Question

我没有对象列表，例如

List<Product> products = [product1, product2, product1, product2, product1, product1]

如何从列表中获取唯一对象的列表及其编号？

import 'package:built_value/serializer.dart';
import 'package:built_value/built_value.dart';

part 'product.g.dart';

abstract class Product implements Built<Product, ProductBuilder>{
  int get id;
  String get title;
  String get image;
  double get price;
  int get volume;

  static Serializer<Product> get serializer => _$productSerializer;
  Product._();
  factory Product([updates(ProductBuilder b)]) = _$Product;
}

我想获取其他带有对象的列表：

class OrderPosition {
int id;
String title;
int count; // number of unique elements from list 'products'
}

例如：

List<OrderPosition> = [
OrderPosition(1, title1, 4),
OrderPosition(2, title2, 2)
]

Answer 1

class Product {
  Product(this.id); // for operator == to work properly
  final int id;     // make at least the id immutable

  String title;
  String image;
  double price;
  int volume;

  bool operator ==(Object other) => identical(this, other) || (other as Product).id == id;

  int get hashCode => id.hashCode;
}

var uniqueProducts = products.toSet().toList();
var result = <OrderPosition>[];
for(var i = 0; i < uniqueProducts.length; i++) {
  result.add(
    OrderPosition(i, 
                  uniqueProducts[i].title, 
                  products.where((e) => e == uniqueProducts[i]).length)));
}

Answer 2

使用Set.putIfAbsent的另一种方法是在一次迭代中收集信息：

var uniqueProductMap = Map<Product, OrderPosition>.identity();
for (var product in products) {
  var position = uniqueProductMap.putIfAbsent(product, 
     () => OrderPosition(product.id, product.title, 0);
  position.count++;
}
var uniqueProducts = uniqueProductMap.values.toList();

这使用identical来确定哪些元素是唯一的。如果要改用相等，则仅用<Product, OrderPosition>{}替换映射。如果要使用对象本身不支持的其他等效项，则可以使用：

var uniqueProductMap = LinkedHashMap<Product, OrderPosition>(
    equals: (p1, p2) => p1.id == p2.id,
    hashCode: (p) => p.id.hashCode);

（我完全支持将id用作最终比较的建议）。

Answer 3

  Map<int, OrderPosition> _positionList = Map(); //map for orders (unique products and heir number)

  List<Product> _orderList = [p1, p1, p2, p1, p3, p3, p2]; // list of products
..........
//
  void addPosition(Product product) {
    var uniqueProducts = _orderList.toSet().toList();
    for(int i = 0; i < uniqueProducts.length; i++){
      var value = OrderPosition(
          uniqueProducts[i].title,
          getCount(uniqueProducts[i].id));
      _positionList[uniqueProducts[i].id] = value;
    }
  }

如何从List <object>获取唯一对象的列表及其编号

3 个答案: