CODEWITHSUNDEEP
angulartypescripthttpasync-awaitrxjs
Taras Rohovets
Taras Rohovets

Reputation: 31

Await recursive forkJoin in Angular

Trying to find out can I await for all http calls in a recursive forkJoin. How can I await for the all the http calls in forkJoin and receive the final result?

Currently I have something like the following

public myMethod(urls: string[]) {

...

return forkJoin(observables)
        .pipe(map(() => {
          if (json.urls !== 0) {
            return this.myMethod(json.urls);
          }
        }),   catchError((err) => {
          console.log(err);
          return of(null);
        })
        );
}

And then I subscribe ...

public mainMehtod(){
  return this.myMethod([json]).pipe(map(() => {
      ...some stuff here. But I need here the final result from the finished recursion
    }));

EDIT

Well the solution I found was, collecting all the observables in a recursion and then calling the forkjoin.

Upvotes: 1

Views: 1648

Answers (2)

cuddlemeister
cuddlemeister

Reputation: 1785

There's an expand operator, that solves your problem.

Here's how you should use it:

import { fromEvent, of, forkJoin, EMPTY } from 'rxjs';
import { expand, delay, map, reduce, last } from 'rxjs/operators';

const DATABASE: Element[] = [
  {id:'one', childId: 'three'},
  {id:'two', childId: 'four'},
  {id:'three', childId: 'five'},
  {id:'four', childId: 'six'},
  {id:'five', childId: null},
  {id:'six', childId: null}
];

mainMethod(['one', 'two']).subscribe(value => {
  console.log(value.map(e => e.id));
})

function mainMethod(ids: string[]) {
  return forkJoin(ids.map(id => getElementById(id))).pipe(
    expand(values => {
      if (values.some(el => el.childId)) {
        return forkJoin(values
          .filter(v => v.childId)
          .map(el => getElementById(el.childId))
        )
      }
      return EMPTY;
    }),
    last() // use this if you want only last emission. Be careful, last throws an error, if no value is passed
    // reduce((acc, values) => acc.concat(values), []) // use this if you want everything
  )
}

function getElementById(id: string) {
  return of(DATABASE.find(el => el.id === id)).pipe(
    delay(1000)
  )
}

interface Element {id: string, childId: string | null}

Working example https://stackblitz.com/edit/e499mk

Upvotes: 0

bryan60
bryan60

Reputation: 29325

expand is a good way to go here. tough to make it precise with the provided code but it'd look like this:

private _myMethod(urls: string[]) {

  ...

  return forkJoin(observables).pipe(
    catchError(err => {
      console.log(err)
      return of(null)
    })
  )
}

public myMethod(urls: string[]) {
  return this._myMethod(urls).pipe(
    // expand will feed the value of outter observable to start, and then call this function recursively with the result of subsequent calls inside expand
    expand(json => {
      if (json.urls.length) {
        // expand calls itself recursively
        return this._myMethod(json.urls);
      }
      return EMPTY; // break
    }),
    // expand emits outter result and then results of recursion one by one. how you collect this is up to you
    // reduce is an option to collect all values in an array like this (like a forkJoin of your forkJoins)
    // reduce((acc, val) => acc.concat([val]), []) 
    // last is an option if you only want the last recursive call (or first if none from recursion)
    // last()
    // or omit these entirely if you want the results one by one.
  )
}

Upvotes: 1

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