输入'列表<动态>'不是“List<Widget>"类型的子类型 [英] type &#39;List&lt;dynamic&gt;&#39; is not a subtype of type &#39;List&lt;Widget&gt;&#39;

问题描述

我有一段从 Firestore 示例中复制的代码:

I have a snippet of code which I copied from Firestore example:

Widget _buildBody(BuildContext context) {
    return new StreamBuilder(
      stream: _getEventStream(),
      builder: (context, snapshot) {
        if (!snapshot.hasData) return new Text('Loading...');
        return new ListView(
          children: snapshot.data.documents.map((document) {
            return new ListTile(
              title: new Text(document['name']),
              subtitle: new Text("Class"),
            );
          }).toList(),
        );
      },
    );
  }

但我收到此错误

type 'List<dynamic>' is not a subtype of type 'List<Widget>'

这里出了什么问题?

推荐答案

这里的问题是类型推断以意想不到的方式失败.解决方案是为 map 方法提供一个类型参数.

The problem here is that type inference fails in an unexpected way. The solution is to provide a type argument to the map method.

snapshot.data.documents.map<Widget>((document) {
  return new ListTile(
    title: new Text(document['name']),
    subtitle: new Text("Class"),
  );
}).toList()

更复杂的答案是，虽然 children 的类型是 List，但该信息不会返回到 map> 调用.这可能是因为 map 后面跟着 toList 并且因为没有办法对闭包的返回进行类型注释.

The more complicated answer is that while the type of children is List<Widget>, that information doesn't flow back towards the map invocation. This might be because map is followed by toList and because there is no way to type annotate the return of a closure.

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