Angular - Correctly using RXJS expand operator to make recursive http calls

问题

I am attempting to make recursive http calls to Reddit's API using a value from the previous call. The problem is that the previous call is not finished before the next one starts, so duplicate calls are being made. The "after" value should be updated for every call until the "after" value is undefined. I found this related post and have attempted to use the solution described, but I can't figure out how to make sure the previous call is finished before making the next call. Below is my actual code:

private getSavedPostsForAuthenticatedUser(username: string, after: string, userPosts: any) {
    const headers = new Headers();
    if (!userPosts) {
        userPosts = [];
    }
    headers.append('Authorization', `Bearer ${this._token}`);
    const redditUrl = `${RetainerConfig.redditOauthUrl}user/${username}/saved`;
    const url = after ? `${redditUrl}/?after=${after}` : redditUrl;
    return this._http.get(url, { headers: headers })
        .map(response => response.json())
        .expand(response => {
            if (response.data) {
                for (const post of response.data.children) {
                    userPosts.push(post);
                }
                if (response.data.after) {
                    return this.getSavedPostsForAuthenticatedUser(username, response.data.after, userPosts);
                }
            }
            return Observable.of(userPosts);
        });

回答1:

Returning the same function getSavedPostsForAuthenticatedUser will cause recursive expands. To solve this you need to separate the http observable.

  private getSavedPostsForAuthenticatedUser(username: string, after: string, userPosts: any) {
    const request$ = this._getRequest(username, after, userPosts);
    if (!userPosts) {
      userPosts = [];
    }
    return request$
      .expand(response => {
        if (response.data) {
          for (const post of response.data.children) {
            userPosts.push(post);
          }
          if (response.data.after) {
            return this._getRequest(username, response.data.after, userPosts);
          }
        }
        return Observable.of(userPosts);
      });
  }

  private _getRequest(username: string, after: string) {
    const headers = new Headers();
    headers.append('Authorization', `Bearer ${this._token}`);
    const redditUrl = `${RetainerConfig.redditOauthUrl}user/${username}/saved`;
    const url = after ? `${redditUrl}/?after=${after}` : redditUrl;

    return this._http.get(url, {headers: headers})
      .map(response => response.json());
  }

To stop the expanding you may use Observable.empty(). Please refer to this post.

来源：https://stackoverflow.com/questions/44981974/angular-correctly-using-rxjs-expand-operator-to-make-recursive-http-calls