How to Handle Sequential AJAX Requests in Angular with Impure Observables

Learn how to effectively combine two impure observables in Angular to handle sequential AJAX requests without losing state management.
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How to Handle Sequential AJAX Requests in Angular with Impure Observables

Introduction

When working with Angular applications, you might often find yourself needing to make multiple AJAX requests that return data from a shared endpoint. This is particularly challenging when the observables you're working with are impure, meaning they rely on specific states or configurations. For instance, in a situation where one observable needs filtered data and another requires unfiltered data, you may encounter complications in state management and data flow.

In today's post, we will explore how to gracefully manage two AJAX requests using impure observables while preserving the application's state. We'll also discuss an efficient workaround using RxJS operators to ensure smooth processing of both filtered and unfiltered data.

The Problem at Hand

Let’s say you have a service that provides a saleCounts$() method, which retrieves data based on certain filters. However, it's designed in such a way that the filter's state affects its output. In this case, you might want to make two requests: one that retrieves filtered data and another that retrieves unfiltered data.

Here’s the crux of the issue:

Impure Observables: The method this.service.saleCounts$() doesn't take arguments; it relies on this.service.filters, hence the need for state management.

State Management: When you change the filters (by deleting properties like status), you need to restore them afterwards to avoid affecting further operations.

Thus, simply using combineLatest to merge two observables isn't an option since you’ll compromise your filter state.

The Solution

To handle this situation, you can use RxJS operators to control the flow of your observable chains. Specifically, you can utilize switchMap to execute the second observable only after the first one completes, allowing you to preserve the data from both requests. Here’s a structured breakdown of how you can achieve this:

Step-by-Step Approach

Store Original Filter State: Before initiating the first request, store the original state of the filter.

Delete the Necessary Filter: Temporarily remove the filter that interferes with your data request.

Make the First Data Request: Invoke the observable to retrieve filtered data.

Restore the Filter State: Use the finalize operator to ensure that your original filter state is restored after the first request concludes.

Make the Second Data Request: Utilize switchMap to initiate the second request for unfiltered data.

Combine Results: Use the map operator to combine the results from both requests into an array.

Example Code Implementation

Here’s how you could implement this in your Angular service:

[[See Video to Reveal this Text or Code Snippet]]

Conclusion

Combining two impure observables in a clean yet effective manner can indeed be challenging, but employing the right RxJS operators can help you navigate these complexities. While the solution may not seem "beautiful" due to the necessary state management, it is a practical approach to achieve your goal of handling both filtered and unfiltered data seamlessly.

With this method, you ensure that your Angular application remains responsive and maintains its state while effectively retrieving data from your service. So, next time you encounter similar issues, remember the steps we laid out and leverage the power of RxJS to your advantage!