可选参数可以是位置参数，也可以是命名参数，但不能同时是位置参数和命名参数.

默认情况下，命名参数是可选的，因此您不必指定默认值.

如果参数是可选的，但不能为空，请提供default value.

With null safety

class TextInputWithIcon extends StatefulWidget {
  final String iconPath;
  final String placeHolder;
  final Function(bool)? onFocusChange; // nullable and optional
  
  const TextInputWithIcon(
      {Key? key,
      required this.iconPath, // non-nullable and required
      this.placeHolder = "", // non-nullable but optional with a default value
      this.onFocusChange, // nullable and optional
      })
      : super(key: key);

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

}

Without null safety

const TextInputWithIcon(
      {Key key,
      @required this.iconPath,
      this.placeHolder = "",
      this.onFocusChange
})
      : super(key: key);

用法:

void _focusChanged(bool value) {

    // using null-aware operator (for both with and without null safety)
    onFocusChange?.call(value);
    
    // or without null-aware operator 
    
    // with null safety
    if(onFocusChange != null) {
      onFocusChange!(value);
    }

    // without null safety
    if(onFocusChange != null) {
      onFocusChange(value);
    }

  }

请看一下Optional Parameters，以便更好地了解.

编辑:谢谢乔纳·威廉姆斯的澄清.