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workspace_javascript
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performance
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6.html

<!DOCTYPE html>
<html lang="zh-CN">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>代码优化 - 虚拟滚动</title>
    <style>
        * {
            margin: 0;
            padding: 0;
            box-sizing: border-box;
        }
        body {
            font-family: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif;
            background: linear-gradient(135deg, #667eea 0%, #764ba2 100%);
            min-height: 100vh;
            padding: 20px;
        }
        .container {
            max-width: 1200px;
            margin: 0 auto;
            background: white;
            border-radius: 15px;
            padding: 30px;
            box-shadow: 0 20px 60px rgba(0,0,0,0.3);
        }
        h1 {
            text-align: center;
            color: #333;
            margin-bottom: 10px;
        }
        .subtitle {
            text-align: center;
            color: #666;
            margin-bottom: 30px;
        }
        .demo-section {
            display: grid;
            grid-template-columns: 1fr 1fr;
            gap: 30px;
            margin-bottom: 30px;
        }
        .demo-box {
            border: 2px solid #e0e0e0;
            border-radius: 10px;
            padding: 20px;
            background: #f9f9f9;
        }
        .demo-box h3 {
            color: #667eea;
            margin-bottom: 15px;
            display: flex;
            align-items: center;
            gap: 10px;
        }
        .demo-box.bad {
            border-color: #ff6b6b;
        }
        .demo-box.good {
            border-color: #51cf66;
        }
        .demo-box.bad h3::before {
            content: '❌';
        }
        .demo-box.good h3::before {
            content: '✅';
        }
        .list-container {
            height: 400px;
            overflow-y: auto;
            border: 1px solid #ddd;
            border-radius: 8px;
            background: white;
            position: relative;
        }
        .list-item {
            padding: 15px;
            border-bottom: 1px solid #eee;
            display: flex;
            align-items: center;
            gap: 15px;
            transition: background 0.2s;
        }
        .list-item:hover {
            background: #f5f5f5;
        }
        .list-item .avatar {
            width: 40px;
            height: 40px;
            border-radius: 50%;
            background: linear-gradient(135deg, #667eea 0%, #764ba2 100%);
            display: flex;
            align-items: center;
            justify-content: center;
            color: white;
            font-weight: bold;
            flex-shrink: 0;
        }
        .list-item .content {
            flex: 1;
        }
        .list-item .name {
            font-weight: 600;
            color: #333;
            margin-bottom: 5px;
        }
        .list-item .email {
            color: #666;
            font-size: 14px;
        }
        .list-item .status {
            padding: 4px 12px;
            border-radius: 12px;
            font-size: 12px;
            font-weight: 500;
        }
        .list-item .status.online {
            background: #d4edda;
            color: #155724;
        }
        .list-item .status.offline {
            background: #f8d7da;
            color: #721c24;
        }
        .virtual-content {
            position: absolute;
            top: 0;
            left: 0;
            right: 0;
        }
        .controls {
            display: flex;
            gap: 10px;
            flex-wrap: wrap;
            margin-top: 15px;
        }
        button {
            padding: 10px 20px;
            border: none;
            border-radius: 5px;
            cursor: pointer;
            font-size: 14px;
            transition: all 0.3s;
        }
        .btn-primary {
            background: #667eea;
            color: white;
        }
        .btn-primary:hover {
            background: #5a6fd6;
            transform: translateY(-2px);
        }
        .btn-danger {
            background: #ff6b6b;
            color: white;
        }
        .btn-danger:hover {
            background: #fa5252;
        }
        .btn-success {
            background: #51cf66;
            color: white;
        }
        .btn-success:hover {
            background: #40c057;
        }
        .stats {
            display: grid;
            grid-template-columns: repeat(auto-fit, minmax(200px, 1fr));
            gap: 15px;
            margin-top: 20px;
        }
        .stat-card {
            background: white;
            padding: 15px;
            border-radius: 8px;
            box-shadow: 0 2px 8px rgba(0,0,0,0.1);
            text-align: center;
        }
        .stat-card h4 {
            color: #666;
            font-size: 14px;
            margin-bottom: 10px;
        }
        .stat-value {
            font-size: 24px;
            font-weight: bold;
            color: #667eea;
        }
        .stat-value.high {
            color: #ff6b6b;
        }
        .stat-value.low {
            color: #51cf66;
        }
        .code-section {
            background: #f8f9fa;
            border-radius: 10px;
            padding: 20px;
            margin-top: 30px;
        }
        .code-section h3 {
            color: #333;
            margin-bottom: 15px;
        }
        pre {
            background: #2d2d2d;
            color: #f8f8f2;
            padding: 15px;
            border-radius: 8px;
            overflow-x: auto;
            font-size: 13px;
            line-height: 1.5;
        }
        .tips {
            background: #e7f3ff;
            border-left: 4px solid #667eea;
            padding: 15px;
            margin-top: 20px;
            border-radius: 0 8px 8px 0;
        }
        .tips h4 {
            color: #667eea;
            margin-bottom: 10px;
        }
        .tips ul {
            margin-left: 20px;
            color: #555;
        }
        .tips li {
            margin: 5px 0;
        }
        .performance-info {
            background: #fff3cd;
            border-left: 4px solid #ffc107;
            padding: 15px;
            margin-top: 15px;
            border-radius: 0 8px 8px 0;
        }
        .performance-info h4 {
            color: #856404;
            margin-bottom: 10px;
        }
        .performance-info p {
            color: #856404;
            font-size: 14px;
            line-height: 1.6;
        }
    </style>
</head>
<body>
    <div class="container">
        <h1>代码优化 - 虚拟滚动</h1>
        <p class="subtitle">虚拟滚动只渲染可视区域内的元素，大幅提升长列表性能</p>

