我正在尝试使用 Streambuilder 来显示 gridview，但它返回此错误，我正在调用方法 classList() 到脚手架主体中的容器
数据库 = FirebaseDatabase.instance
我想在 gridview 中以 RaisedButtons 显示 classData，显示 0,1,2..etc('className','classSection' 在每个 child 的一个按钮中)
执行下面的代码时出现这个错误
I/flutter ( 5433): ══╡ EXCEPTION CAUGHT BY WIDGETS LIBRARY ╞═══════════════════════════════════════════════════════════
I/flutter ( 5433): The following NoSuchMethodError was thrown building HomePage(dirty, state: _HomePage#055be):
I/flutter ( 5433): The method 'split' was called on null.
I/flutter ( 5433): Receiver: null
I/flutter ( 5433): Tried calling: split("/")
I/flutter ( 5433):
I/flutter ( 5433): When the exception was thrown, this was the stack:
I/flutter ( 5433): #0 Object.noSuchMethod (dart:core/runtime/libobject_patch.dart:50:5)
I/flutter ( 5433): #1 DatabaseReference.child (file:///G:/flutter/.pub-cache/hosted/pub.dartlang.org/firebase_database-2.0.3/lib/src/database_reference.dart:24:58)
I/flutter ( 5433): #2 _HomePage.classList (package:barcode_scan_example/home_page.dart:169:44)
I/flutter ( 5433): #3 _HomePage.build (package:barcode_scan_example/home_page.dart:265:30)
I/flutter ( 5433): #4 StatefulElement.build (package:flutter/src/widgets/framework.dart:3825:27)
I/flutter ( 5433): #5 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3736:15)
I/flutter ( 5433): #6 Element.rebuild (package:flutter/src/widgets/framework.dart:3559:5)
I/flutter ( 5433): #7 ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:3716:5)
I/flutter ( 5433): #8 StatefulElement._firstBuild (package:flutter/src/widgets/framework.dart:3864:11)
I/flutter ( 5433): #9 ComponentElement.mount (package:flutter/src/widgets/framework.dart:3711:5)
I/flutter ( 5433): #10 Element.inflateWidget (package:flutter/src/widgets/framework.dart:2956:14)
I/flutter ( 5433): #11 Element.updateChild (package:flutter/src/widgets/framework.dart:2759:12)
I/flutter ( 5433): #12 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3747:16)
I/flutter ( 5433): #13 Element.rebuild (package:flutter/src/widgets/framework.dart:3559:5)
I/flutter ( 5433): #14 BuildOwner.buildScope (package:flutter/src/widgets/framework.dart:2273:33)
I/flutter ( 5433): #15 _WidgetsFlutterBinding&BindingBase&GestureBinding&ServicesBinding&SchedulerBinding&PaintingBinding&SemanticsBinding&RendererBinding&WidgetsBinding.drawFrame (package:flutter/src/widgets/binding.dart:700:20)
I/flutter ( 5433): #16 _WidgetsFlutterBinding&BindingBase&GestureBinding&ServicesBinding&SchedulerBinding&PaintingBinding&SemanticsBinding&RendererBinding._handlePersistentFrameCallback (package:flutter/src/rendering/binding.dart:268:5)
I/flutter ( 5433): #17 _WidgetsFlutterBinding&BindingBase&GestureBinding&ServicesBinding&SchedulerBinding._invokeFrameCallback (package:flutter/src/scheduler/binding.dart:988:15)
I/flutter ( 5433): #18 _WidgetsFlutterBinding&BindingBase&GestureBinding&ServicesBinding&SchedulerBinding.handleDrawFrame (package:flutter/src/scheduler/binding.dart:928:9)
I/flutter ( 5433): #19 _WidgetsFlutterBinding&BindingBase&GestureBinding&ServicesBinding&SchedulerBinding._handleDrawFrame (package:flutter/src/scheduler/binding.dart:840:5)
I/flutter ( 5433): #23 _invoke (dart:ui/hooks.dart:209:10)
I/flutter ( 5433): #24 _drawFrame (dart:ui/hooks.dart:168:3)
I/flutter ( 5433): (elided 3 frames from package dart:async)
classList() {
StreamBuilder(
stream:
database.reference().child('user').child(userUid).child('classData').orderByKey().onValue,
builder: (BuildContext context, AsyncSnapshot<Event> snapshot) {
if (snapshot.hasData) {
if (snapshot.data.snapshot.value != null) {
Map<dynamic, dynamic> map = snapshot.data.snapshot.value;
List<dynamic> list = map.values.toList();
print("list is : $list");
return GridView.builder(
gridDelegate: SliverGridDelegateWithFixedCrossAxisCount(
crossAxisCount: 3),
itemCount: list.length,
padding: EdgeInsets.all(2.0),
itemBuilder: (BuildContext context, int index) {
print("print job ${list[index]["className"]}");
return Container(
child: RaisedButton(
onPressed: null,
child: Text(
