迁移到 null safety 后的 dart closure return 类型问题

Question

以下代码在迁移到 Flutter 2 和 null safety 之前可以正常工作。我只是不知道正确的 return 类型应该是什么。我得到的错误是：

The return type 'List<PopupMenuItem<dynamic>?>' isn't a 'List<PopupMenuEntry<dynamic>>', as required by the closure's context.

代码是：

 Widget _parentMenuSelector() {
    return PopupMenuButton<dynamic>(
      child: Padding(
        padding: const EdgeInsets.all(16.0),
        child: Row(
          mainAxisAlignment: MainAxisAlignment.spaceEvenly,
          children: [
            Icon(Icons.content_copy),
            Text('Copy to another menu'),
          ],
        ),
      ),
      onSelected: (dynamic selectedMenu) {
        Map<String, dynamic> selMenu = selectedMenu;
        print(selMenu['id']);
        model!.copyMenuItem(selMenu['id']);
      },
      itemBuilder: (BuildContext context) {
        return model!.restaurant!.restaurantMenus!.values.map((dynamic item) { //ERROR
          Map<String, dynamic> itemMap = item;                                 //ERROR
          if (itemMap['name'] != model!.menu!.name) {                          //ERROR
            return PopupMenuItem<dynamic>(                                     //ERROR
              child: Text('Copy to ${itemMap['name']}'),                       //ERROR
              value: item,                                                     //ERROR
            );                                                                 //ERROR
          }                                                                    //ERROR
        }).toList();                                                           //ERROR
      },
    );
  }

Answer 1

正如 jamesdlin 所建议的，这至少在语法上对我有用。我还不确定它是否会正常工作 - 我还没有构建应用程序，因为我仍在修复 null-safety 迁移问题。

  Widget _parentMenuSelector() {
    return PopupMenuButton<dynamic>(
      child: Padding(
        padding: const EdgeInsets.all(16.0),
        child: Row(
          mainAxisAlignment: MainAxisAlignment.spaceEvenly,
          children: [
            Icon(Icons.content_copy),
            Text('Copy to another menu'),
          ],
        ),
      ),
      onSelected: (dynamic selectedMenu) {
        Map<String, dynamic> selMenu = selectedMenu;
        print(selMenu['id']);
        model!.copyMenuItem(selMenu['id']);
      },
      itemBuilder: (BuildContext context) {
        return model!.restaurant!.restaurantMenus!.values.map((dynamic item) {
          Map<String, dynamic> itemMap = item;
          if (itemMap['name'] != model!.menu!.name) {
            return PopupMenuItem<dynamic>(
              child: Text('Copy to ${itemMap['name']}'),
              value: item,
            );
          } else {
            return PopupMenuItem<dynamic>(
              child: null,
            );
          }
        }).toList();
      },
    );
  }

迁移到 null safety 后的 dart closure return 类型问题

Dart closure return type problem after migrating to null safety

dart

flutter

null-safety