Resolving LiveData Issues in Android Fragments with Shared ViewModel

Learn how to solve the problem of inconsistent `LiveData` values between fragments in your Android app by using a shared ViewModel with activity scope.
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This video is based on the question https://stackoverflow.com/q/65870598/ asked by the user 'Babanel' ( https://stackoverflow.com/u/15070557/ ) and on the answer https://stackoverflow.com/a/65870927/ provided by the user 'adnandann' ( https://stackoverflow.com/u/9147199/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Resolving LiveData Issues in Android Fragments with Shared ViewModel

When working on an Android application, you may encounter an issue where LiveData values become inconsistent when used across multiple fragments. This post will delve into a specific scenario where a score value is updated from different fragments and the inconsistencies it causes. We’ll explore the problem, explain the root cause, and provide a clear solution for maintaining consistent LiveData values in your app.

The Problem

Imagine you have an Android app with an activity that contains two fragments, each with its own button to update a score. You expect pressing the button in either fragment to update the same score value. However, you've noticed the following:

When using a button in the main activity (buttonTest), pressing it increments the score consistently.

When using a button in the fragment (button1), the score increments independently of the main activity button.

The logs indicate that LiveData is storing separate values based on where the click event is initiated.

Your activity observer does not update when the score is changed from the fragment.

This leads to confusion and unexpected results for users, as the app displays different scores based on which button is pressed.

Understanding the Cause

The root cause of this issue lies in how ViewModel scoping works in Android. By default, when you instantiate a ViewModel in a fragment using by viewModels(), it is scoped to that particular fragment. This results in each fragment having its own instance of the ViewModel, leading to independent LiveData values.

The Solution: Using Activity-scoped ViewModel

To solve this problem, you need to change the scope of your ViewModel to be shared across fragments within the same activity. Here’s how you can achieve that:

Step 1: Update the ViewModel Declaration in Fragment

To share the ViewModel across multiple fragments:

Open the fragment class where you're currently creating the ViewModel instance.

Replace the fragment-scoped ViewModel declaration with an activity-scoped one.

Here's what you need to change:

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

Using activityViewModels() allows the fragment to share the ViewModel instance with its parent activity.

Step 2: Ensure Consistent LiveData Updates

Once you've made this change, whenever you update the score from either the main activity or any fragment, the LiveData will remain consistent:

The MainActivity observer will trigger updates accordingly regardless of which button is pressed.

Both fragments will see the same score value reflected when any of the buttons is clicked.

Example Code Adjustment

Here’s a snippet of the modified fragment code for clarity:

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

Conclusion

Ensuring that you utilize a shared ViewModel through activityViewModels() is crucial for managing state across different fragments consistently. This approach removes the discrepancies in LiveData values, leading to a smoother user experience. By following the steps outlined above, you'll be able to maintain a consistent score that reflects updates from any button in your activity or fragments.

Now you can enhance your app's functionality without facing issues caused by independent fragment states. Happy coding!

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