"${list[index]["className"]}\n ${list[index]["classSection"]}"),
),
padding: EdgeInsets.all(2.0),
);
},
);
} else {
return Container(
child: Center(
child: Text(
"There's no Class registered in the system",
style: TextStyle(fontSize: 20.0, color: Colors.grey),
textAlign: TextAlign.center,
)));
}
} else {
return CircularProgressIndicator();
}
});
}
整个飞镖文件
import 'package:flutter/material.dart';
import 'auth.dart';
import 'auth_provider.dart';
import 'package:firebase_auth/firebase_auth.dart';
import 'package:flutter/services.dart';
import 'make_class.dart';
import 'dart:async';
import 'package:barcode_scan/barcode_scan.dart';
import 'package:firebase_database/firebase_database.dart';
import 'qr_screen.dart';
class HomePage extends StatefulWidget {
@override
_HomePage createState() => _HomePage();
}
enum ScaffoldType { student, lecturer }
ScaffoldType _scaffoldType = ScaffoldType.lecturer;
class _HomePage extends State<HomePage> {
final FirebaseDatabase database = FirebaseDatabase.instance;
String barcode = "";
String userUid;
var userClass;
Future<void> _signOut(BuildContext context) async {
try {
final BaseAuth auth = AuthProvider.of(context).auth;
await auth.signOut();
} catch (e) {
print(e);
}
}
@override
void initState() {
super.initState();
userUi();
setState(() async* {
FirebaseDatabase database;
database = FirebaseDatabase.instance;
database.setPersistenceEnabled(true);
database.setPersistenceCacheSizeBytes(10000000);
});
}
Future reUser() async {
FirebaseUser user = await FirebaseAuth.instance.currentUser();
print(user.uid);
var userUid = user.uid;
var userWho = await database
.reference()
.child('user')
.child(userUid)
.once()
.then((DataSnapshot snapshot) {
Map<dynamic, dynamic> data = snapshot.value;
var isL = data.values.toList();
print(isL[1]);
print(data.values);
if (isL[1].toString().toLowerCase() == "true") {
_scaffoldType = ScaffoldType.lecturer;
print('lecturer');
} else if (isL[1].toString().toLowerCase() == "false") {
_scaffoldType = ScaffoldType.student;
print("student");
}
});
// print(userWho);
}
userUi() async {
FirebaseUser user = await FirebaseAuth.instance.currentUser();
print(user.uid);
return user.uid;
}
classList(userUid) {
print(userUid);
StreamBuilder(
stream:
database.reference().child('user').child(userUid).child('classData').orderByKey().onValue,
builder: (BuildContext context, AsyncSnapshot<Event> snapshot) {
if (snapshot.hasData) {
if (snapshot.data.snapshot.value != null) {
Map<dynamic, dynamic> map = snapshot.data.snapshot.value;
List<dynamic> list = map.values.toList();
print("list is : $list");
return GridView.builder(
gridDelegate: SliverGridDelegateWithFixedCrossAxisCount(
crossAxisCount: 3),
itemCount: list.length,
padding: EdgeInsets.all(2.0),
itemBuilder: (BuildContext context, int index) {
print("print job ${list[index]["className"]}");
return Container(
child: RaisedButton(
onPressed: (){},
child: Text(
"${list[index]["className"]}\n ${list[index]["classSection"]}"),
),
padding: EdgeInsets.all(2.0),
);
},
);
} else {
return Container(
child: Center(
child: Text(
"There's no Class registered in the system",
style: TextStyle(fontSize: 20.0, color: Colors.grey),
textAlign: TextAlign.center,
)));
}
} else {
return CircularProgressIndicator();
}
});
}
@override
Widget build(BuildContext context) {
// var userUid = userUi();
reUser();
return Scaffold(
appBar: AppBar(
title: Text('Welcome'),
actions: <Widget>[
FlatButton(
child: Icon(Icons.exit_to_app),
onPressed: () => _signOut(context),
)
],
),
bottomNavigationBar: BottomAppBar(
notchMargin: 8.0,
child: Padding(
padding: const EdgeInsets.all(8.0),
child: Padding(
padding: const EdgeInsets.fromLTRB(35, 0, 35, 0),
child: Row(
mainAxisSize: MainAxisSize.max,
mainAxisAlignment: MainAxisAlignment.spaceBetween,
children: <Widget>[
Column(
mainAxisSize: MainAxisSize.min,
mainAxisAlignment: MainAxisAlignment.center,
children: buttonBelow(),
),
Column(
mainAxisSize: MainAxisSize.min,
mainAxisAlignment: MainAxisAlignment.center,
children: buttonBelow2(),
),
],
),
),
),
shape: CircularNotchedRectangle(),
