我正在开发一个餐厅应用程序，需要帮助来创建对象以充当可保存实时数据的购物车，但是，我对此有些迷失。这是我到目前为止所做的：
class Cart {
int resId;
String productImage;
CartDetails orderDetails;
Cart(this.resId, this.orderDetails, this.productImage);
}
class CartDetails {
int productId;
int quantity;
CartDetails(this.productId, this.quantity);
}
然后为了保存数据：
// //Cart Data
List<Cart> _cart = [];
List<Cart> get userCart => _cart;
Map<String, dynamic> orderDetail;
我通过以下方式向我的对象添加项目：
void addToCart(resId, proudctName, prodId, qty) {
orderDetail = {'prodId': prodId, 'quantity': qty};
try {
List<Map> list;
list.map((i) {
Cart i = Cart as Cart;
i.resId = resId;
i.productName = proudctName;
i.orderDetails.productId = prodId;
i.orderDetails.quantity = qty;
_cart.add(i);
}).toList();
print(_cart.toList());
} catch (e) {
print('Sumptin went wrong bruh');
print(e);
}
print(userCart);
}
然后我将它们全部与此一起：
addToCart(widget.resId, widget.prodName, prodId, cartVal);
当我这样做时，我得到一个错误：
NoSuchMethodError: The method 'map' was called on null.
Receiver: null
Tried calling: map<Null>(Closure: (Map<dynamic, dynamic>) => Null)
我不确定从何处将所有项目添加到地图，然后访问应用程序内不同位置的数据。
答案 0 :(得分：1)
假设您的json字符串如下所示
{
"resId":123,
"productImage":"http",
"CartDetails" :
[
{"productId":1,
"quantity":2},
{"productId":3,
"quantity":4}
]
}
代码片段以创建对象，将对象转换为json并将json字符串转换为对象
List<CartDetail> listCart = [];
listCart.add(CartDetail(productId: 1, quantity: 2));
listCart.add(CartDetail(productId: 3, quantity: 4));
Payload payload = Payload(resId: 1, productImage: "", cartDetails: listCart);
print('${payload.cartDetails[0].productId.toString()}');
String payloadStr = payloadToJson(payload);
print('${payloadStr}');
final payload1 = payloadFromJson(jsonString);
print('${payload1.cartDetails[0].productId.toString()}');
相关课程
// To parse this JSON data, do
//
// final payload = payloadFromJson(jsonString);
import 'dart:convert';
Payload payloadFromJson(String str) => Payload.fromJson(json.decode(str));
String payloadToJson(Payload data) => json.encode(data.toJson());
class Payload {
int resId;
String productImage;
List<CartDetail> cartDetails;
Payload({
this.resId,
this.productImage,
this.cartDetails,
});
factory Payload.fromJson(Map<String, dynamic> json) => Payload(
resId: json["resId"] == null ? null : json["resId"],
productImage: json["productImage"] == null ? null : json["productImage"],
cartDetails: json["CartDetails"] == null ? null : List<CartDetail>.from(json["CartDetails"].map((x) => CartDetail.fromJson(x))),
);
Map<String, dynamic> toJson() => {
"resId": resId == null ? null : resId,
"productImage": productImage == null ? null : productImage,
"CartDetails": cartDetails == null ? null : List<dynamic>.from(cartDetails.map((x) => x.toJson())),
};
}
class CartDetail {
int productId;
int quantity;
CartDetail({
this.productId,
this.quantity,
});
factory CartDetail.fromJson(Map<String, dynamic> json) => CartDetail(
productId: json["productId"] == null ? null : json["productId"],
quantity: json["quantity"] == null ? null : json["quantity"],
);
Map<String, dynamic> toJson() => {
"productId": productId == null ? null : productId,
"quantity": quantity == null ? null : quantity,
};
}
完整代码
import 'package:flutter/material.dart';
// To parse this JSON data, do
//
// final payload = payloadFromJson(jsonString);
import 'dart:convert';
Payload payloadFromJson(String str) => Payload.fromJson(json.decode(str));
String payloadToJson(Payload data) => json.encode(data.toJson());
class Payload {
int resId;
String productImage;
List<CartDetail> cartDetails;
Payload({
this.resId,
this.productImage,
this.cartDetails,
});
factory Payload.fromJson(Map<String, dynamic> json) => Payload(
resId: json["resId"] == null ? null : json["resId"],
productImage: json["productImage"] == null ? null : json["productImage"],
cartDetails: json["CartDetails"] == null ? null : List<CartDetail>.from(json["CartDetails"].map((x) => CartDetail.fromJson(x))),
);
Map<String, dynamic> toJson() => {
"resId": resId == null ? null : resId,
"productImage": productImage == null ? null : productImage,
"CartDetails": cartDetails == null ? null : List<dynamic>.from(cartDetails.map((x) => x.toJson())),
};
}
class CartDetail {
int productId;
int quantity;
CartDetail({
this.productId,
this.quantity,
});
factory CartDetail.fromJson(Map<String, dynamic> json) => CartDetail(
productId: json["productId"] == null ? null : json["productId"],
quantity: json["quantity"] == null ? null : json["quantity"],
);
Map<String, dynamic> toJson() => {
"productId": productId == null ? null : productId,
"quantity": quantity == null ? null : quantity,
};
}
void main() => runApp(MyApp());
class MyApp extends StatelessWidget {
// This widget is the root of your application.
