How can I prevent UI freeze when memoizing the renderItem function in a large FlatList?

Below are a few recommendations to try in order to stop the UI from freezing. Often, the culprit is that functions and data objects are being recreated on every render, forcing expensive re-renders—even if you’ve already memoized some components. Here are some steps and an example based on your code:

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1. Memoize Your Data

Every time your parent component renders, you call your helper function (BleSections) to generate a new array for your data. Even if the content is the same, a new reference causes FlatList to think the list has changed. Use useMemo to memoize your sections data.

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2. Memoize Your renderItem Function

Define your renderItem function outside of your rendered JSX and wrap it with useCallback to ensure that its reference stays the same between renders:

JAVASCRIPTCopied
const renderItem = useCallback(({ item }) => {
  return <BleSectionRender item={item} />;
}, []);

Even though you’ve already memoized BleSectionRender using memo, wrapping the function passed to FlatList helps avoid unnecessary function re-creations.

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3. Combine Both Data and renderItem Memoization in Your Parent

For example, your parent component might look like this:

JAVASCRIPTCopied
import React, { useCallback, useMemo } from "react";
import { FlatList } from "react-native";
import BleSectionRender from "./BleSectionRender";
import BleSections from "./BleSections";

const MyComponent = ({ bleDevices, bleDevice }) => {
  // Memoize your device extraction so that it only changes when bleDevices or bleDevice changes.
  const device = useMemo(() => {
    return Object.values(bleDevices).find(
      (device) => bleDevice && device.bleDeviceAddress === bleDevice.id
    );
  }, [bleDevices, bleDevice]);

  // Memoize your sections list; this prevents recreating the array unless 'device' changes.
  const sections = useMemo(() => {
    return BleSections({ device });
  }, [device]);

  // Memoize your renderItem function so it isn’t recreated on every render.
  const renderItem = useCallback(({ item }) => {
    return <BleSectionRender item={item} />;
  }, []);

  return (
    <FlatList
      data={sections}
      renderItem={renderItem}
      keyExtractor={(item, index) => index.toString()}
      contentContainerStyle={{ /* your style here */ }}
    />
  );
};

export default MyComponent;

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4. Additional Checks

• Key Extraction: Make sure your keyExtractor is returning truly unique keys. Even though using the index may work for a static list, a change in the data order might cause issues.
• FlatList props: In lists with many items, consider setting props like initialNumToRender, windowSize, and removeClippedSubviews to help with performance.
• Infinite re-renders: If any parent state causes an infinite loop by redefining functions or objects, it will freeze the UI. Verify that no extra state or context triggers repeated renders.

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By ensuring that both your list data (sections) and your renderItem function remain stable using useMemo and useCallback, you reduce unnecessary work in each render cycle. This should help prevent the UI freeze you are seeing.

If the problem persists after these changes, double-check any other parts of your component tree or additional logic that might be causing an infinite loop or unnecessary re-renders.