如何使用 Angular 2 在下拉列表中仅显示唯一值

我有一个 JSON 数据，我使用 *ngFor 将“accountNumber”显示到下拉列表中。由于 JSON 数据中有多个条目具有相同的帐号，因此我在下拉列表中多次看到相同的帐号。在此输入图像描述

html:

<div class="btn btn-default dropdown-toggle" type="button" 
  id="dropdownMenu" data-toggle="dropdown" aria-expanded="true">

  <span>Select</span>
  <span class="caret"></span>
  <ul class="select-menu" aria-labelledby="dropdownMenu">
    <li *ngFor="#account of accounts">{{account.accountNumber}}</li>
  </ul>
</div>

json:

`[
{
    "accountNumber": 7890,
    "transactionDate": "4/2/2016",
    "postingDate": "4/3/2016",
    "description": "Pok Pok Thai",
    "category": "Restaurants",
    "amount": 15.00
},
{
    "accountNumber": 7890,
    "transactionDate": "4/3/2016",
    "postingDate": "4/4/2016",
    "description": "Pok Pok Hai",
    "category": "Hotel",
    "amount": 25.00
},

{
    "accountNumber": 8901,
    "transactionDate": "4/6/2016",
    "postingDate": "4/7/2016",
    "description": "Pok Pok Fai",
    "category": "Dairy",
    "amount": 55.00
},
{
    "accountNumber": 8901,
    "transactionDate": "4/7/2016",
    "postingDate": "4/8/2016",
    "description": "Pok Pok Aai",
    "category": "Automotive",
    "amount": 65.00
},

{
    "accountNumber": 4567,
    "transactionDate": "4/9/2016",
    "postingDate": "4/10/2016",
    "description": "Pok Pok Cai",
    "category": "Healthcare",
    "amount": 85.00
},
{
    "accountNumber": 4567,
    "transactionDate": "4/10/2016",
    "postingDate": "4/11/2016",
    "description": "Pok Pok Dai",
    "category": "Healthcare",
    "amount": 95.00
},

{
    "accountNumber": 8901,
    "transactionDate": "4/12/2016",
    "postingDate": "4/13/2016",
    "description": "sit amet",
    "category": "Software",
    "amount": 115.00
}
 ]`

如何避免在下拉列表中显示帐号的重复值？我假设它将需要自定义管道，但不确定如何做到这一点。

我是 Angular 2 的新手，尝试寻找解决方案，但找不到任何适合我需要的东西。

已经有关于管道的基本解释后的示例：如何将过滤器应用于 *ngFor

查看适合您情况的工作插件http://plnkr.co/edit/E7HlWeNJV2N3zwPfI51Q?p=preview.. 我用过lodash图书馆及其uniqBy函数，那么管道就真的那么简单：

declare var _: any; // lodash, not strictly typed

@Pipe({
    name: 'uniqFilter',
    pure: false
})
@Injectable()
    export class UniquePipe implements PipeTransform {
        transform(items: any[], args: any[]): any {

        // lodash uniqBy function
        return _.uniqBy(items, args);
    }
}

..以及您组件中的用法：

<div>
    <ul>
        <li *ngFor="let account of accounts | uniqFilter: 'accountNumber'">{{ account.accountNumber }}</li>
    </ul>
</div>

EDIT：我已将 plunker 更新到最新的 Angular 版本，并向管道添加了过滤参数。