How to Collect Data Inside a Loop Using Flow and Coroutines in Kotlin

Learn how to effectively use `flow` and `coroutines` in Kotlin to collect and process data within a loop, ensuring your UI updates correctly without interruptions.
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This video is based on the question https://stackoverflow.com/q/67535464/ asked by the user 'Rafael J Silva' ( https://stackoverflow.com/u/7980826/ ) and on the answer https://stackoverflow.com/a/67535934/ provided by the user 'Róbert Nagy' ( https://stackoverflow.com/u/4487858/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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How to Collect Data Inside a Loop Using Flow and Coroutines in Kotlin

If you're venturing into the world of Kotlin and coroutines, you might find yourself facing some challenges in managing asynchronous data collection, especially when you want to gather data within a loop. One common scenario developers encounter involves collecting data from multiple sources while ensuring the UI reflects these changes accurately. In this guide, we will walk through a specific problem and provide a detailed solution using flow and coroutines.

The Problem

Let's say you have two methods: useCases.getScheduleList() which retrieves a list of schedules, and useCases.getScheduleDetails(scheduleCode) that fetches details for each schedule item. The task at hand is to gather the schedule list, loop through each item, and collect their corresponding details effectively. However, a major challenge arises — the UI gets updated before the details are actually collected. This can lead to an incomplete view and a frustrating user experience.

Understanding the Initial Attempt

Here's a simplified version of the initial code you might be using:

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

In this approach, updateUI(schedulesWithDetails) executes too early, often resulting in an empty or incomplete list being displayed.

The Solution

To resolve this, we can utilize the power of Kotlin coroutines and the async functionality, which allows for concurrent computations. This way, we can ensure all detail fetching completes before updating the UI. Let’s break down the solution step-by-step.

Step-by-Step Solution

Use map: Modify the flow to map the list of schedules and fetch the corresponding details in a non-blocking way.

Leverage async: Use async to initiate the fetching of details concurrently for each schedule without blocking the main thread.

Await Completion: Finally, utilize awaitAll() to ensure all detail requests have been completed before progressing.

Here's how your revised code should look:

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

Explanation of the Revised Code

map: The outer map collects the schedule list, while the inner map asynchronously fetches details using async. This makes the data collection concurrent.

awaitAll(): It waits for all coroutine jobs to finish before continuing. This ensures that we have all necessary detail data ready before we update the UI.

UI Update: The updateUI(schedulesWithDetails) call now occurs after all data collection is complete, ensuring a seamless user experience.

Conclusion

Using Kotlin's coroutines and flow helps streamline data collection, especially in scenarios that require fetching related data within loops. By restructuring our approach with async and awaitAll(), we can ensure our UI only updates with fully collected and prepared data. This not only improves the performance of your application but also enhances the overall user experience.

As you develop your app, consider these strategies to tackle similar challenges. Happy coding!

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