如何跳过 .map ionic 4 observables 中的迭代

Question

我正在开发一个 Ionic 4 应用程序，它从 wordpress API 中提取 posts。我正在使用 mergeMap 和 forkjoin 获取特色图片，以便在 posts 主页上获得 posts 特色图片。

当有特色图片要获取时，以下代码工作正常，但如果没有特色图片，那么我只会从我的 throwError 中得到一个错误。我似乎无法记录错误。

ERROR Something went wrong ;)

这是我的 home.page.ts 个文件

import { Component } from '@angular/core';
import { Router } from '@angular/router';
import { LoadingController } from '@ionic/angular';
import { WordpressRestapiService, Post } from '../services/wordpress-restapi.service';

@Component({
  selector: 'app-home',
  templateUrl: 'home.page.html',
  styleUrls: ['home.page.scss'],
})
export class HomePage {

  categoryId: number;
  private posts : Post[] = [];

  constructor(
    public loadingController: LoadingController, 
    private router: Router,
    private wordpressService: WordpressRestapiService) { }

  async ngOnInit() {
    const loading = await this.loadingController.create();
    await loading.present();

    this.loadPosts().subscribe((posts: Post[]) => {
      this.posts = posts;
      loading.dismiss();
    });
  }

  loadPosts() {
    return this.wordpressService.getRecentPosts(this.categoryId);
  }

  openPost(postId) {
    this.router.navigateByUrl('/post/' + postId);
  }
}

这是我使用 wordpress 的服务文件 api

import { Injectable } from '@angular/core';
import { HttpClient } from '@angular/common/http';
import { Observable, forkJoin, throwError, empty } from 'rxjs';
import { catchError, map, mergeMap } from 'rxjs/operators';

@Injectable({
  providedIn: 'root'
})
export class WordpressRestapiService {

  baseRestApiUrl: string = 'http://example.com/wp-json/wp/v2/';

  constructor(private httpClient: HttpClient) { }

  getRecentPosts(categoryId: number, page: number = 1): Observable<any[]> {
    // Get posts by a category if a category id is passed
    let category_url = categoryId ? ("&categories=" + categoryId) : "";

    return this.httpClient.get(this.baseRestApiUrl + "posts?page=" + page + category_url).pipe(
      map((res: any) => res),
      mergeMap((posts: any[]) => {
        if (posts.length > 0) {
          return forkJoin(
            posts.map((post: any) => {
              if (post.featured_media === 0) {
                console.log('fired');
                post.media = {};
                return new Post(post);
              }
              else {
                return this.httpClient.get(this.baseRestApiUrl + "media/" + post.featured_media).pipe(
                  map((res: any) => {
                    let media: any = res;
                    post.media =  new Media(media);
                    return new Post(post);
                  }),
                  catchError(error => {
                    return throwError('Something went wrong ;)');
                  })
                );
              }
            })
          );
        }
        return empty();
      }),
      catchError(error => {
        return throwError('Something went wrong ;)');
      })
    );
  }
}

export class Post {
  author: number;
  categories: number[];
  comment_status: string;
  content: object;
  date: string;
  date_gmt: string;
  excerpt: object;
  featured_media: number;
  format: string;
  guid: object;
  id: number;
  link: string;
  media: object;
  meta: object;
  modified: string;
  modified_gmt: string;
  ping_status: string;
  slug: string;
  status: string;
  sticky: boolean;
  tags: number[];
  template: string;
  title: object;
  type: string;
  _links: object;

  constructor(values: Object = {}) {
    Object.assign(this, values);
  }
}

export class Media {
  date: string;
  date_gmt: string;
  guid: object;
  id: number;
  link: string;
  modified: string;
  modified_gmt: string;
  slug: string;
  status: string;
  type: string;
  title: object;
  author: number;
  comment_status: string;
  ping_status: string;
  meta: object;
  template: string;
  alt_text: string;
  caption: object;
  description: object;
  media_type: string;
  mime_type: string;
  media_details: object;
  post: number;
  source_url: string;

  constructor(values: Object = {}) {
    Object.assign(this, values);
  }
}

我检查了一下，如果 post.featured_media === 0 那么就 return post 否则调用 api 来获取特色图片，但这似乎从来没有 return post 秒。 console.log('fired') 被调用但 loadingCtrl 永远不会关闭并且 posts 永远不会显示。

如果不存在特色图像但 return 所有 post，我如何才能 return post.media 的空对象？

更新： 根据大卫所说，我已经更新了 WordpresRestapi 服务中的 getRecentPosts() 函数，内容如下。这给了我想要的结果。

getRecentPosts(categoryId: number, page: number = 1): Observable<any> {
// Get posts by a category if a category id is passed
let category_url = categoryId ? ("&categories=" + categoryId) : "";

return this.httpClient.get(this.baseRestApiUrl + "posts?page=" + page + category_url).pipe(
  map((res: any) => res),
  mergeMap((posts: Post[]) => {
    if (posts.length > 0) {
      return forkJoin(
        posts.map((post: Post) => {
          if (post.featured_media === 0) {
            post.media = new Media;
            return of(new Post(post));
          }
          else {
            return this.httpClient.get(this.baseRestApiUrl + "media/" + post.featured_media).pipe(
              map((res: any) => {
                post.media = new Media(res);
                return new Post(post);
              }),
              catchError(val => of(val))
            );
          }
        })
      );
    }
    return empty();
  }),
  catchError(val => of(val))
);

}

Answer 1

我认为问题在于您要将 Post 而不是 Post 的 Observable 添加到要传递给 forkJoin 的数组中。

你可以试试，

import { Observable, forkJoin, throwError, empty, of } from 'rxjs';
...
if (post.featured_media === 0) {
  console.log('fired');
  post.media = {};
  return of(new Post(post));
}

在旁注中，我会避免那样使用 catchError。它本质上是吃掉任何 javascript 错误（以及不成功的 HTTP 响应）。这就是为什么在这种情况下您没有看到有用的错误。

参考资料：（这些不是官方文档，但我发现它们更具可读性） Stack overflow 认为这些链接是代码...

https://www.learnrxjs.io/operators/combination/forkjoin.html
https://www.learnrxjs.io/operators/creation/of.html
https://www.learnrxjs.io/operators/error_handling/catch.html

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