Flutter - 必须向 Text 小部件提供非空字符串答案

【问题标题】：Flutter - A non-null String must be provided to a Text widgetFlutter - 必须向 Text 小部件提供非空字符串
【发布时间】：2020-04-14 15:21:21
【问题描述】：

关于使用 futureBuilder 从 firestore 获取数据时出现错误。当我在不调用 Text 小部件的情况下打印 snapshot.data 时，我得到了一个“用户”实例。

不调用 author.fullname

body: ListView.builder(
    itemCount: _posts.length,
    itemBuilder: (BuildContext context, int index) {
      Post post = _posts[index];
      return FutureBuilder(
        future: DatabaseService.getUserWithId(post.authorid),
        builder: (BuildContext context, AsyncSnapshot snapshot) {
          if (snapshot.connectionState == ConnectionState.done) {
            if (snapshot.hasData) {
              print(snapshot.data);
              User author = snapshot.data;
              return Column(
                children: <Widget>[],
              );
            }
          }
          return SizedBox.shrink();

输出

Performing hot restart...                                               
Restarted application in 1,146ms.
flutter: Instance of 'User'
flutter: Instance of 'User'

但是，当我尝试从 firestore 调用用户的全名时，它返回 null

body: ListView.builder(
    itemCount: _posts.length,
    itemBuilder: (BuildContext context, int index) {
      Post post = _posts[index];
      return FutureBuilder(
        future: DatabaseService.getUserWithId(post.authorid),
        builder: (BuildContext context, AsyncSnapshot snapshot) {
          if (snapshot.connectionState == ConnectionState.done) {
            if (snapshot.hasData) {
              print(snapshot.data);
              User author = snapshot.data;
              return Column(
                children: <Widget>[
                  Text(author.fullname)
                ],
              );
            }
          }
          return SizedBox.shrink();

输出

Performing hot restart...                                               
Restarted application in 1,042ms.
flutter: Instance of 'User'
flutter: ══╡ EXCEPTION CAUGHT BY WIDGETS LIBRARY   
flutter: The following assertion was thrown building FutureBuilder<User>(dirty, state:
flutter: _FutureBuilderState<User>#23d74):
flutter: A non-null String must be provided to a Text widget.
flutter: 'package:flutter/src/widgets/text.dart':
flutter: Failed assertion: line 285 pos 10: 'data != null'
flutter:
flutter: Either the assertion indicates an error in the framework itself, or we should provide substantially
flutter: more information in this error message to help you determine and fix the underlying cause.
flutter: In either case, please report this assertion by filing a bug on GitHub:
flutter:   https://github.com/flutter/flutter/issues/new?template=BUG.md
flutter:
flutter: The relevant error-causing widget was:
flutter:   FutureBuilder<User> file:///Users/momo/Desktop/combine/lib/screens/home.dart:65:18
flutter:
flutter: When the exception was thrown, this was the stack:
flutter: #2      new Text (package:flutter/src/widgets/text.dart:285:10)
flutter: #3_HomeState.build.<anonymous closure>.<anonymous closure> (package:combine/screens/home.dart:75:23)
flutter: #4      _FutureBuilderState.build (package:flutter/src/widgets/async.dart)
flutter: #5      StatefulElement.build (package:flutter/src/widgets/framework.dart:4334:27)
flutter: #6      ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4223:15)
flutter: #7      Element.rebuild (package:flutter/src/widgets/framework.dart:3947:5)
flutter: #8      BuildOwner.buildScope (package:flutter/src/widgets/framework.dart:2432:33)
flutter: #9      WidgetsBinding.drawFrame (package:flutter/src/widgets/binding.dart:773:20)
flutter: #10     RendererBinding._handlePersistentFrameCallback (package:flutter/src/rendering/binding.dart:283:5)
flutter: #11     SchedulerBinding._invokeFrameCallback (package:flutter/src/scheduler/binding.dart:1102:15)
flutter: #12     SchedulerBinding.handleDrawFrame (package:flutter/src/scheduler/binding.dart:1041:9)
flutter: #13     SchedulerBinding._handleDrawFrame (package:flutter/src/scheduler/binding.dart:957:5)
flutter: #17     _invoke (dart:ui/hooks.dart:259:10)
flutter: #18     _drawFrame (dart:ui/hooks.dart:217:3)
flutter: (elided 5 frames from class _AssertionError and package dart:async)

用户类

class User {
  final String id;
  final String email;
  final String password;
  final String fullname;

User(
    {this.id,
    this.email,
    this.password,
    this.fullname,});

factory User.fromDoc(DocumentSnapshot doc) {
  return User(
    id: doc.documentID,
    email: doc['email'],
    fullname: doc['fullname'],
    );
  }
}

【问题讨论】：

接收到的数据好像是User类的对象实例，如果没有问题请提供它的代码。
@Sparks 我已对其进行了编辑以包含 User 类
您只有一个文本小部件，即 Text(author.fullname)，因此很可能 author.fullname 为空。使用打印（作者。全名）；在构建方法中验证它。试试 Text(author.fullname==null?'--':author.fullname)
@Dev 正在执行热重启...在 1,248 毫秒内重新启动了应用程序。颤振：“用户”的实例颤振：null 颤振：“用户”的实例颤振：null
打印 (snapshot.data.fullname) 在控制台中为您提供什么？

标签： firebase flutter dart google-cloud-firestore widget

【解决方案1】：

如果名称为空，则必须提供空文本

return Column(
                children: <Widget>[
                  Text(author.fullname == null? '' : author.fullname)
                ],
              );

【讨论】：

颤动：空颤动：null。