How to Resolve the Cannot create an instance of class ViewModel class Error in Android MVVM

Discover how to fix the common issue of not being able to create instances of ViewModel in Android's MVVM architecture with a simple step-by-step guide.
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This video is based on the question https://stackoverflow.com/q/65523837/ asked by the user 'Sumit Shukla' ( https://stackoverflow.com/u/7254873/ ) and on the answer https://stackoverflow.com/a/65523860/ provided by the user 'ΓDΛ' ( https://stackoverflow.com/u/5595926/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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How to Resolve the Cannot create an instance of class ViewModel class Error in Android MVVM

When developing Android applications using the Model-View-ViewModel (MVVM) architecture, you may encounter the frustrating error message: Cannot create an instance of class DemoViewModel class. This issue typically arises when the ViewModel is instantiated without the necessary parameters, particularly when your ViewModel has constructors requiring dependencies.

In this guide, we’ll break down the solution to this problem into manageable steps, ensuring you can smoothly work with ViewModels in your Android app.

Understanding the Problem

In MVVM, ViewModels are intended to manage the data for a UI component and survive configuration changes such as screen rotations. However, if your ViewModel requires dependencies (like a repository), you can’t simply instantiate it with default parameters.

The Example

Let's look at the provided code:

DemoFragment.kt:

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DemoViewModel.kt:

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From the code above, you can see that DemoViewModel has a constructor that requires a DemoRepository instance. However, in DemoFragment, the code attempts to create an instance of DemoViewModel without providing any parameters.

The Solution: Use ViewModelFactory

To resolve this issue, you need to implement a custom ViewModelFactory that supplies the required dependencies when the ViewModel is created.

Step-by-Step Implementation

Create a ViewModelFactory Class
The factory class creates the ViewModel instance and supplies it with the necessary parameters.

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Use the Factory in Your Fragment
Modify the fragment code to instantiate the ViewModel using the new factory.

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Conclusion

By implementing a ViewModelFactory, you ensure that your ViewModel has access to all the necessary dependencies. The above steps will help you avoid the common Cannot create an instance of class ViewModel class error and allow you to harness the full power of the MVVM architecture in your Android applications.

Following best practices like this can significantly streamline your app development process, improve maintainability, and enhance the overall user experience.

Feel free to reach out if you have any further questions or run into additional issues while implementing ViewModels in your application!