react-performance（React 性能）

React Performance Optimization Skill

Overview

Master React performance optimization techniques including memoization, code splitting, lazy loading, and profiling tools.

Learning Objectives

• Identify performance bottlenecks
• Use React DevTools Profiler
• Implement memoization strategies
• Apply code splitting techniques
• Optimize bundle size

React DevTools Profiler

Profiling Steps

1. Open React DevTools
2. Go to Profiler tab
3. Click record button
4. Interact with app
5. Stop recording
6. Analyze flame graph

Reading Results

• Render duration: Time spent rendering
• Commits: Number of renders
• Flame graph: Visual render tree
• Ranked chart: Components by render time

Memoization

React.memo

// Prevent re-render if props unchanged
const ExpensiveComponent = React.memo(function ExpensiveComponent({ data }) {
  return <div>{data.value}</div>;
});

// Custom comparison
const Component = React.memo(
  ({ user }) => <div>{user.name}</div>,
  (prevProps, nextProps) => {
    return prevProps.user.id === nextProps.user.id;
  }
);

useMemo

function ProductList({ products, searchTerm }) {
  // Only re-filter when dependencies change
  const filteredProducts = useMemo(() => {
    console.log('Filtering...');
    return products.filter(p =>
      p.name.toLowerCase().includes(searchTerm.toLowerCase())
    );
  }, [products, searchTerm]);

  return <div>{filteredProducts.map(renderProduct)}</div>;
}

useCallback

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

  // Stable function reference
  const handleClick = useCallback(() => {
    setCount(c => c + 1);
  }, []);

  return <MemoizedChild onClick={handleClick} />;
}

const MemoizedChild = React.memo(({ onClick }) => {
  console.log('Child rendered');
  return <button onClick={onClick}>Click</button>;
});

Code Splitting

Route-Based Splitting

import { lazy, Suspense } from 'react';
import { Routes, Route } from 'react-router-dom';

const Home = lazy(() => import('./pages/Home'));
const Dashboard = lazy(() => import('./pages/Dashboard'));
const Settings = lazy(() => import('./pages/Settings'));

function App() {
  return (
    <Suspense fallback={<LoadingSpinner />}>
      <Routes>
        <Route path="/" element={<Home />} />
        <Route path="/dashboard" element={<Dashboard />} />
        <Route path="/settings" element={<Settings />} />
      </Routes>
    </Suspense>
  );
}

Component-Based Splitting

function ProductPage() {
  const [showReviews, setShowReviews] = useState(false);

  const Reviews = lazy(() => import('./Reviews'));

  return (
    <div>
      <ProductInfo />
      <button onClick={() => setShowReviews(true)}>
        Show Reviews
      </button>
      {showReviews && (
        <Suspense fallback={<Spinner />}>
          <Reviews />
        </Suspense>
      )}
    </div>
  );
}

List Virtualization

React Window

import { FixedSizeList } from 'react-window';

function VirtualList({ items }) {
  return (
    <FixedSizeList
      height={600}
      itemCount={items.length}
      itemSize={50}
      width="100%"
    >
      {({ index, style }) => (
        <div style={style}>
          {items[index].name}
        </div>
      )}
    </FixedSizeList>
  );
}

Image Optimization

Lazy Loading

function LazyImage({ src, alt }) {
  const [isLoaded, setIsLoaded] = useState(false);
  const [isInView, setIsInView] = useState(false);
  const imgRef = useRef();

  useEffect(() => {
    const observer = new IntersectionObserver(
      ([entry]) => {
        if (entry.isIntersecting) {
          setIsInView(true);
          observer.disconnect();
        }
      },
      { threshold: 0.1 }
    );

    if (imgRef.current) observer.observe(imgRef.current);

    return () => observer.disconnect();
  }, []);

  return (
    <div ref={imgRef}>
      {isInView && (
        <img
          src={src}
          alt={alt}
          onLoad={() => setIsLoaded(true)}
          style={{ opacity: isLoaded ? 1 : 0 }}
        />
      )}
    </div>
  );
}

Bundle Optimization

Tree Shaking

// Good: Named imports
import { Button, Input } from 'components';

// Bad: Import everything
import * as Components from 'components';

Dynamic Imports

async function exportData() {
  const XLSX = await import('xlsx');
  // Use XLSX only when needed
}

Bundle Analysis

# Create React App
npm install --save-dev source-map-explorer
npm run build
npx source-map-explorer 'build/static/js/*.js'

# Vite
npm install --save-dev rollup-plugin-visualizer

Common Mistakes

// ❌ Overusing useMemo
const greeting = useMemo(() => `Hello ${name}`, [name]);

// ✅ Just compute
const greeting = `Hello ${name}`;

// ❌ Inline objects in deps
useEffect(() => {
  fetchData(filters);
}, [filters]); // New object every render!

// ✅ Memoize object
const filters = useMemo(() => ({ category, sort }), [category, sort]);

// ❌ Creating functions in render
{items.map(item => <Item onClick={() => handle(item.id)} />)}

// ✅ Use useCallback
const handle = useCallback((id) => {...}, []);
{items.map(item => <Item onClick={handle} id={item.id} />)}

Web Vitals

Measuring Performance

import { getCLS, getFID, getFCP, getLCP, getTTFB } from 'web-vitals';

function sendToAnalytics(metric) {
  console.log(metric);
}

getCLS(sendToAnalytics);
getFID(sendToAnalytics);
getFCP(sendToAnalytics);
getLCP(sendToAnalytics);
getTTFB(sendToAnalytics);

Best Practices

1. Profile before optimizing
2. Use React.memo for expensive components
3. Memoize only when beneficial
4. Split code at route boundaries
5. Virtualize long lists
6. Lazy load images
7. Monitor bundle size

When to Optimize

DO Optimize

• Frequent renders
• Large lists (>100 items)
• Expensive calculations
• Heavy component trees

DON'T Optimize

• Simple components
• Rare renders
• Cheap calculations
• Small lists (<50 items)

Practice Exercises

1. Profile app with React DevTools
2. Optimize large product list
3. Implement code splitting
4. Build virtualized table
5. Optimize image gallery
6. Reduce bundle size
7. Measure Web Vitals

Resources

• React Performance
• Web Vitals
• React Window

Difficulty: Advanced Estimated Time: 2-3 weeks Prerequisites: React Hooks, Component Architecture