Flutter ListView.builder报错：操作数非空与返回类型不匹配求助

出现的错误信息 #

The left operand can't be null, so the right operand is never executed.
Try removing the operator and the right operand

The return type 'Object' isn't a 'Widget', as required by the closure's context

相关代码 #

newNotifications 列表 #

List newNotification = ['liked', 'follow'];

ListView.builder 代码 #

ListView.builder(
              itemCount: newNotification.length,
              itemBuilder: (context, index) {
                return newNotification[index] == 'follow' ?? PictureTile():NotificationTile1();
            },),

PictureTile 组件 #

class PictureTile extends StatelessWidget {
  const PictureTile({super.key});

  @override
  Widget build(BuildContext context) {
    return Row(
      children: [
        SizedBox(
          height: 70,
          width: 70,
          child: Stack(
            children: const [
              Padding(
                padding: EdgeInsets.only(left: 10),
                child: CircleAvatar(
                  radius: 25,
                  backgroundImage:
                      AssetImage('assets/images/ico-modified-rem.png'),
                ),
              ),
              Positioned(
                bottom: 10,
                child: CircleAvatar(
                  radius: 25,
                  backgroundImage: AssetImage('assets/images/sha-modified.png'),
                ),
              ),
            ],
          ),
        ),
        const SizedBox(width: 10),
        Expanded(
          child: Column(
            mainAxisSize: MainAxisSize.min,
            crossAxisAlignment: CrossAxisAlignment.start,
            children: const [
              Text(
                'Order Confirmation',
                style: TextStyle(
                  fontSize: 16,
                  fontWeight: FontWeight.bold,
                ),
              ),
              SizedBox(
                height: 5,
              ),
              Text(
                'Order Cancelled',
                style: TextStyle(
                  fontSize: 13,
                  color: Colors.black54,
                ),
              ),
              const SizedBox(height: 5),
              Text(
                'You ordered Shawarma',
                style: TextStyle(
                  fontSize: 13,
                ),
              ),
            ],
          ),
        ),
        Image.asset(
          'assets/images/chicken-shawarma-modified.png',
          height: 95,
          width: 91,
        ),
      ],
    );
  }
}

NotificationTile1 组件 #

class NotificationTile1 extends StatefulWidget {
  const NotificationTile1({super.key});

  @override
  State<NotificationTile1> createState() => _NotificationTile1State();
}

class _NotificationTile1State extends State<NotificationTile1> {
  @override
  Widget build(BuildContext context) {
    return Row(
      children: [
        TileImg('assets/images/ico-modified-rem.png'),
        const SizedBox(
          width: 15,
        ),
        Column(
          mainAxisSize: MainAxisSize.min,
          crossAxisAlignment: CrossAxisAlignment.start,
          children: const [
            Text(
              'Order Confirmation',
              style: TextStyle(
                fontSize: 16,
                fontWeight: FontWeight.bold,
              ),
            ),
            SizedBox(
              height: 5,
            ),
            Text(
              'Order Successful',
              style: TextStyle(
                fontSize: 13,
                color: Colors.black54,
              ),
            ),
            SizedBox(
              height: 5,
            ),
            Text(
              'You ordered Shawarma',
              style: TextStyle(
                fontSize: 13,
              ),
            ),
          ],
        ),
      ],
    );
  }
}

问题原因与解决 #

核心错误 #

你把三元运算符和空合并运算符搞混了：

• 三元运算符的正确写法是 条件 ? 表达式1 : 表达式2，用于根据布尔条件返回两个值中的一个
• ??是空合并运算符，作用是当左侧为null时返回右侧值，否则返回左侧值。你这里左侧是newNotification[index] == 'follow'（布尔值，不可能为null），所以编译器报错说右侧永远不会执行

同时，错误的运算符写法导致编译器无法识别返回的是Widget类型，所以提示返回类型是Object不符合要求。

修正后的代码 #

把??改成?即可，修正后的ListView.builder代码：

ListView.builder(
  itemCount: newNotification.length,
  itemBuilder: (context, index) {
    return newNotification[index] == 'follow' ? PictureTile() : NotificationTile1();
  },
),

关于可空类型的疑问 #

不需要把PictureTile()或NotificationTile1()设为可空类型，因为这里的问题完全是运算符使用错误导致的，和可空性无关。这两个组件本身都是非空的Widget，符合itemBuilder的返回要求。

内容的提问来源于stack exchange，提问作者shimomaru