Collaborating with various sequential asynchronous services

In my component, I am injecting multiple services, two of which provide lists of objects needed by a third service. The problem is that the asynchronous calls to these services are not always executed in order. Nesting the calls inside each other does not seem to be effective in every case, and it would not make sense to nest them anyway. How can I ensure that all values from my services are loaded so that I can use them properly? I attempted to load the services and their responses in the constructor, and then access those responses in ngOnInit(), but that approach did not work either.

Here is an example using Typescript:

 constructor(     
    private stateService: StatesService, 
    private zoneService: ZoneDetailService, 
    private countyService: CountyService) { 

      // Retrieve states from service
      this.stateService.GetStates().subscribe(response => {
        this.statesResults = response;
        // Perform some action with the response
       });

      // Gather counties from service
      this.countyService.GetCounties().subscribe(response => {
        this.countyResults = response;       

          // Fetch details from service
          this.zoneService.GetZoneDetails(this.prodPointId).subscribe(response => { 

            this.state = response.stateCode;                
            this.apiCountyCode = response.apiCountyCode;              

            // Filter based on the fetched data
            let answer = this.countyResults.filter(c => c.countyCode === response.apiCountyCode).map(c => {
              return c.id;
            });

            let answer2 = this.statesResults.filter(c => c.stateCode === response.stateCode).map(c => {
              return c.id;
            });

As evident from the code snippet, the stateService does not load the stateResults correctly resulting in 'undefined', causing issues with filtering. I need guidance on how to ensure that both stateService and countyService complete before proceeding with detailService. Any assistance would be highly appreciated. Thank you.

Answer №1

Hey there! If you need to run multiple observables simultaneously and wait for all of them to complete before proceeding, you can use Observable.forkJoin(). Here's an example to help illustrate:

const requests: Observable<any>[] = [];

requests.push(this.countyService.GetCounties());
requests.push(this.stateService.GetStates());          
requests.push(this.zoneService.GetZoneDetails(this.prodPointId));

Observable.forkJoin(requests)
  .take(1)
  .subscribe(results => { 
     this.countyResults = results[0]
     this.statesResults = results[1]
     this.zoneDetails = results[2]
  }

I hope this explanation is useful to you!

Answer №2

If you're looking for a solution, consider using a forkJoin method. This approach involves passing an array of observables and waiting for all of them to complete before proceeding:

import {forkJoin} from 'rxjs';
import {map} from 'rxjs/operators';

forkJoin([this.statesService.getStates(), this.countyService.getCounties(), this.zoneService.GetZoneDetails(this.prodPointId)])
    .subscribe(res => {
        // Handle the results accordingly
})

Keep in mind that directly subscribing to observables within services may not be the best practice. It's recommended to handle subscriptions within components and utilize rxjs functions within the service for better code organization.

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