Replicating useLayoutEffect Behavior for DOM Measurements

This challenge asks you to recreate the functionality of React's useLayoutEffect hook. Understanding useLayoutEffect is crucial for scenarios where you need to read from or write to the DOM after the browser has performed its layout calculations but before it has painted the screen. This is essential for tasks like measuring element dimensions or synchronizing with DOM changes.

Problem Description

Your task is to create a custom hook, useCustomLayoutEffect, that mimics the behavior of useLayoutEffect. This hook should accept the same arguments as useEffect: a callback function and an optional dependency array.

Key Requirements:

1. Execution Timing: The callback function provided to useCustomLayoutEffect must execute synchronously after all DOM mutations have been committed by React, but before the browser has a chance to paint. This is the defining characteristic of useLayoutEffect.
2. Cleanup Function: The callback function can optionally return a cleanup function. This cleanup function should be invoked before the effect runs again (due to dependency changes) or when the component unmounts.
3. Dependency Array: Similar to useEffect, the effect should only re-run if the values in the dependency array have changed. If no dependency array is provided, the effect should run after every render.
4. No Additional Rendering: Your hook should not introduce any extra re-renders beyond what React's core rendering process dictates.

Expected Behavior:

When a component uses useCustomLayoutEffect, the provided callback should be executed in the following sequence:

1. React commits DOM updates.
2. Browser calculates layout (e.g., element sizes, positions).
3. Your useCustomLayoutEffect callback runs.
4. Browser paints the screen.

If a cleanup function is returned, it should run before the next execution of the effect, or when the component unmounts.

Examples

Example 1: Simple DOM Measurement

import React, { useRef, useState } from 'react';
import { useCustomLayoutEffect } from './useCustomLayoutEffect'; // Assuming your hook is here

function MeasureComponent() {
  const [height, setHeight] = useState(0);
  const divRef = useRef<HTMLDivElement>(null);

  useCustomLayoutEffect(() => {
    if (divRef.current) {
      // This should measure the height *after* the DOM is updated but before paint
      setHeight(divRef.current.getBoundingClientRect().height);
      console.log('Layout effect ran, measured height:', height); // Note: height here might be stale from previous render
    }
    // No cleanup needed for this simple case
  }, []); // Empty dependency array: runs once after initial mount

  return (
    <div>
      <div ref={divRef} style={{ padding: '20px', backgroundColor: 'lightblue' }}>
        This div's height needs to be measured.
      </div>
      <p>Measured Height: {height}px</p>
    </div>
  );
}

Explanation:

The MeasureComponent uses useCustomLayoutEffect to measure the height of a div. The setHeight call inside the effect will update the state. Because useCustomLayoutEffect runs synchronously after DOM updates, the p tag displaying "Measured Height" should reflect the correct height shortly after the div is rendered, without a visible flicker or delay caused by a separate paint cycle. The console.log will demonstrate when the effect executes.

Example 2: Cleanup Function

import React, { useState } from 'react';
import { useCustomLayoutEffect } from './useCustomLayoutEffect';

function TimerComponent() {
  const [count, setCount] = useState(0);

  useCustomLayoutEffect(() => {
    console.log('Timer effect started.');
    const intervalId = setInterval(() => {
      setCount(prevCount => prevCount + 1);
    }, 1000);

    return () => {
      console.log('Timer effect cleanup.');
      clearInterval(intervalId);
    };
  }, []); // Runs once on mount, cleans up on unmount

  return (
    <div>
      <h2>Count: {count}</h2>
    </div>
  );
}

Explanation:

The TimerComponent starts an interval when it mounts. useCustomLayoutEffect ensures the interval is set up synchronously after the initial render. When the component unmounts, the cleanup function (clearInterval) is called synchronously before the browser paints the unmounted component, preventing potential memory leaks or errors.

Example 3: Dependency Array Changes

import React, { useState, useRef } from 'react';
import { useCustomLayoutEffect } from './useCustomLayoutEffect';

function ResizableComponent({ text }: { text: string }) {
  const [width, setWidth] = useState(0);
  const divRef = useRef<HTMLDivElement>(null);

  useCustomLayoutEffect(() => {
    if (divRef.current) {
      // Measure width whenever the 'text' prop changes, as that might affect width
      setWidth(divRef.current.getBoundingClientRect().width);
      console.log('Layout effect ran, measured width for text:', text);
    }
    // No cleanup needed
  }, [text]); // Re-run effect when 'text' changes

  return (
    <div>
      <div ref={divRef} style={{ border: '1px solid black', padding: '10px' }}>
        {text}
      </div>
      <p>Measured Width: {width}px</p>
    </div>
  );
}

function App() {
  const [displayText, setDisplayText] = useState("Short");

  return (
    <div>
      <button onClick={() => setDisplayText("This is a much longer piece of text")}>
        Change Text
      </button>
      <button onClick={() => setDisplayText("Short")}>
        Reset Text
      </button>
      <ResizableComponent text={displayText} />
    </div>
  );
}

Explanation:

The ResizableComponent measures its width. The useCustomLayoutEffect is configured to re-run only when the text prop changes. When the "Change Text" button is clicked, React re-renders ResizableComponent with a new text prop. useCustomLayoutEffect will then execute synchronously after the DOM is updated with the new text, measuring and displaying the new width. The console.log will show the text that triggered the effect.

Constraints

• Your useCustomLayoutEffect hook must be implemented in TypeScript.
• You must not use the actual useLayoutEffect from React. Your hook should simulate its behavior using other React hooks or primitives available in a typical React environment.
• The solution should aim for efficiency and avoid unnecessary re-renders or computations.
• Assume a standard React environment where useEffect and useRef are available.

Notes

• The core challenge lies in ensuring the effect runs synchronously after DOM updates and before paint. Think about how React's rendering pipeline works and what hooks can help you tap into those stages.
• Consider the implications of synchronous execution. Why is useLayoutEffect used instead of useEffect for certain tasks?
• Your custom hook should have the same signature as useEffect: (effect: EffectCallback, deps?: DependencyList) => void.
• An EffectCallback is a function that can optionally return a cleanup function. A DependencyList is an array of values.