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.

Solution

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

Comments

[Max]

List<Map> list <==you are creating object that has null value. on next line you are trying to map items in null map..

[Max]

apparently its not a restaurant app: void addToCart(dispensaryId)

[Doing Things Occasionally]

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