flutter RaisedButton怎样禁用

const RaisedButton({
    Key key,
    @required VoidCallback onPressed,
    VoidCallback onLongPress,
    ValueChanged<bool> onHighlightChanged,
    MouseCursor mouseCursor,
    ButtonTextTheme textTheme,
    Color textColor,
    Color disabledTextColor,
    Color color,
    Color disabledColor,
    Color focusColor,
    Color hoverColor,
    Color highlightColor,
    Color splashColor,
    Brightness colorBrightness,
    double elevation,
    double focusElevation,
    double hoverElevation,
    double highlightElevation,
    double disabledElevation,
    EdgeInsetsGeometry padding,
    VisualDensity visualDensity,
    ShapeBorder shape,
    Clip clipBehavior = Clip.none,
    FocusNode focusNode,
    bool autofocus = false,
    MaterialTapTargetSize materialTapTargetSize,
    Duration animationDuration,
    Widget child,
  }) 

 /// The fill color of the button when the button is disabled.
  ///
  /// The default value of this color is the theme's disabled color,
  /// [ThemeData.disabledColor].
  ///
  /// See also:
  ///
  ///  * [color] - the fill color of the button when the button is [enabled].
  final Color disabledColor;
 
 
 
 /// Whether the button is enabled or disabled.
  ///
  /// Buttons are disabled by default. To enable a button, set its [onPressed]
  /// or [onLongPress] properties to a non-null value.
  bool get enabled => onPressed != null || onLongPress != null;

RaisedButton(
    disabledColor: Res.COLOR.text_999999,
    color: Res.COLOR.title_bg,
    padding: EdgeInsets.only(top: 16, bottom: 16),
    child: const Text('提交',style: TextStyle(color: Colors.white)),
    onPressed: _submit(),
    )                
 
 
void Function() _submit() {
    return selectedContactList.isNotEmpty ? () => onBtnPressed() : null;
}
 
void onBtnPressed() {
    var params = <String, dynamic>{
      'select_data': '123'
    };
    FlutterBoost.singleton.closeCurrent(result: params);
  }