Understanding the Difference Between useEffect and useLayoutEffect in React Native

When building mobile apps with React Native, you often need to run side effects like fetching data, setting up event listeners, or measuring layout sizes. React Native provides two powerful hooks for handling side effects:

• useEffect
• useLayoutEffect

What is useEffect in React Native?

useEffect runs asynchronously after the component has rendered and the UI is visible to the user. It doesn't block the UI, making it ideal for non-UI-related tasks.

Common Use Cases for useEffect in React Native

• Fetching data from an API
• Managing state or side effects
• Listening for network status changes
• Subscribing to events (e.g., device orientation changes)

import React, { useEffect, useState } from 'react';
import { View, Text, Button } from 'react-native';

const ExampleScreen = () => {
  const [count, setCount] = useState(0);

  useEffect(() => {
    console.log('useEffect - Count:', count);
  }, [count]);

  return (
    <View>
      <Text>Count: {count}</Text>
      <Button title="Increment" onPress={() => setCount(count + 1)} />
    </View>
  );
};        

When useEffect Runs in React Native

• After the component renders and the screen is visible.
• Does not block UI rendering.
• Great for background tasks like data fetching.

What is useLayoutEffect in React Native?

useLayoutEffect runs synchronously after all DOM mutations (in React Native’s case, after the native view tree updates) but before the UI is painted to the screen. It blocks the UI rendering until it completes, which is useful for layout measurements and animations.

Common Use Cases for useLayoutEffect in React Native

• Measuring UI elements using onLayout or measure()
• Synchronous animations or transitions
• Dynamically adjusting UI components before rendering
• Preventing layout shifts

import React, { useLayoutEffect, useRef, useState } from 'react';
import { View, Text } from 'react-native';

const LayoutExample = () => {
  const [width, setWidth] = useState(0);
  const viewRef = useRef<View>(null);

  useLayoutEffect(() => {
    if (viewRef.current) {
      viewRef.current.measure((x, y, width) => {
        setWidth(width);
      });
    }
  }, []);

  return (
    <View>
      <View ref={viewRef} style={{ width: '80%', height: 100, backgroundColor: 'skyblue' }} />
      <Text>Width: {width}px</Text>
    </View>
  );
};        

When useLayoutEffect Runs in React Native

• After React Native updates the native view tree.
• Before the screen is drawn to the user.
• Useful for layout manipulations without flickering.

When to Use Which Hook in React Native?

? Use useEffect When:

• You are fetching data from an API.
• You are setting up listeners or subscriptions.
• You are performing side effects that do not affect the UI directly.

? Use useLayoutEffect When:

• You need to measure or manipulate UI elements before rendering.
• You are performing animations that depend on layout dimensions.
• You want to prevent layout shifts or visual flickering.