为什么 dart 泛型类不能根据构造函数确定其类型

Question

I am writing my own MyFutureBuilder for my FutureBuilders do the same thing every time (except working with the snapshot data for which I specify my separate function).

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());
      },
    );
  }
}

When I instantiate it with

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

the type of snapshotData is dynamic and not Article . Can't the type T be automatically be determined from the type of _article, Future?

When I seperately give the type to MyFutureBuilder (which is IMHO redundant)

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

the type of snapshotData is of course Article.

Answer 1

In 1st code You should cast the returned snapshotData to the type you need before using it

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