),
resizeToAvoidBottomInset: true,
floatingActionButtonLocation: FloatingActionButtonLocation.centerDocked,
floatingActionButton: Container(
height: 80,
width: 80,
child: FittedBox(
child: FloatingActionButton(
onPressed: scan,
child: Icon(
Icons.camera,
size: 35,
),
elevation: 2.0,
),
),
),
body: Container(child: classList(userUi()),),
);
}
List<Widget> buttonBelow() {
if (_scaffoldType == ScaffoldType.lecturer) {
return <Widget>[
FlatButton(
child: Icon(
Icons.add_circle,
semanticLabel: "Add Class",
size: 45,
),
onPressed: () {
Navigator.push(
context,
MaterialPageRoute(builder: (context) => MakeClass()),
);
},
shape:
RoundedRectangleBorder(borderRadius: BorderRadius.circular(180)),
),
Text("Add Class")
];
} else {
return <Widget>[
Opacity(
opacity: 0,
child: FlatButton(
onPressed: () {},
child: Icon(
Icons.add_circle,
semanticLabel: "Add Class",
size: 45,
),
shape: RoundedRectangleBorder(
borderRadius: BorderRadius.circular(180)),
),
),
Opacity(opacity: 0, child: Text("Add Class"))
];
}
}
List<Widget> buttonBelow2() {
if (_scaffoldType == ScaffoldType.lecturer) {
return <Widget>[
FlatButton(
child: Icon(
Icons.blur_on,
semanticLabel: "QRCode",
size: 45,
),
onPressed: () {
Navigator.push(
context,
MaterialPageRoute(builder: (context) => QrScreen()),
);
},
shape:
RoundedRectangleBorder(borderRadius: BorderRadius.circular(180)),
),
Text("QRCode")
];
}
}
Future scan() async {
try {
String barcode = await BarcodeScanner.scan();
setState(() => this.barcode = barcode);
} on PlatformException catch (e) {
if (e.code == BarcodeScanner.CameraAccessDenied) {
setState(() {
this.barcode = 'The user did not grant the camera permission!';
});
} else {
setState(() => this.barcode = 'Unknown error: $e');
}
} on FormatException {
setState(() => this.barcode =
'null (User returned using the "back"-button before scanning anything. Result)');
} catch (e) {
setState(() => this.barcode = 'Unknown error: $e');
}
}
}
最佳答案
主要问题是当我们在 firebase 的“.child()”方法中传递空白或空字符串时，它会给我们 split("/") 错误，因为在内部它会尝试从字符串中拆分并尝试获取数据 从它。
现在您的代码中的问题是您在使用 StreamBuilder() 从 firebase userid 获取数据之前在小部件覆盖方法中调用“classList(userUid)”，但 userUid 为空，正如我上面所说的， 如果我们在该“.child()”方法中传递空白或 null，firebase 将给我们 split("/") 错误。
所以结论是我们必须等待 firebase userid，直到我们从异步结果中获取它，因为要获取 firebase userid，这是异步任务，我们必须等待它直到使用 await 关键字得到结果。
所以这是答案，如何做到这一点: 您必须使用 future<> 替换小部件中的正文部分，如下所示:
@override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text('Welcome'),
actions: <Widget>[
FlatButton(
child: Icon(Icons.exit_to_app),
onPressed: () => _signOut(context),
)
],
),
bottomNavigationBar: BottomAppBar(
notchMargin: 8.0,
child: Padding(
padding: const EdgeInsets.all(8.0),
child: Padding(
padding: const EdgeInsets.fromLTRB(35, 0, 35, 0),
child: Row(
mainAxisSize: MainAxisSize.max,
mainAxisAlignment: MainAxisAlignment.spaceBetween,
children: <Widget>[
Column(
mainAxisSize: MainAxisSize.min,
mainAxisAlignment: MainAxisAlignment.center,
children: buttonBelow(),
),
Column(
mainAxisSize: MainAxisSize.min,
mainAxisAlignment: MainAxisAlignment.center,
children: buttonBelow2(),
),
],
),
),
),
shape: CircularNotchedRectangle(),
),
resizeToAvoidBottomInset: true,
floatingActionButtonLocation: FloatingActionButtonLocation.centerDocked,
floatingActionButton: Container(
height: 80,
width: 80,
child: FittedBox(
child: FloatingActionButton(
onPressed: scan,
child: Icon(
Icons.camera,
size: 35,
),
elevation: 2.0,
),
),
),
body: new FutureBuilder<FirebaseUser>(
future: FirebaseAuth.instance.currentUser(),
builder: (BuildContext context, AsyncSnapshot<FirebaseUser> snapshot) {
if (snapshot.connectionState == ConnectionState.done) {
return new Container(child: classList(snapshot.data.uid));
}
else {
return new Text('Loading...');
}
},
),
);
}
此外，最佳实践是您必须仅在“initState()”方法中获取 firebase 用户 ID，该方法在生命周期内仅调用一次。所以在你的类中全局声明“FirebaseUser 用户”，然后调用你的“userUi()” 仅在“initState()”方法中。
关于firebase-realtime-database - flutter 如何让 gridview 像按钮一样显示 firebase 数据，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/55775499/
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