Resolving null Variables in ViewModel Coroutines: Understanding Construct Injection Conflicts

Discover how to address the issue of null variables in ViewModel when using coroutines with constructor and field injection in Kotlin.
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Resolving null Variables in ViewModel Coroutines: Understanding Construct Injection Conflicts

In the world of Android development, particularly when using Kotlin and Dagger Hilt for dependency injection, developers may occasionally stumble upon perplexing problems. One such issue arises when working with API calls in a ViewModel using coroutines, specifically when you notice that an injected variable is unexpectedly null in your ViewModel's coroutine functions.

In this guide, we will dive into the root of this problem and provide an effective solution to ensure your API calls operate smoothly without encountering null values.

The Problem Defined

You may encounter a situation where you run an API call in your ViewModel using a repository object, and despite the repository being accessible in the init block of a coroutine, you face issues when trying to invoke it from a non-suspend function. Here's a breakdown of the symptoms:

Issue: The repository object (repo) is being referenced successfully in the coroutine's init block but behaves unexpectedly—appearing null when accessed later.

Workaround: The developer starts calling the repository's functions directly from exposed public functions, avoiding the init block altogether.

Example Scenario

Here is a snippet representing how things might look in code when this issue arises:

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

Unraveling the Cause

The Conflict

The primary issue here stems from the use of both constructor and field injection for the same object (i.e., the repo object). In Kotlin, this scenario can lead to undefined behaviors, including null references.

Suggested Solution

To resolve this problem, you should adopt a consistent approach for dependency injection. By sticking exclusively to constructor injection, you eliminate the potential for nullification of the injected variable. Here is a revised version of the code:

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

Why This Works

Constructor Injection: By using constructor injection consistently, you ensure that the injected repository is correctly initialized and remains non-null throughout the ViewModel's lifecycle.

Cleaner Architecture: This approach aligns with clean architecture principles by clearly defining dependencies at the constructor level.

Conclusion

When you encounter the strange behavior of injected variables being null in your ViewModel, remember to examine your dependency injection strategy. Relying solely on constructor injection for your dependencies, particularly in coroutine scenarios, will provide a stable foundation for your ViewModel logic.

This clean approach not only resolves your immediate issue but also enhances the overall quality of your code, making it easier to read and maintain. Embrace the advantages of proper dependency management in your ViewModel implementations and enjoy smoother application performance!

If you have other questions or encounters with dependency injection issues in your Android applications, feel free to share!