Create an Object and encode data Flutter

Question

I'm working on a restaurant app and need help creating object to function as a cart that will hold live data but, I'm a little lost on this. This is what I've made so far:

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);
}

Then in order to hold the data:

// //Cart Data

List<Cart> _cart = [];

List<Cart> get userCart => _cart;
Map<String, dynamic> orderDetail;

I add items to my object with this:

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);
}

then I bring them all together with this:

          addToCart(widget.resId, widget.prodName, prodId, cartVal);

When I do that, I get an error :

NoSuchMethodError: The method 'map' was called on null.
Receiver: null
Tried calling: map<Null>(Closure: (Map<dynamic, dynamic>) => Null)

I'm not sure where to go from here to add all the items to the map and then access the data in different places within the app.

List<Map> list <==you are creating object that has null value. on next line you are trying to map items in null map.. — Max
– Max, Commented Oct 24, 2019 at 0:10
apparently its not a restaurant app: void addToCart(dispensaryId) — Max
– Max, Commented Oct 24, 2019 at 0:30

chunhunghan · Accepted Answer · 2019-10-24 08:50:11Z

Suppose your json string look like this

{
    "resId":123,
   "productImage":"http",
   "CartDetails" : 
   [
       {"productId":1,
        "quantity":2},
       {"productId":3,
        "quantity":4}
  ]
}

code snippet to create object , convert object to json and convert json string to object

    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()}');

related class

    // 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,
    };
}

full code

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.
    );
  }
}

Output

I/flutter ( 9822): 1
I/flutter ( 9822): {"resId":1,"productImage":"","CartDetails": 
 [{"productId":1,"quantity":2},{"productId":3,"quantity":4}]}
I/flutter ( 9822): 1

I get the error: Converting object to an encodable object failed: Instance of 'Cart'

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