Nested Observable with unknown nesting depth

问题

When sending a Get request to Sharepoint URI (e.g. http://company.com.au/teams/Telstra/_api/web/Lists/getbytitle('${listTitle}')/items), it returns a finite number of items (e.g. 30) and provides a link to query to retrieve the next 30 items like so:

I am writing an Angular app to retrieve all items. The pseudo code is like so:

public GetByDomainOwner(listTitle: string, domainNameOwner: string): Observable<any[]> {

    let dataToReturn = new Array<any>();

    let URI = `http://company.com.au/teams/Telstra/_api/web/Lists/getbytitle('${listTitle}')/items?$filter=DomainOwnerText eq '${domainNameOwner}'`;

    var headers: HttpHeaders = new HttpHeaders();
    headers.append('Accept', 'application/json;odata=verbose');

    while(URI !== undefined){
        const records = this.http.get(URI, {
            headers: headers,
            withCredentials: true
        });

        records.subscribe(r => {
            let data = <SharepointItem> r;

            dataToReturn.push(data.d.results);

            if(data.d.__next){
                URI = data.d.__next;
            } else{
                URI = undefined;
            }
        });
    }       

    console.log("Escaped while loop");

    return new Observable<any[]>(dataToReturn); // This is invalid obviously, but it describes how I want to make one Observable call to retrieve all data.
}

I want to make the above one single Observable call where it will read and subscribe to d.__next to retrieve the next finite number of items

Using Observable flatmap seems to be quite promising for situations where the depth of nesting is known (e.g. C subscribes to B's stream, of which B subscribes to A's stream). I want to make a flatMap (open to other suggestions) that can repeatedly check and subscribe to more streams without specifying a depth.

回答1:

As @KiraAG commented, expand is probably what you want.

This blog post should have the details you are after: https://blog.angularindepth.com/rxjs-understanding-expand-a5f8b41a3602

The key part is:

import { empty } from "rxjs/observable/empty";
import { concatMap, expand } from "rxjs/operators";
import { get } from "./get";

const url = "https://api.github.com/users/sindresorhus/repos";
const repos = get(url).pipe(
  expand(({ next }) => next ? get(next) : empty()),
  concatMap(({ content }) => content)
);
repos.subscribe(repo => console.log(repo));

where get(url) returns an observable with the ajax results (content) and the "next" url (to get the next results - held in __next in your example)

来源：https://stackoverflow.com/questions/56520156/nested-observable-with-unknown-nesting-depth

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typescript

rxjs