Maintaining the order of subscribers during asynchronous operations can be achieved by implementing proper synchronization

In my Angular setup, there is a component that tracks changes in its route parameters. Whenever the params change, it extracts the ID and triggers a function to fetch the corresponding record using a promise. Once the promise resolves, the component updates a property for data binding purposes.

However, I have noticed instances where responses arrive out of order, causing confusion. Here's a snippet of the code:

ngOnInit() {
  this.route.params.subscribe((params) => {
    const id = params['id'];
    console.log('processing', id);
    this.service.getPerson(id).then((person) => {
      console.log('processed id', id);
      this.person = person; // issues with order here
    }
  });
}

For example, imagine I am filtering results by name in a textbox while the app navigates, fetching IDs associated with names. Sometimes, the sequence looks like this:

processing 1

processing 2

processed 2

processed 1 <---- ISSUE

How can I elegantly tackle this problem? One solution could involve transforming promises into observables and attempting cancellation, but that might be cumbersome. I'm exploring options for serializing subscriptions to prevent overlapping 'processing' operations as one completes before the next starts.

Answer №1

To efficiently handle asynchronous requests in this scenario, it is recommended to utilize either concatMap or switchMap. Using concatMap ensures that the previous request completes before proceeding with the next one. On the other hand, switchMap maintains order but will cancel processing of the current request if a newer request arrives.

ngOnInit() {
  this.route.params.pipe(
      map(params => params['id']),
      tap(id => console.log('processing', id)),
      concatMap(id => fromPromise(this.service.getPerson(id)).pipe(
        tap(person => console.log('processed id', id))
      )), 
      tap(person => this.person = person)
  )
  .subscribe();
}

Answer №2

Consider including a unique 'request identifier' for each API call, which will be sent back to you in the response. By comparing this identifier with the last processed one in the callback function, you can effectively manage and track requests.

This method may not be suitable for small requests with simple integer responses. In that case, I decided to embed my identifier within the headers and used an interceptor/filter mechanism to transmit this information back to the client with every subsequent request.

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