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flutter - 在颤振应用程序中从firebase检索数据时

问题描述

这是代码

import 'package:flutter/material.dart';
import 'Authentication.dart';
import 'photoUpload.dart';
import 'Posts.dart';
import 'package:firebase_database/firebase_database.dart';
// import 'package:flutter_blogapp/Authentication.dart';
// import 'package:flutter_blogapp/photoUpload.dart';


class HomePage extends StatefulWidget
{
  HomePage 
(
  {
    this.auth,
    this.onSignedOut,
  }
);
final AuthImplementation auth;
final VoidCallback onSignedOut;

  @override
  State<StatefulWidget> createState() 
  {
    return _HomePageState();
  }
}

class _HomePageState extends State<HomePage>
{

  List<Posts> postsList = [];

  @override
  void initState() 
  {
    super.initState();

    DatabaseReference postsRef = FirebaseDatabase.instance.reference().child("Posts");

    postsRef.once().then((DataSnapshot snap)
    {
      var KEYS = snap.value.keys;
      var DATA = snap.value;

      postsList.clear();

      for(var individualKey in KEYS)
      {
        Posts posts = new Posts
        (
          DATA[individualKey]['image'],
          DATA[individualKey]['desctiption'],
          DATA[individualKey]['data'],
          DATA[individualKey]['time'], 
        );

        postsList.add(posts);
      }

      setState(() 
      {
        print('Length : $postsList.length');
      });
    });
  }

  void _logoutUser() async
  {
    try 
    {
      await widget.auth.signOut();
      widget.onSignedOut();
    } 
    catch (e) 
    {
      print(e.toString());
    }
  }

  @override
  Widget build(BuildContext context) 
  {
    return new Scaffold
    (
      appBar:  new AppBar
      (
        title: new Text('Home'),
      ),
      
      
      body : new Container
      (
        child: postsList.length == 0 ? new Text(" No Post available ") : new ListView.builder
        (
          itemCount: postsList.length,
          itemBuilder: (_, index)
          //itemBuilder: (BuildContext _, int index ) //<-----
          {
            return PostsUI(postsList[index].image, postsList[index].description, postsList[index].date, postsList[index].time);
          }
        ),
      ),

      bottomNavigationBar: new BottomAppBar
      (
        color: Colors.pink,

        child: new Container
        (
          margin: const EdgeInsets.only(left: 70.0, right: 70.0),
          child: new Row
          (
            mainAxisAlignment: MainAxisAlignment.spaceBetween,
            mainAxisSize: MainAxisSize.max,

            children: <Widget>
            [
              new IconButton
              (
                icon: new Icon(Icons.local_car_wash),
                iconSize: 50,
                color: Colors.white, 

                onPressed: _logoutUser,

              ),

              new IconButton
              (
                icon: new Icon(Icons.add_a_photo),
                iconSize: 50,
                color: Colors.white,

                onPressed: ()
                {
                  Navigator.push
                  (
                    context, 
                    MaterialPageRoute(builder: (context)
                    {
                      return new UploadPhotoPage();
                    })
                  );
                },

              ),
            ],

          ),
        ),
      ),


    );
  }

                                                  // Designing Posts UI <remove from Text field><??'defaut value'>

  Widget PostsUI(String image, String description, String date, String time)
  {
    return new Card
    (
      elevation: 10.0,
      margin: EdgeInsets.all(15.0),


      child: new Container
      (
        padding: new EdgeInsets.all(14.0),

        child: new Column
        (
          crossAxisAlignment: CrossAxisAlignment.start,

          children: <Widget>
          [
            new Row
            (
              mainAxisAlignment: MainAxisAlignment.spaceBetween,

              children: <Widget>
              [
                new Text
                (
                  date,
                  style: Theme.of(context).textTheme.subtitle,
                  textAlign: TextAlign.center,
                ),

                new Text
                (
                  time,
                  style: Theme.of(context).textTheme.subtitle,
                  textAlign: TextAlign.center,
                ),                  //<----
              ],
            ),

            SizedBox(height: 10.0,),

            new Image.network(image, fit:BoxFit.cover),

            SizedBox(height: 10.0,),

            new Text
                (
                  description,  //= null ?  "true" : "False", //??'defaut value'
                  style: Theme.of(context).textTheme.subhead,
                  textAlign: TextAlign.center,
                ),


          ],

        )



      ) 

    );

  }
}

我得到的错误

构建时抛出了以下断言：必须向 Text 小部件提供非空字符串。'package:flutter/src/widgets/text.dart'：断言失败：第 285 行 pos 10：'data != null'

断言表明框架本身存在错误，或者我们应该在此错误消息中提供更多信息，以帮助您确定和修复根本原因。无论哪种情况，请通过在 GitHub 上提交错误来报告此断言：
https ://github.com/flutter/flutter/issues/new?template=BUG.md

抛出异常时，这是堆栈

2 个新的文本包：flutter/…/widgets/text.dart:285

3 _HomePageState.PostsUI 包：flutter_blogapp/HomePage.dart:184

4 _HomePageState.build。包：flutter_blogapp/HomePage.dart:100

5 SliverChildBuilderDelegate.build 包：flutter/…/widgets/sliver.dart:446

6 SliverMultiBoxAdaptorElement._build。包：flutter/…/widgets/sliver.dart:1260

我可以做些什么来检索我的应用程序中的数据？我已经尝试了我能做的。有人可以帮我找出我哪里错了吗？我最近开始学习了。

标签： flutterfirebase-realtime-databasedart

解决方案

首先，将代码从 initState 中拉出并放入不同的函数。这将使您的 initState 保持清洁。我可以看到您正在尝试更新 Firestore 中的数据。您可以做的是定义一个新函数，如下所示：

final databaseReference = Firestore.instance;

setPostsData() async {
  DocumentSnapshot snapshot = await databaseReference
      .collection("NAME_OF_COLLECTION")
      .document("NAME_OF_DOCUMENT")
      .get();
  // use this DocumentSnapshot snapshot to get the current data that is there in the document inside of your collection.
  var currentData = snapshot.data;
  print(currentData); // to check whats actually there and if its working...

  //lets assume newPostsList is the data that you want to put in this referenced document.
  //to update the data in firestore:
  await databaseReference
      .collection("NAME_OF_COLLECTION")
      .document("NAME_OF_DOCUMENT")
      .updateData({newPostsList});
}

然后您可以将 setPostsData 放入 initState 中。

为此，您可能需要重新设计数据库结构，例如：它应该类似于用户 --> 帖子 --> List_of_all_posts。如果这样做，NAME_OF_COLLECTION 将是用户，NAME_OF_DOCUMENT 将是帖子。

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