dart 構造函數中的默認值

Question

我有以下 class 需要 unpressedImage 字段。 如果未指定pressedImage，我需要將unpressedImage 值設置為pressedImage 字段。 我怎樣才能以最好的方式實現這一目標？

class ImageButton extends StatefulWidget {
  final Image unpressedImage;
  final Image pressedImage;
  final onTap;

  const ImageButton({
    Key key,
    @required this.unpressedImage,
    this.pressedImage,
    this.onTap,
  }) : super(key: key);

  @override
  _ImageButtonState createState() => _ImageButtonState();
}

Answer 1

將其設置在初始化列表中，並使用條件來選擇要使用的值：

class ImageButton extends StatefulWidget {
  final Image unpressedImage;
  final Image pressedImage;
  final onTap;

  const ImageButton({
    Key key,
    @required this.unpressedImage,
    Image pressedImage,
    this.onTap,
  })  : this.pressedImage =
            pressedImage == null ? unpressedImage : pressedImage,
        super(key: key);

  @override
  _ImageButtonState createState() => _ImageButtonState();
}