Acessing data from instance of a class

Question

I have a question, I started learning dart/flutter, and when passing data from one screen to another I access the data like this

final orderData = ModalRoute.of(context).settings.arguments;

OrderData class looks like this

class OrderItem {
  final String id;
  final String name;
  final String date;
  final String address;
  final String recordNumber;

  OrderItem({
    @required this.id,
    @required this.name,
    @required this.date,
    @required this.address,
    @required this.recordNumber,
  });

  Map<String, dynamic> toMap() {
    return {
      'id': id,
      'name': name,
      'date': date,
      'address': address,
      'recordNumber': recordNumber,
    };
  }

  factory OrderItem.fromMap(Map<String, dynamic> map) {
    if (map == null) return null;
  
    return OrderItem(
      id: map['id'],
      name: map['name'],
      date: map['date'],
      address: map['address'],
      recordNumber: map['recordNumber'],
    );
  }

  String toJson() => json.encode(toMap());

  factory OrderItem.fromJson(String source) => OrderItem.fromMap(json.decode(source));

  @override
  String toString() {
    return 'OrderItem(id: $id, name: $name, date: $date, address: $address, recordNumber: $recordNumber)';
  }

  @override
  bool operator ==(Object o) {
    if (identical(this, o)) return true;
  
    return o is OrderItem &&
      o.id == id &&
      o.name == name &&
      o.date == date &&
      o.address == address &&
      o.recordNumber == recordNumber;
  }

  @override
  int get hashCode {
    return id.hashCode ^
      name.hashCode ^
      date.hashCode ^
      address.hashCode ^
      recordNumber.hashCode;
  }
}

My question is two fold(this is what vs code plugin generates for class). First is how can I access the data in the instance of the class(do I need for each specific getter), and second can somebody explain what toMap(), fromMap(), toJson(), fromJson(), bool operator ==(Object o), and hashCode getter do.

Answer 1

Looks like you're trying to use Flutter navigation methods.

Have a look at this blog post that explains how it works.

In summary, to "push" state to be used in the next screen, do something like this:

final arguments = OrderItem(
    id = 'id',
    name = 'name',
    date = 'date',
    address = 'address',
    recordNumber = '10',
  );
Navigator.pushNamed(context, NamedPagePassed.route, arguments: arguments);

You can later access it like this:

final OrderItem args = ModalRoute.of(context).settings.arguments;

print('The id of the order is ${args.id}');

You seem to have a whole lot of generated code in your class. The fromJson, toJson, fromMap and toMap methods are used for serialization (i.e. turn a Dart object into something that can be "transferred" to/from another language/network/etc).

The == operator and hashCode are used to check if an object instance is equal to another (which is very common in Flutter as Flutter wants to know if your UI state has been modified). hashCode allows a fast way to check that two objects are definitely not equal (you can know for sure that two objects are not equal if their hash-codes are different... if the hash-codes are equal, the objects may or may not be equal, but the probability they are NOT equal will be low because hash functions try to avoid "collisions", which is when two different objects have the same hash-code).

Hash-code and == are normally implemented together to give your class "identity". Just google around and you'll see how this all works.

I would recommend you don't use code generation from your IDE like this. Instead, get familiar with how Dart builders work, then use a codegen library that will automatically create these methods for you every time you compile (so changes to the data model are immediately reflected in the implementation of all these generated methods).

My recommendation is to use freezed for that.

Your code will be much more maintainable this way.