Flutter/Dart how to groupBy list of maps

Question

I have this list of maps.

[
    {title: 'Avengers', release_date: '10/01/2019'},
    {title: 'Creed', release_date: '10/01/2019'}
    {title: 'Jumanji', release_date: '30/10/2019'},
]

I would like to write a code that would group the list of movies by release_date like that.

[
    {'10/01/2019':  [
                  {title: 'Avengers'},
                  {title: 'Creed'}
              ]
  },

   {'30/10/2019': [
                  {title: 'Jumanji'}
              ]
 }
]

Answer 1

如果你有Dart 2.7，你可以扩展 Iterable添加一个有用的groupBy方法：

extension Iterables<E> on Iterable<E> {
  Map<K, List<E>> groupBy<K>(K Function(E) keyFunction) => fold(
      <K, List<E>>{},
      (Map<K, List<E>> map, E element) =>
          map..putIfAbsent(keyFunction(element), () => <E>[]).add(element));
}

现在，您是List of Maps，可以使用以下内容进行分组：

final releaseDateMap = listOfMaps.groupBy((m) => m['release_date'])

像这样的数据：

[
    {title: 'Avengers', release_date: '10/01/2019'},
    {title: 'Creed', release_date: '10/01/2019'}
    {title: 'Jumanji', release_date: '30/10/2019'},
]

会变成：

{
  '10/01/2019': [
    {title: 'Avengers', release_date: '10/01/2019'},
    {title: 'Creed', release_date: '10/01/2019'}
  ],
  '30/10/2019': [
    {title: 'Jumanji', release_date: '30/10/2019'},
  ]
}

Answer 2

包集合实现该groupBy功能。

对于按日期分组：

import "package:collection/collection.dart";

main(List<String> args) {
  var data = [
    {"title": 'Avengers', "release_date": '10/01/2019'},
    {"title": 'Creed', "release_date": '10/01/2019'},
    {"title": 'Jumanji', "release_date": '30/10/2019'},
  ];


  var newMap = groupBy(data, (obj) => obj['release_date']);

  print(newMap);
}

release_date要从每个地图条目中删除密钥：

var newMap = groupBy(data, (obj) => obj['release_date']).map(
    (k, v) => MapEntry(k, v.map((item) { item.remove('release_date'); return item;}).toList()));

更改密钥：

var newMap = groupBy(data, (obj) => obj['release_date']).map(
    (k, v) => MapEntry(k, v.map((item) => {'name': item['title']}).toList()));

Answer 3

我不知道为什么没有人提到如何通过基本的内置函数和方法来实现它，例如：

main() {
  List<Map> data = [
    {'title': 'Avengers', 'release_date': '10/01/2019'},
    {'title': 'Creed', 'release_date': '10/01/2019'},
    {'title': 'Jumanji', 'release_date': '30/10/2019'},
  ];

  // Loop through empty {} map and check if the release date exists, if not
  // add as the key and empty list as the value, then fill the list with element
  // itself, removing 'release_date' before adding. Then you can map the map
  // to a list of maps.
  List result = data
      .fold({}, (previousValue, element) {
        Map val = previousValue as Map;
        String date = element['release_date'];
        if (!val.containsKey(date)) {
          val[date] = [];
        }
        element.remove('release_date');
        val[date]?.add(element);
        return val;
      })
      .entries
      .map((e) => {e.key: e.value})
      .toList();

  print(result);
}

输出：

C:\Users\duoqu\Desktop>dart run test.dart
[{10/01/2019: [{title: Avengers}, {title: Creed}]}, {30/10/2019: [{title: Jumanji}]}]