Stateful Widget 上的提供程序 - 参数类型问题

Question

I have a class RedditAPIService where i am putting all the items methods related to DRAW plugin for Reddit.

I created an object for the class in a Stateless widget. (below the class _RedditAuthState extends State portion)

RedditAPIService reddit = RedditAPIService();

I need this reddit object to be available on multiple Widgets downstream so i wanted to use Provider to expose the object:

  @override
  Widget build(BuildContext context) {
    return Provider<RedditAPIService>(
      create: (_) => RedditAPIService(),
      builder: (context) {
        Scaffold(
          appBar: GlobalAppBar(
            appbarTitle: 'Authorize ReadStories',
          ),
          body: SafeArea(
              child: Center(
            child: haveRedditAuthCode
                ? CircularProgressIndicator()
                : WebviewScaffold(
                    url: reddit.getAuthURL(),
                    hidden: true,
//                initialChild: Center(child: Text('data')),
                  ),
          )),
        );
      },
    );
  }
}

I am currently getting the error:

"error: The argument type 'Null Function(BuildContext)' can't be assigned to the parameter type 'Widget Function(BuildContext, Widget)'."

What am i doing wrong?

Answer 1

I believe that you are getting this error because you are not returning anything from the builder callback function, hence the Null in the error message. Try adding a return before the Scaffold widget and adding the child parameter to the callback function as below:

@override
Widget build(BuildContext context) {
  return Provider<RedditAPIService>(
    create: (_) => RedditAPIService(),
    builder: (context, child) {
      return Scaffold(
        appBar: GlobalAppBar(
          appbarTitle: 'Authorize ReadStories',
        ),
        body: SafeArea(
            child: Center(
              child: haveRedditAuthCode
                  ? CircularProgressIndicator()
                  : WebviewScaffold(
                url: reddit.getAuthURL(),
                hidden: true,
//              initialChild: Center(child: Text('data')),
              ),
            )),
      );
    },
  );
}

The error is referencing two separate issues which I will attempt to explain below:

Null -> Widget

This error is caused by the fact that the builder callback function is not returning anything, however a Widget is expected to be returned. This Widget is what gets displayed in the User Interface of the application and is much the same as returning a Widget from the overridden build method. See here and here for more information.

(BuildContext) -> (BuildContext, Widget)

This error is caused by the fact that the builder callback function only has one parameter, however two parameters are expected. The second parameter is a child Widget , which can be used if you have a particularly large child Widget in the tree that does not need to be recreated each time that the state changes. See here and here for more information.