Make Function parameter optional in custom widget flutter

Question

I try to create some custom widget with some parameter in the constructor. This widget has some optional and required parameter. how can make Function type paramete

Answer 1

You can use a default value that does nothing:

class TextInputWithIcon extends StatefulWidget {
  final String iconPath;
  final String placeHolder;
  final Function(bool) onFocusChange;
  const TextInputWithIcon(
      {Key key,
      @required this.iconPath,
      this.placeHolder = "",
      this.onFocusChange = _dummyOnFocusChange})
      : assert(onFocusChange != null), super(key: key);

  @override
  _TextInputWithIconState createState() => _TextInputWithIconState();

  static dynamic _dummyOnFocusChange(bool val) {}
}

I created a static named function instead of just a closure as default value, because closures are not const and currently default values need to be const.

I added the assert(...) to ensure that an error is shown when null is passed explicitly.