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Learn how to effectively manage state in Jetpack Compose using ViewModel, ensuring data persistence across navigation.
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This video is based on the question https://stackoverflow.com/q/73186300/ asked by the user 'Brian' ( https://stackoverflow.com/u/461631/ ) and on the answer https://stackoverflow.com/a/73186424/ provided by the user 'Ma3x' ( https://stackoverflow.com/u/2422224/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Managing State in Jetpack Compose with ViewModel

When building Android apps with Jetpack Compose, managing state effectively becomes crucial, especially when navigating between different screens. One common issue faced by developers is the recreation of composable functions, which can lead to unnecessary data retrieval and processing. In this guide, we will tackle a specific problem: how to save data in a composable function using a ViewModel.

The Problem

Imagine you have a composable function for the home screen of your app. Each time you navigate away from the home screen and then return, the home screen gets recreated. This results in the app calling a method to retrieve data every time you re-enter the screen, which can be inefficient and problematic.

Key Questions

Do I need to move the ViewModel somewhere higher up in the hierarchy?

How can I ensure that I don't trigger data retrieval again upon returning to the home screen?

These questions are common among Android developers working with Jetpack Compose and ViewModel.

The Solution

To solve this problem, we need to ensure that the ViewModel retains its state even after the composable function is recreated. Below are the steps to accomplish this:

Step 1: Correctly Initialize Your ViewModel

Make sure you are retrieving your ViewModel correctly. By calling viewModel() within your composable, you ensure that you're accessing the same instance as long as the ViewModel's lifecycle owner is unchanged.

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By using viewModel() this way, your ViewModel instance persists during navigation as long as it’s scoped to the same NavController.

Step 2: Avoid Unnecessary Data Calls

The code provided in the original question suggests that data retrieval happens every time the HomeScreen is displayed. To avoid this, implement a mechanism that tracks whether the data has already been fetched:

Add a Flag in the ViewModel: To track if the data has already been fetched, consider adding a fetched state variable in your ViewModel.

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Use remember() in Composable: If you still need to track it within the composable, leverage remember combined with your ViewModel's state to ensure you don’t fetch data unnecessarily.

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Step 3: Preserve the ViewModel Across Navigation

Ensure that your ViewModel is tied to the NavGraph:

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By following these steps, you will ensure that your ViewModel maintains its state even when navigating back and forth between composables.

Conclusion

Managing state in Jetpack Compose is all about understanding the lifecycle of your components and effectively utilizing the ViewModel architecture. By following the practices detailed above, you can significantly reduce redundant data fetching and provide a smoother user experience. Remember, always scope your ViewModel appropriately and manage state efficiently!

With the right strategies and understanding, you can ensure that your composables behave intuitively, letting users navigate your app seamlessly.