Understanding Retrofit API Call Issues in Kotlin Flow: Why Your Function Might Not Be Triggering

Dive into the common problem of Retrofit API calls not executing within Kotlin Flow. Learn how to solve this issue with a simple and clear explanation.
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This video is based on the question https://stackoverflow.com/q/76046969/ asked by the user 'Rahul Surendran' ( https://stackoverflow.com/u/8969422/ ) and on the answer https://stackoverflow.com/a/76048779/ provided by the user 'Tenfour04' ( https://stackoverflow.com/u/506796/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Understanding Retrofit API Call Issues in Kotlin Flow

When working with APIs in Android development, particularly using Retrofit and Kotlin Flow, you might encounter a frustrating problem: your function inside the flow is not getting invoked. This often leaves developers puzzled, especially when everything else works perfectly. In this guide, we'll explore the reasons why this happens and how to resolve the issue.

The Problem at a Glance

You've set up your Retrofit API calls and are utilizing Kotlin Flow to handle the responses. However, during execution, the code inside your flow isn't being executed as expected. As a result, you are not able to retrieve any API data, while a regular Retrofit call would work fine.

Sample Execution Flow

To illustrate the problem, let's take a look at the specific code structure you're working with:

ViewModel Execution

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Use Case

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Repository and Data Source

The repository fetches from remote sources, and the key part is how you're returning a Flow of Result.

Understanding the Solution

The issue arises in your AppConfigRemoteDataSourceImpl.toResultFlow() method. Let's break down why the function isn't being triggered and how to fix it.

Identifying the Core Issue

In your current implementation, the flow is set up to only emit a loading state:

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This means that while your Flow starts executing, it never completes. It never calls apiService.getAppConfig(), which is the root of your problem.

The Fix

You need to modify the toResultFlow function to include the actual API call within the flow. Here’s how to implement it correctly:

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Suggestions for Improvement

Use Flow Result T Instead of Flow Result T ? :

It's a good idea to return Flow<Result<T>> instead of Flow<Result<T>?>. Using nullable types can lead to complexities and potentially make it difficult for collectors to manage their outputs effectively.

Summary

To sum up, ensuring that your flow emits both a loading state and the result from the API call is essential in making your Retrofit function work correctly inside Flow. By simply modifying how you set up your flow to include the API call, you can fix this issue and ensure that your application retrieves data as expected.

If you encounter similar challenges, remember to trace through your flow and check for any missing calls that may inhibit execution. Happy coding!

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