Question

我有一个列表，我需要按其子项列表中的最高时间戳对其进行排序。

所以流程应该是这样的：

从MainItem对象获取子项列表中的最新DateTime added
按{1的最高值排序MainItem对象

class MainItem {
  final String title;
  final List<SubItem> subItem;

  MainItem(this.title, this.subItem);

}

class SubItem {
  final DateTime added;

  SubItem(this.added);
}

我有工作代码，不确定执行多种排序是否是最有效的方法？

      final items = <MainItem>[...]
      items.sort((a, b) {
        a.subItem.sort((a, b) => b.added.compareTo(a.added));
        b.subItem.sort((a, b) => b.added.compareTo(a.added));
        return b.subItem.last.added.compareTo(a.subItem.last.added);
      });

Answer 1

import 'package:queries/collections.dart';

void main() {
  var items = col(_getMainItems());
  var query1 = items.orderBy((mi) =>
      col(mi.subItem).orderBy((si) => si.added).firstOrDefault()?.added);
  var list1 = query1.toList();
  print('Ascending');
  for (var element in list1) {
    print(element);
  }

  var query2 = items.orderByDescending((mi) =>
      col(mi.subItem).orderBy((si) => si.added).firstOrDefault()?.added);
  var list2 = query2.toList();
  print('Descending');
  for (var element in list2) {
    print(element);
  }
}

Collection<E> col<E>(Iterable<E> sequence) => Collection<E>(sequence);

List<MainItem> _getMainItems() {
  var result = <MainItem>[];  
  var day = 0;
  for (var i = 0; i < 3; i++) {
    var subItems = <SubItem>[];
    for (var j = 0; j < 3; j++) {
      var subItem = SubItem(DateTime.now().subtract(Duration(days: day++)));
      subItems.add(subItem);
    }

    var title = 'Item ${i}';
    var item = MainItem(title, subItems);
    result.add(item);
  }

  result.add(MainItem('Item w/o subitems', []));
  return result;
}

class MainItem {
  final String title;
  final List<SubItem> subItem;

  MainItem(this.title, this.subItem);

  String toString() {
    return '${title} : ${subItem}';
  }
}

class SubItem {
  final DateTime added;

  SubItem(this.added);

  String toString() {
    return '${added}';
  }
}

结果：

Ascending Item w/o subitems : [] Item 2 : [2019-03-15 20:44:07.065185, 2019-03-14 20:44:07.065185, 2019-03-13 20:44:07.065185] Item 1 : [2019-03-18 20:44:07.065185, 2019-03-17 20:44:07.065185, 2019-03-16 20:44:07.065185] Item 0 : [2019-03-21 20:44:07.063184, 2019-03-20 20:44:07.064184, 2019-03-19 20:44:07.064184] Descending Item 0 : [2019-03-21 20:44:07.063184, 2019-03-20 20:44:07.064184, 2019-03-19 20:44:07.064184] Item 1 : [2019-03-18 20:44:07.065185, 2019-03-17 20:44:07.065185, 2019-03-16 20:44:07.065185] Item 2 : [2019-03-15 20:44:07.065185, 2019-03-14 20:44:07.065185, 2019-03-13 20:44:07.065185] Item w/o subitems : []

按内部对象列表对对象列表进行排序

1 个答案: