Скачать или смотреть How to Repeat a Request Until You Collect Enough Data in RxJava

Learn how to effectively use RxJava to repeatedly fetch data from an API until a set amount of items are collected.
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This video is based on the question https://stackoverflow.com/q/65291131/ asked by the user 'Tomislav Turcic' ( https://stackoverflow.com/u/1841941/ ) and on the answer https://stackoverflow.com/a/65291478/ provided by the user 'akarnokd' ( https://stackoverflow.com/u/61158/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Mastering RxJava: How to Repeat a Request Until You Collect Enough Data

In the world of Android development, working with APIs is a common task. However, sometimes fetching data from an API can be unpredictable. You may encounter situations where the API returns fewer items than expected. This can lead to unnecessary problems if your application depends on a specific amount of data. In this guide, we're going to tackle a specific scenario where you need to repeat a request to an API until a minimum number of items have been gathered.

The Problem

Consider you are developing an application that fetches data from an API. Your requirements are straightforward:

Fetch items from the API. The API can return anywhere from 0 to 10 items.

If fewer than 10 items are returned, request more items from the API.

Repeat this process up to 5 times or until you have collected 10 or more items.

This can be challenging especially when you are trying to ensure that no useful data is lost during the fetching process. For example, if the API returns 9 items followed by a request that successfully returns 10 items, you want to ensure that all 19 items are preserved, not just the latest 10.

The Initial Approach

You may have started with a simple RxJava Observable that looks something like this:

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

This setup works reasonably well, but it has a flaw: If the API returns 9 items in one request and later 10 in another, you end up discarding the initial 9 items. We need a better solution to handle this scenario.

The Proposed Solution

To overcome this challenge, you can utilize a shared list that collects all items across multiple requests. Here’s how to implement it step-by-step:

Step 1: Set Up the Collection

You can achieve this by using a Single.defer function to ensure that your collection starts fresh each time.

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

Step 2: Fetch Data with Conditional Repeat

Now, use the observable to fetch data and collect items into your list:

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

Step 3: Implement the Repeat Condition

The core of our solution lies in the repeat condition. Use repeatUntil to check both for the number of items collected and the number of attempts made:

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

Step 4: Finalize and Return the Collected List

Finally, complete the observable chain by ignoring elements and returning your list:

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

Complete Code Snippet

Combining all the steps, your complete implementation looks like this:

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

Conclusion

By applying the method above, you ensure that your application collects all necessary data while adhering to the specified conditions. This approach not only prevents data loss but also maintains a clean structure for your observables. If you're working with RxJava, mastering these patterns will help you harness the power of reactive programming more effectively. Now you can fetch data confidently, knowing that you’ll get the required amount without missing crucial items!

Feel free to reach out in the comments below if you have any questions or need further clarification about using RxJava to manage your API calls!