How to Fix the FlatList Not Rerendering Issue in React Native When Data Prop Changes

Discover an effective solution to the problem of `FlatList` in React Native not rerendering despite changes in the data prop, especially using Redux.
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Understanding the FlatList Rerendering Problem in React Native

React Native is a powerful framework for building mobile applications, allowing developers to craft reliable and performant user interfaces. However, like any technology, it comes with its share of challenges. One common issue developers face is getting the FlatList component to rerender correctly when the data it's displaying changes—especially when using state management libraries like Redux.

In this post, we'll tackle a specific scenario where the FlatList does not rerender when items are removed from a list, even though the Redux state is being updated correctly. Understanding this issue and addressing it effectively can greatly enhance the performance and usability of your application.

The Problem

Consider a scenario where you have a FlatList component that displays a list of active users. You use Redux to keep track of these users. Pseudo-code might look like this:

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

You transform the Redux state into a usable array for your FlatList:

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

The issue arises when users are added or removed from the list. While the FlatList correctly updates when a user is added, it fails to rerender when a user is removed, even though Redux shows the correct updates.

Why Is Your FlatList Not Rerendering?

After thorough investigation, it appears that the problem lies within the ACTIVE_USER_CHILD_REMOVED case in your Redux reducer. Specifically, you were mutating the existing state object rather than returning a new object. This is crucial because React's rendering logic relies on detecting changes in the state.

To illustrate, the reducer you've set up for removing a user looks like this:

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

The Consequences of State Mutation

Mutating the state prevents React from recognizing that any change has occurred, leading to the failure of the FlatList to rerender. React's useState and Redux's state management require immutability to trigger updates effectively. By default, Redux only triggers updates when the references change, not when the contents change.

The Solution

To solve the problem, you'll need to modify the ACTIVE_USER_CHILD_REMOVED case in your reducer to ensure that a new state object is returned. Here's the fixed version of the code:

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

Key Changes Made

Avoiding Direct Mutation: Instead of deleting from the oldState, this solution destructures the current activeUsers state, effectively creating a new object containing the remaining users.

Returning a New Object: It ensures the returned state is always a new object, allowing Redux and React to recognize the change and trigger re-renders as needed.

Conclusion

This adjustment will ensure that the FlatList rerenders appropriately whenever users are added or removed from the active users list. When working with React Native, always remember to keep data immutable when using Redux or similar state management solutions. This not only helps in maintaining application flow but also optimizes performance by preventing unnecessary renders.

By following this guide, you should now have a clear understanding of how to troubleshoot and resolve the rerendering issue with FlatList in your React Native applications. Happy coding!