Angular 2中异步可观察和管道上的安全导航操作员 [英] Safe navigation operator on an async observable and pipe in Angular 2

问题描述

在异步加载的可观察对象上使用安全导航运算符时，我遇到了将空值(而不是讲座数组)传递给管道的问题:

I'm running into a problem with a null value (instead of an array of Lectures) being passed to a pipe when using the safe navigation operator on an async loaded observable:

<div *ngFor="let lecture of ((lecturesObservable | async)?.lectures | lectureType: 'main')" class="list-group-item">

lecture-type.pipe.ts

lecture-type.pipe.ts

import { Pipe, PipeTransform } from '@angular/core';

import { Lecture } from './lecture';

@Pipe({name: 'lectureType'})
export class LectureTypePipe implements PipeTransform {
    transform(allLectures: Lecture[], lectureType: string): Lecture[]{
        return allLectures.filter(lecture => lecture.type==lectureType);
    }
}

一旦异步加载了这些讲义，就不用管道就可以反复进行这些讲义.这只是我在ng2中必须忍受的东西吗?

The lectures are iterated through fine without the pipe, once they are loaded by async. Is this just something I have to live with in ng2?

推荐答案

当异步管道的输入可观察到的值尚无值时，该管道将根据设计解析为null.所以是的，您将不得不通过设计管道来处理空输入来忍受它.

The async pipe resolves to null by design when its input observable does not yet have a value. So yes, you will have to live with it, by designing your pipe to handle a null input.

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