Question

我正在为自己的FutureBuilder编写自己的MyFutureBuilder（除了使用我为其指定单独功能的快照数据时）。

typedef MyFutureBuilderBuildFunction<T> = Widget Function(T snapshotData);

class MyFutureBuilder<T> extends StatefulWidget {
  final MyFutureBuilderBuildFunction<T> builder;
  final Future<T> future;

  MyFutureBuilder({this.future, @required this.builder})
      : assert(builder != null);

  @override
  State<StatefulWidget> createState() {
    return MyFutureBuilderState(future: future, builder: builder);
  }
}

class MyFutureBuilderState<T> extends State<MyFutureBuilder<T>> {
  MyFutureBuilderBuildFunction<T> builder;
  Future<T> future;

  MyFutureBuilderState({this.future, @required this.builder})
      : assert(builder != null);

  @override
  Widget build(BuildContext context) {
    return FutureBuilder<T>(
      future: future,
      builder: (context, snapshot) {
        if (snapshot.hasData) {
          return builder(snapshot.data);
        } else if (snapshot.hasError) {
          return Text('${snapshot.error}');
        }

        return Center(child: CircularProgressIndicator());
      },
    );
  }
}

当我用

实例化它时

Future<Article> _article;
MyFutureBuilder(
    future: _article,
    builder: (snapshotData) {

snapshotData的类型为动态，而不是 Article 。无法根据_article的类型自动确定T类型吗？

当我将类型分别提供给MyFutureBuilder时（恕我直言，是多余的）

Future<Article> _article;
MyFutureBuilder<Article>(
    future: _article,
    builder: (snapshotData) {

snapshotData的类型当然是Article。

Answer 1

在第一个代码中，应将返回的snapshotData转换为使用前所需的类型

Future<Article> _article;
MyFutureBuilder(
  future: _article,
  builder: (snapshotData) {
    Article article = snapshotData as Article;
  }
)

为什么dart泛型类无法基于构造函数确定其类型

1 个答案: