在flutter应用程序中从Firebase检索数据时
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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,
),
],
)
)
);
}
}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
下面的断言被构建：一个非空字符串必须是提供给“文本”小部件。 'package：flutter / src / widgets / text.dart'：断言失败：第285行pos 10：'data！= null'
要么断言表明框架本身有错误，要么我们应该在此错误消息中提供更多的信息，以帮助您确定并解决根本原因。在任一情况下，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
引发异常时，这是堆栈
2个新文本包：flutter /.../ widgets / text.dart：285
3 _HomePageState.PostsUI软件包：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
我该怎么做才能检索应用程序中的数据？我已经尽力了。有人可以帮助我找到我哪里错了吗？我最近开始学习。
答案
[第一件事，将代码从initState中拉出，并放入另一个函数中。这将使您的initState保持干净。我看到的是您正在尝试更新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放在initState中。
为此，您可能希望重新设计数据库结构，例如：它应该像用户->帖子-> List_of_all_posts。如果这样做，则NAME_OF_COLLECTION将成为用户，而NAME_OF_DOCUMENT将成为帖子。
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