@override
Widget build(BuildContext context) {
return MaterialApp(
title: 'Flutter Demo',
theme: ThemeData(
// This is the theme of your application.
//
// Try running your application with "flutter run". You'll see the
// application has a blue toolbar. Then, without quitting the app, try
// changing the primarySwatch below to Colors.green and then invoke
// "hot reload" (press "r" in the console where you ran "flutter run",
// or simply save your changes to "hot reload" in a Flutter IDE).
// Notice that the counter didn't reset back to zero; the application
// is not restarted.
primarySwatch: Colors.blue,
),
home: MyHomePage(title: 'Flutter Demo Home Page'),
);
}
}
class MyHomePage extends StatefulWidget {
MyHomePage({Key key, this.title}) : super(key: key);
// This widget is the home page of your application. It is stateful, meaning
// that it has a State object (defined below) that contains fields that affect
// how it looks.
// This class is the configuration for the state. It holds the values (in this
// case the title) provided by the parent (in this case the App widget) and
// used by the build method of the State. Fields in a Widget subclass are
// always marked "final".
final String title;
@override
_MyHomePageState createState() => _MyHomePageState();
}
class _MyHomePageState extends State<MyHomePage> {
int _counter = 0;
String jsonString = '''
{
"resId":123,
"productImage":"http",
"CartDetails" :
[
{"productId":1,
"quantity":2},
{"productId":3,
"quantity":4}
]
}
''';
void _incrementCounter() {
List<CartDetail> listCart = [];
listCart.add(CartDetail(productId: 1, quantity: 2));
listCart.add(CartDetail(productId: 3, quantity: 4));
Payload payload = Payload(resId: 1, productImage: "", cartDetails: listCart);
print('${payload.cartDetails[0].productId.toString()}');
String payloadStr = payloadToJson(payload);
print('${payloadStr}');
final payload1 = payloadFromJson(jsonString);
print('${payload1.cartDetails[0].productId.toString()}');
setState(() {
// This call to setState tells the Flutter framework that something has
// changed in this State, which causes it to rerun the build method below
// so that the display can reflect the updated values. If we changed
// _counter without calling setState(), then the build method would not be
// called again, and so nothing would appear to happen.
_counter++;
});
}
@override
Widget build(BuildContext context) {
// This method is rerun every time setState is called, for instance as done
// by the _incrementCounter method above.
//
// The Flutter framework has been optimized to make rerunning build methods
// fast, so that you can just rebuild anything that needs updating rather
// than having to individually change instances of widgets.
return Scaffold(
appBar: AppBar(
// Here we take the value from the MyHomePage object that was created by
// the App.build method, and use it to set our appbar title.
title: Text(widget.title),
),
body: Center(
// Center is a layout widget. It takes a single child and positions it
// in the middle of the parent.
child: Column(
// Column is also a layout widget. It takes a list of children and
// arranges them vertically. By default, it sizes itself to fit its
// children horizontally, and tries to be as tall as its parent.
//
// Invoke "debug painting" (press "p" in the console, choose the
// "Toggle Debug Paint" action from the Flutter Inspector in Android
// Studio, or the "Toggle Debug Paint" command in Visual Studio Code)
// to see the wireframe for each widget.
//
// Column has various properties to control how it sizes itself and
// how it positions its children. Here we use mainAxisAlignment to
// center the children vertically; the main axis here is the vertical
// axis because Columns are vertical (the cross axis would be
// horizontal).
mainAxisAlignment: MainAxisAlignment.center,
children: <Widget>[
Text(
'You have pushed the button this many times:',
),
Text(
'$_counter',
style: Theme.of(context).textTheme.display1,
),
],
),
),
floatingActionButton: FloatingActionButton(
onPressed: _incrementCounter,
tooltip: 'Increment',
child: Icon(Icons.add),
), // This trailing comma makes auto-formatting nicer for build methods.
);
}
}
输出
I/flutter ( 9822): 1
I/flutter ( 9822): {"resId":1,"productImage":"","CartDetails":
[{"productId":1,"quantity":2},{"productId":3,"quantity":4}]}
I/flutter ( 9822): 1