Angular 2:如何返回属于多个类的对象，扩展一个数组中的超类

Question

我正在尝试在 Angular 2-Native Script 应用程序中创建一个服务，该服务定义了新闻项数组，并具有多个对相同数据进行操作的方法。这些新闻项目可以是不同的类型，例如大，小等，每个新闻可能具有不同的参数。

我已经定义了如下接口和服务。

import { Injectable } from "@angular/core";
export interface NewsItem {
type: "big" | "small" |"referral";
title: string;
text: string;
imageUrl?: string;
}
export interface Referral extends NewsItem {
type: "referral";
username: string;
}

@Injectable()
export class NewsService {
    private data = [ 
        { type: "big", title: "This is one big item!", text: "Lorem ipsum     dolor sit amet, consectetur adipiscing elit. Phasellus iaculis, turpis vitae ornare accumsan, arcu tortor ultrices nunc, eu aliquam libero sapien vitae tellus.", imageUrl: "https://octodex.github.com/images/octocat-de-los-muertos.jpg" }, 
        { type: "small", title: "Yes, we code!", text: "Vivamus a sem eget erat feugiat hendrerit at quis massa. Nullam varius ac eros non dignissim. Fusce gravida arcu libero.", imageUrl: "https://octodex.github.com/images/baracktocat.jpg" },
        { type: "referral", title: "Yes, we code!", username:"Vasanthi Subramaniam", text: "Vivamus a sem eget erat feugiat hendrerit at quis massa. Nullam varius ac eros non dignissim. Fusce gravida arcu libero.", imageUrl: "https://octodex.github.com/images/baracktocat.jpg" }
    ]   
private getItems(): NewsItem {
        let result:NewsItem[] = this.data;
    return result;
}
}

我收到如下错误：

Type '({ type: string; title: string; text: string; imageUrl: string; } 
| { type: string; title: string...' is not assignable to type 'NewsItem[]'

如何定义数组，使其可分配给新闻项？

Answer 1

只需将对象显式强制转换为 Referral 类：

private data = [ 
    { type: "big", title: "This is one big item!", text: "Lorem ipsum     dolor sit amet, consectetur adipiscing elit. Phasellus iaculis, turpis vitae ornare accumsan, arcu tortor ultrices nunc, eu aliquam libero sapien vitae tellus.", imageUrl: "https://octodex.github.com/images/octocat-de-los-muertos.jpg" }, 
    { type: "small", title: "Yes, we code!", text: "Vivamus a sem eget erat feugiat hendrerit at quis massa. Nullam varius ac eros non dignissim. Fusce gravida arcu libero.", imageUrl: "https://octodex.github.com/images/baracktocat.jpg" },
    <Referral>{ type: "referral", title: "Yes, we code!", username:"Vasanthi Subramaniam", text: "Vivamus a sem eget erat feugiat hendrerit at quis massa. Nullam varius ac eros non dignissim. Fusce gravida arcu libero.", imageUrl: "https://octodex.github.com/images/baracktocat.jpg" }
]