How to Fix the Cannot create an instance of class Error in ViewModel with Constructor in Android

Описание: Discover effective tips to resolve the `Cannot create an instance of class` error when using ViewModel with a constructor in your Android app.
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This video is based on the question https://stackoverflow.com/q/69657473/ asked by the user 'RaceyT' ( https://stackoverflow.com/u/17080993/ ) and on the answer https://stackoverflow.com/a/69657544/ provided by the user 'End User' ( https://stackoverflow.com/u/10033423/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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How to Fix the Cannot create an instance of class Error in ViewModel with Constructor in Android

If you're developing an Android application using Kotlin and the MVVM architecture, you might run into a frustrating issue when trying to create an instance of a ViewModel with a constructor that has parameters. The error message "Cannot create an instance of class" can leave you scratching your head and wondering what went wrong.

In this post, we will explore the root cause of this error and provide a simple solution to help you get your app back on track.

Understanding the Problem

What Happens When You Use a Constructor in ViewModel?

In Android, the ViewModel class is designed to store and manage UI-related data in a lifecycle-conscious way. However, when you create your own ViewModel with a constructor that takes parameters, like an Application instance, it complicates things. The default ViewModelProvider cannot instantiate such a ViewModel because it doesn't know how to provide the necessary parameters.

Consider the following implementation of a ViewModel:

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

And in your activity:

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

When the activity tries to retrieve the MyViewModel, it throws the error because it can't find a suitable way to create an instance of MyViewModel with the given constructor.

The Simple Solution

Use AndroidViewModel Instead

To fix the error, you can extend your ViewModel from AndroidViewModel instead of ViewModel. The AndroidViewModel is designed to accept an Application context, making it perfect for scenarios where you need the application context in your ViewModel.

Update your ViewModel class as follows:

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

Benefits of Using AndroidViewModel

Access to Application Context: Since AndroidViewModel requires an Application instance, it gives you direct access to the application context, allowing you to perform operations such as fetching the package name.

Lifecycle Awareness: Like the regular ViewModel, AndroidViewModel is also lifecycle-aware, meaning it can survive configuration changes such as screen rotations.

Conclusion

Encountering the "Cannot create an instance of class" error when working with ViewModels can be a common pitfall for Android developers, especially when constructors are involved. By extending your ViewModel from AndroidViewModel, you can easily resolve the issue while retaining access to the application context needed for your app.

Take note of this approach the next time you find yourself in a similar situation, and you'll save yourself time and frustration in your Android development journey!

Happy Coding!