為什么 dart 泛型類不能根據構造函數確定其類型

Question

我正在為我的 FutureBuilders 編寫自己的 MyFutureBuilder，每次都做同樣的事情（除了使用我為其指定單獨函數的快照數據）。

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());
      },
    );
  }
}

當我實例化它時

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

snapshotData 的類型是動態的，而不是Article 。 不能從_article，Future的類型中自動確定類型T嗎？

當我將類型單獨提供給 MyFutureBuilder 時（恕我直言，這是多余的）

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

snapshotData 的類型當然是文章。

Answer 1

在第一個代碼中，您應該在使用之前將返回的 snapshotData 轉換為您需要的類型

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