如何解决颤振Dart-NoSuchMethodError:方法“ map”在null上被调用
在构建时收到错误:
NoSuchMethodError:方法“ map”在null上被调用。 接收者:null 尝试致电: 地图(关闭:Transaction =>容器)
════════ Exception caught by widgets library ═══════════════════════════════════════════════════════
The following NoSuchMethodError was thrown building TransactionList(dirty):
The method 'map' was called on null.
Receiver: null
Tried calling: map<Container>(Closure: (Transaction) => Container)
The relevant error-causing widget was:
TransactionList file:///C:/.../AndroidStudioProjects/money_tracker/lib/Widgets/user_transactions.dart:44:9
When the exception was thrown,this was the stack:
#0 Object.noSuchMethod (dart:core-patch/object_patch.dart:53:5)
#1 TransactionList.build (package:money_tracker/models/transaction_list.dart:14:30)
#2 StatelessElement.build (package:flutter/src/widgets/framework.dart:4576:28)
#3 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4502:15)
#4 Element.rebuild (package:flutter/src/widgets/framework.dart:4218:5)
...
════════════════════════════════════════════════════════════════════════════════════════════════════
我相信该错误与user_transactions.dart文件中的TransactionList(_userTransactions)行有关,但不确定如何解决。
代码:
来自main.dart:
import 'package:flutter/cupertino.dart';
import 'package:flutter/material.dart';
import 'package:gradient_app_bar/gradient_app_bar.dart';
import 'package:money_tracker/Widgets/user_transactions.dart';
void main() {
runApp(Home());
}
class Home extends StatelessWidget {
@override
Widget build(BuildContext context) {
return MaterialApp(
home: Scaffold(
appBar: GradientAppBar(
centerTitle: true,gradient: LinearGradient(
colors: [Colors.green,Color(0xff01655e)],),title: Text(
'Money Tracker',style: TextStyle(color: Colors.white),// backgroundColorStart: Color(0xff01655e),actions: <Widget>[
Icon(Icons.add),],body: Column(
mainAxisAlignment: MainAxisAlignment.start,children: <Widget>[
Container(
width: double.infinity,margin: EdgeInsets.all(15.0),child: Card(
//Card is a freestyle container
color: Color(0xff01655e),child: Text('Chart'),elevation: 5.0,UserTransactions(),);
}
}
来自user_transactions.dart:
import 'package:flutter/material.dart';
import 'package:money_tracker/models/transaction_list.dart';
import 'new_transaction.dart';
import '../models/transaction.dart';
class UserTransactions extends StatefulWidget {
@override
_UserTransactionsState createState() => _UserTransactionsState();
}
class _UserTransactionsState extends State<UserTransactions> {
final List<Transaction> _userTransactions = [
Transaction(
id: '1',title: 'course',amount: 11.99,dateTime: DateTime.now(),Transaction(
id: '2',title: 'milk',amount: 2.87,];
void _addNewTransaction(String txTitle,double amount) {
final newTx = Transaction(
title: txTitle,amount: amount,id: DateTime.now().toString());
setState(() {
_userTransactions.add(newTx);
});
}
@override
Widget build(BuildContext context) {
return Column(
children: <Widget>[
NewTransaction(_addNewTransaction),TransactionList(_userTransactions),);
}
}
从transaction_list.dart:
import 'package:flutter/material.dart';
import 'transaction.dart';
import 'package:intl/intl.dart';
class TransactionList extends StatelessWidget {
TransactionList(List<Transaction> userTransactions,{this.transactions});
final List<Transaction> transactions;
@override
Widget build(BuildContext context) {
return Column(
children: transactions.map((tx) {
return Container(
margin: EdgeInsets.fromLTRB(20.0,10.0,20.0,10.0),padding: EdgeInsets.all(0.5),decoration: BoxDecoration(
border: Border.all(color: Colors.grey,width: 2.0),borderRadius: BorderRadius.all(
Radius.circular(15.0),child: Row(
mainAxisAlignment: MainAxisAlignment.spaceEvenly,children: <Widget>[
Container(
child: Text(
'\$${tx.amount}',style: TextStyle(
fontWeight: FontWeight.bold,fontSize: 20.0,Column(
crossAxisAlignment: CrossAxisAlignment.start,children: <Widget>[
Text(tx.title,style: TextStyle(
fontWeight: FontWeight.bold,fontSize: 18.0)),Text(
DateFormat().format(tx.dateTime),style: TextStyle(color: Colors.grey),)
],);
}).toList(),);
}
}
来自transaction.dart:
class Transaction {
Transaction(
{@required this.id,@required this.title,@required this.amount,@required this.dateTime});
final String id;
final String title;
final double amount;
final DateTime dateTime;
}
来自new_transaction.dart:
class NewTransaction extends StatelessWidget {
final Function addTx;
final title = TextEditingController();
final amount = TextEditingController();
NewTransaction(addNewTransaction,{this.addTx});
@override
Widget build(BuildContext context) {
return Card(
elevation: 5.0,child: Container(
width: double.infinity,child: Column(
children: <Widget>[
Text(
'Add New Transaction',style: TextStyle(
fontWeight: FontWeight.bold,TextField(
decoration: InputDecoration(labelText: 'Title'),controller: title,// onChanged: (value) {
// title = value;
// },TextField(
decoration: InputDecoration(labelText: 'Amount'),controller: amount,// onChanged: (value) {
// amount = value;
// },Padding(
padding: EdgeInsets.all(8.0),child: FlatButton(
padding: EdgeInsets.all(10.0),color: Colors.green,onPressed: () {
addTx(
title.text,double.parse(amount.text),);
},child: Text(
'Add',style: TextStyle(
color: Colors.white,fontWeight: FontWeight.bold,)
],);
}
}
解决方法
该错误表示transactions
是null
。您没有将任何内容传递给该参数。
您的TransactionList
构造函数有2个参数,但是仅使用了一个。更改为
TransactionList(List<Transaction> userTransactions,{this.transactions});
到
TransactionList(this.transactions);
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