android - FutureBuilder 错误：返回类型“对象？” 不是闭包上下文所要求的“小部件”

Question

我是 Flutter 的新手，我正在研究 Future Builder，当我在以下行运行代码时：return snapshot.data;返回以下错误：The return type 'Object?' isn't a 'Widget', as required by the closure's context.

我认为这与 Null Safety 有关，因为当我更改行时：它return snapshot.data;会return Text(snapshot.data.toString());呈现文本：ListView(ScrollDirection: vertical...ScrollPhysics)但我对 Null 安全性可能是错误的。

下面的代码来自一个 youtube 教程：https://www.youtube.com/watch?v=y-IypschepA我正在看，我在学习时逐字复制代码，我已经验证了近 20 次我的代码正是导师编写并成功运行的代码。

我不确定如何解决此错误。

import 'package:flutter/material.dart';

class FutureBuilderTest extends StatefulWidget {
  @override
  _FutureBuilderTestState createState() => _FutureBuilderTestState();
}

class _FutureBuilderTestState extends State<FutureBuilderTest> {
  Future<void> loadList(BuildContext context) async {
    return Future.delayed(
      Duration(seconds: 1),
      () {
        return ListView.builder(
          itemCount: 40,
          itemBuilder: (context, index) {
            return Card(
              elevation: 8,
              child: Text(
                "Dummy Text $index",
                style: TextStyle(fontSize: 17, fontWeight: FontWeight.bold),
              ),
            );
          },
        );
      },
    );
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text("Future Builder Test"),
      ),
      body: FutureBuilder(
        future: loadList(context),
        builder: (context, snapshot) {
          if (snapshot.connectionState == ConnectionState.waiting) {
            return Center(
              child: CircularProgressIndicator(),
            );
          }
          if (snapshot.connectionState == ConnectionState.done) if (snapshot
              .hasError) {
            return Center(
              child: Text(snapshot.data.toString()),
            );
          }
          if (snapshot.data != null) {
            return snapshot.data;        // THIS LINE IS RETURNING THE ERROR
          } else {
            return Center(
              child: CircularProgressIndicator(),
            );
          }
        },
      ),
    );
  }
}

标签： androidflutterdart

Answer 1

FutureBuilder 中的构建器返回一个小部件（就像您在返回 CircularProgressIndicator 时所做的那样）。所以你不应该按原样返回 snapshot.data 。相反，将其放入一些小部件中以显示在屏幕上。所以喜欢，

 if (snapshot.data != null) {
        print(snapshot.data);        
        return Center(
            child:Text(snapshot.data.toString(),
                ),
              ),
            );
}