Implementing a Visibility Sensor in React Native with FlatList

What is a Visibility Sensor?

A visibility sensor is a feature that detects when a component appears or disappears on the screen. This can be useful for tracking user behavior, such as which components are viewed the most, or for triggering events when a component comes into view.

Using FlatList for Visibility Detection

FlatList is a built-in React Native component that provides an efficient way to render large lists of data. It also provides a callback function, onViewableItemsChanged, which is called whenever the visibility of list items changes. We can use this callback function to detect when a component appears or disappears on the screen.

Limitations of the FlatList API

While the onViewableItemsChanged callback function is useful for detecting visibility changes, it has some limitations. The function must be stable, meaning it cannot rely on any dependencies that may change during the app’s lifecycle. If the function is recreated, it will cause a render error.

Workarounds for Common Issues

• Wrap the callback function in a useCallback Hook: By wrapping the callback function in a useCallback Hook with an empty dependency array, we can ensure that the function is only created once and is not recreated during the app’s lifecycle.
• Use the state updater function: If we need to use a state variable inside the callback function, we can use the state updater function to access the previous state and avoid relying on dependencies.
• Ignore ESLint warnings: If we are using a custom Hook or a pure function from another file, we can ignore ESLint warnings about missing dependencies.

Example Use Case

import React, { useState, useCallback } from 'react';
import { FlatList } from 'react-native';

const VisibilitySensor = () => {
  const [visibleItems, setVisibleItems] = useState([]);

  const onViewableItemsChanged = useCallback((info) => {
    const visibleItems = info.viewableItems.map((item) => item.item);
    setVisibleItems(visibleItems);
  }, []);

  return (

}
      onViewableItemsChanged={onViewableItemsChanged}
    />
  );
};

In this example, we use the onViewableItemsChanged callback function to detect when a component appears or disappears on the screen. We then update the state with the visible items using the state updater function.