Question

这是FutureBuilder：

@override
Widget build(BuildContext context) {
  return FutureBuilder(
    future: postsRef
        .document(userId)
        .collection('userPosts')
        .document(postId)
        .get(),
    builder: (context, snapshot) {
      if (!snapshot.hasData) {
        return circularProgress();
      }
      Post post = Post.fromDocument(snapshot.data);
      return Center(
        child: Scaffold(
          appBar: header(context, titleText: post.description),
          body: ListView(
            children: <Widget>[
              Container(
                child: post,
              )
            ],
          ),
        ),
      );
    },
  );
}

引用它的show post方法：

showPost(context) {
  Navigator.push(
    context,
    MaterialPageRoute(
      builder: (context) => PostScreen(
        postId: postId,
        userId: userId,
      ),
    ),
  );
}

遇到错误

方法'[]'在null上被调用。接收者：null 尝试致电：相关的引起错误的小部件是： FutureBuilder文件：///home/testflutter/AndroidStudioProjects/testingflutter/lib/pages/post_screen.dart：15：12

Answer 1

添加else子句

Widget build(BuildContext context) {
    return FutureBuilder(
      future: postsRef
          .document(userId)
          .collection('userPosts')
          .document(postId)
          .get(),
      builder: (context, snapshot) {
        if (!snapshot.hasData) {
          return circularProgress();
        }
        else {
        Post post = Post.fromDocument(snapshot.data);
        return Center(
          child: Scaffold(
            appBar: header(context, titleText: post.description),
            body: ListView(
              children: <Widget>[
                Container(
                  child: post,
                )
              ],
            ),
          ),
        );
        } 
      },
    );

Answer 2

检查快照的ConnectionState，看是否已完成。

Widget build(BuildContext context) {
  return FutureBuilder(
    future: postsRef
        .document(userId)
        .collection('userPosts')
        .document(postId)
        .get(),
    builder: (context, snapshot) {
      if (snapshot.connectionState == ConnectionState.waiting)
        return circularProgress();
      else if (snapshot.connectionState == ConnectionState.done) {
        if (snapshot.hasData && snapshot.data.isNotEmpty) {
          Post post = Post.fromDocument(snapshot.data);
          return Center(
            child: Scaffold(
              appBar: header(context, titleText: post.description),
              body: ListView(
                children: <Widget>[
                  Container(
                    child: post,
                  ),
                ],
              ),
            ),
          );
        } else {
          return Center(child: Text("No data or it's empty"));
        }
      } else {
        return Center(child: Text("Neither waiting or done..."));
      }
    },
  );
}

我已修正代码以使其变得容易，并添加了else子句。

Answer 3

可能的错误是在您的 showPost() 方法中。

在您的 home.dart 文件中初始化一个 User 对象。

User currentUser;

然后像这样更新 showPost() 方法：

showPost(BuildContext context) {
    Navigator.push(
      context,
      MaterialPageRoute(
        builder: (context) =>
            PostScreen(userId: currentUser.id, postId: postId),
      ),
    );
  }

方法'[]'在null上被调用。扑

3 个答案: