class 'List' 没有未命名的构造函数

Question

I am trying to write a model but i keep getting this error. Here is the code.

import 'package:flutter/material.dart';

class CardModel {
late List<CardResults> results;

CardModel({ required this.results});

CardModel.fromJson(Map<String, dynamic> json) {
if (json['cardResults'] != null) {
  results = List<CardResults>();
  json['cardResults'].forEach((v) {
    results.add(CardResults.fromJson(v));
  });
  }
 }
}

Answer 1

List<CardResults>() is no longer valid Dart code as of Dart 3.0. See https://github.com/dart-lang/sdk/issues/49529 for details.

You can instead use <CardResults>[] . But personally, I prefer creating the completed list at the declaration, rather than creating an empty list and building it up.

Answer 2

First, List type doesn't have a default constructor so doing List() is not possible. So, instead you can create an empty list using <CardResults>[] or List<CardResults>.empty() .

You can learn more about available constructors for List type here .

---

Second, you can optimize your code and instead of creating an empty list and calling add for each item you can use map method, like:

CardModel.fromJson(Map<String, dynamic> json) {
  if (json['cardResults'] != null) {
    results = json['cardResults'].map<CardResults>((v) => CardResults.fromJson(v)).toList();
  }
}

You can learn more about map method here .

---

Third, instead of marking a property as late you can make it final and use factory constructor to instantiate an object. Finally, your CardModel should look like this:

class CardModel {
  final List<CardResults> results;

  CardModel({required this.results});

  factory CardModel.fromJson(Map<String, dynamic> json) {
    return CardModel(
      results: json['cardResults']?.map<CardResults>((v) => CardResults.fromJson(v)).toList() ?? [],
    );
  }
}

You can learn more about factory constructors here .

Answer 3

Since you are getting the list of results from a JSON, you can map get the model list like this:

class CardModel {
//assuming your list is a list of strings
List<String> results;

CardModel({required this.results});

factory CardModel.fromJson(Map<String, dynamic> json) {
  if (json['cardResults'] != null) {
  return CardModel(
      results: List<String>.from(json["cardResults"].map((x) => x)));
 }
 // return an empty list if cardResults is empty
 return CardModel(results: []);
 }
}