How to Fix RuntimeException when Accessing ViewModel in Fragments with MVVM in Android

Описание: Struggling with a `RuntimeException` when trying to access a ViewModel instance across fragments and activities in Android? This guide explains the solution and why it works, breaking down the necessary steps to resolve your issue effectively.
---
This video is based on the question https://stackoverflow.com/q/74548705/ asked by the user 'Gowtham K K' ( https://stackoverflow.com/u/9248098/ ) and on the answer https://stackoverflow.com/a/74549543/ provided by the user 'cactustictacs' ( https://stackoverflow.com/u/13598222/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: Caused by: java.lang.RuntimeException: Cannot create an instance of class com.app.MyViewModel

Also, Content (except music) licensed under CC BY-SA https://meta.stackexchange.com/help/l...
The original Question post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license, and the original Answer post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license.

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---
Understanding the RuntimeException in ViewModel Access

If you've found yourself facing the error, "java.lang.RuntimeException: Cannot create an instance of class com.app.MyViewModel" when attempting to access your ViewModel from a fragment in an Android application, you're not alone. This situation often arises when implementing the MVVM (Model-View-ViewModel) design pattern with Kotlin. In this guide, we'll dive into what's causing this issue and how to effectively resolve it.

The Problem: ViewModel Instantiation Error

When working with activities and fragments in Android, it's common to share a single instance of a ViewModel. However, this may lead to problems if the ViewModel requires a factory for instantiation. You might encounter this error primarily due to:

Fragment Accessing First: If your fragment requests the ViewModel instance before the activity accesses it, and the factory isn't provided in the fragment, it defaults to a no-argument constructor, causing the error.

Factory Misconfiguration: Your custom ViewModel needs a factory constructor that matches your parameters, and if this is missing at first request, you'll face issues.

The Solution: Ensuring Proper ViewModel Initialization

Let's break down the solution into clear steps to ensure that you can access your ViewModel without running into exceptions.

1. Use a ViewModelFactory

To provide the necessary parameters for your ViewModel, you need to create a ViewModelFactory. In the provided code, you've already created MyViewModelFactory that takes PaymentDataModel as an argument.

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

2. Requesting the ViewModel in Activity

When you define your ViewModel in the activity, make sure to use the factory just as you have done:

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

3. Access the ViewModel in Fragment

The same factory usage applies to your fragment. Use the activityViewModels delegate and provide the factory:

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

4. Understand ViewModel Lifecycle

It's essential to understand how ViewModels work in Android. When you request a ViewModel:

Each Activity and Fragment can have their own single instance of a ViewModel.

If a ViewModel has not been previously created for the owner, the factory will be called to instantiate it.

5. Why It Matters

If the fragment accesses the ViewModel before the activity, and the factory isn't provided, the system attempts to create a default instance, which leads to the runtime error due to the missing parameters.

Conclusion

Following these steps will help ensure that your ViewModel access works smoothly without running into instantiation issues. The key takeaway is to always provide the factory when you're defining or accessing the ViewModel, regardless of whether it's in an Activity or a Fragment.

A Final Word

Feel free to try out these solutions in your own projects to see how they improve your implementation of the Android MVVM pattern. Happy coding!