How to Extend List<T> in Dart?

June 20, 2024

How to make a custom List constructor in Dart? My attempt was to extend a List but it seems to not work.

The class ‘FooList’ can’t extend ‘List’ because ‘List’ only has
factory constructors (no generative constructors), and ‘FooList’ has
at least one generative constructor

Here is the context:

typedef JsonMap = Map<String, dynamic>;

typedef JsonList = List<JsonMap>;

class Foo {
  final String foo;

  Foo(this.foo);

  factory Foo.fromJson(JsonMap json) {
    return Foo(json['foo']);
  }

  JsonMap toJson() {
    return {'foo': foo};
  }
}

My goal is to fromJson parse a list of JsonMap

typedef FooList = List<Foo>;
void main() {
  final jsonList = [
    {'foo': 'bar'},
    {'foo': 'baz'},
  ];

  final foos = jsonList.map((e) => Foo.fromJson(e)).toList(); // I don't like this, too verbose

  print(foosBetter);
}

In traditional Dart development, the above is ok, but it seems to verbose. Something like this will be ideal.

final foosBetter = FooList.from(jsonList); // I like this better, but this is obviously not working

So my attempt was instead of declaring FooList as a simple alias:

abstract class FooList extends List<Foo> {
  FooList.from(List<JsonMap> jsonList) {
    addAll(jsonList.map((e) => Foo.fromJson(e)));
  }
}

Which leads to the error quoted above.

>Solution :

List in dart is an interface so you can’t just extend it.

You can either extend an implementation of List such as DelegatingList from package collection.

import 'package:collection/collection.dart';

abstract class FooList extends DelegatingList<Foo> {
  FooList.from(List<JsonMap> jsonList)
      : super(jsonList.map(Foo.fromJson).toList());
}

Or you can create an extension type on a List.

extension type FooList(List<Foo> list) implements List<Foo> {
  FooList.from(List<JsonMap> jsonList)
      : this(jsonList.map(Foo.fromJson).toList());
}

Or you can implement the List interface, but that would require providing definitions for all List methods.