Скачать или смотреть How to Set Listener in State for Firestore in React Native

Learn how to properly handle onSnapshot listeners with Firestore in React Native, ensuring effective state management and correct unsubscription.
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This video is based on the question https://stackoverflow.com/q/70140411/ asked by the user 'showtime' ( https://stackoverflow.com/u/17211349/ ) and on the answer https://stackoverflow.com/a/70140784/ provided by the user 'danh' ( https://stackoverflow.com/u/294949/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Understanding Firestore Listeners in React Native

Google's Firestore is an excellent database option, particularly for real-time applications where you want your app to react to changes immediately. One of the features Firestore provides is the ability to listen to changes in your database with onSnapshot listeners. However, managing these listeners can be tricky, especially when incorporating them into your application's state management. In this guide, we'll explore how to set a listener in the state correctly and how to unsubscribe from it.

The Problem: Unsubscribing from the Listener

The issue arises when you set up a listener but struggle with unsubscribing from it after it's no longer needed. The original code snippet shows that a listener is created, but when it's time to unsubscribe, it fails because it’s not being invoked properly.

Code Breakdown

Here's a simplified explanation of how the original setup was implemented:

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

In the code snippet above:

A Firestore query is being created but not directly associated with the unsubscribe function.

this.fetchChatsListener is a query, not the actual unsubscribe function.

When attempting to unsubscribe using:

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

You encounter a TypeError: this.fetchChatsListener is not a function, highlighting that the listener was not stored correctly.

The Solution: Correctly Implementing the Listener

Step 1: Set Up the Listener Correctly

Instead of assigning the query directly to this.fetchChatsListener, you need to store the unsubscribe function returned by the onSnapshot() method.

Here’s how to do it:

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

Step 2: Ensure Proper Unsubscription

When you want to unsubscribe from the listener, you must call it as a function (using parentheses). Here’s the correct way to set up your unsubscription method:

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

Key Points to Remember

Make sure to store the unsubscribe method that onSnapshot() returns.

Always call that unsubscribe method when you want to stop listening to Firestore changes.

Implement checks in your unsubscribe method to ensure you are calling it only if it has been previously set.

Conclusion

By properly managing your Firestore listener and ensuring that you're invoking the unsubscribe function correctly, you can maintain a clean and efficient application state. This practice not only optimizes the performance of your React Native app but also enhances user experience by avoiding unnecessary updates.

Feel free to try out this implementation and fine-tune it to fit your application's needs! If you have further questions or need examples, don't hesitate to reach out.