[英]Why can't I extend an interface “generic method” and narrow its type to my inherited interface “class generic”?
[英]Why can't a dart generic class determine its type based on constructor
我正在为我的 FutureBuilders 编写自己的 MyFutureBuilder,每次都做同样的事情(除了使用我为其指定单独函数的快照数据)。
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());
},
);
}
}
当我实例化它时
Future<Article> _article;
MyFutureBuilder(
future: _article,
builder: (snapshotData) {
snapshotData 的类型是动态的,而不是Article 。 不能从_article,Future的类型中自动确定类型T吗?
当我将类型单独提供给 MyFutureBuilder 时(恕我直言,这是多余的)
Future<Article> _article;
MyFutureBuilder<Article>(
future: _article,
builder: (snapshotData) {
snapshotData 的类型当然是文章。
在第一个代码中,您应该在使用之前将返回的 snapshotData 转换为您需要的类型
Future<Article> _article;
MyFutureBuilder(
future: _article,
builder: (snapshotData) {
Article article = snapshotData as Article;
}
)
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