        <div class="demo-section">
            <div class="demo-box bad">
                <h3>普通滚动（全部渲染）</h3>
                <div id="normalContainer" class="list-container"></div>
                <div class="performance-info">
                    <h4>⚠️ 性能问题</h4>
                    <p>渲染10000个DOM节点，导致页面卡顿、内存占用高</p>
                </div>
                <div class="controls">
                    <button class="btn-danger" onclick="loadNormalList()">加载10000条数据</button>
                    <button class="btn-primary" onclick="clearNormal()">清空</button>
                </div>
            </div>

            <div class="demo-box good">
                <h3>虚拟滚动（按需渲染）</h3>
                <div id="virtualContainer" class="list-container"></div>
                <div class="performance-info" style="background: #d4edda; border-color: #28a745;">
                    <h4 style="color: #155724;">✅ 性能优化</h4>
                    <p>只渲染可视区域的元素，性能提升显著</p>
                </div>
                <div class="controls">
                    <button class="btn-success" onclick="loadVirtualList()">加载10000条数据</button>
                    <button class="btn-primary" onclick="clearVirtual()">清空</button>
                </div>
            </div>
        </div>

        <div class="stats">
            <div class="stat-card">
                <h4>普通渲染DOM数量</h4>
                <div id="normalDomCount" class="stat-value high">0</div>
            </div>
            <div class="stat-card">
                <h4>虚拟渲染DOM数量</h4>
                <div id="virtualDomCount" class="stat-value low">0</div>
            </div>
            <div class="stat-card">
                <h4>DOM减少比例</h4>
                <div id="domReduction" class="stat-value">0%</div>
            </div>
            <div class="stat-card">
                <h4>数据总量</h4>
                <div id="totalData" class="stat-value">0</div>
            </div>
        </div>

        <div class="code-section">
            <h3>代码实现</h3>
            <pre><code>// 虚拟滚动实现
class VirtualScroll {
    constructor(container, options = {}) {
        this.container = container;
        this.itemHeight = options.itemHeight || 70;
        this.totalItems = options.totalItems || 0;
        this.visibleCount = Math.ceil(container.offsetHeight / this.itemHeight) + 2;

        this.init();
    }

    init() {
        // 创建虚拟内容容器
        this.content = document.createElement('div');
        this.content.className = 'virtual-content';
        this.content.style.height = (this.totalItems * this.itemHeight) + 'px';
        this.container.appendChild(this.content);

        // 监听滚动事件
        this.container.addEventListener('scroll', () => {
            this.render();
        }, { passive: true });

        // 初始渲染
        this.render();
    }

    render() {
        const scrollTop = this.container.scrollTop;
        const startIndex = Math.floor(scrollTop / this.itemHeight);
        const endIndex = Math.min(startIndex + this.visibleCount, this.totalItems - 1);

        // 清空当前内容
        this.content.innerHTML = '';

        // 渲染可见区域的元素
        for (let i = startIndex; i <= endIndex; i++) {
            const item = this.createItem(i);
            item.style.position = 'absolute';
            item.style.top = (i * this.itemHeight) + 'px';
            item.style.left = '0';
            item.style.right = '0';
            this.content.appendChild(item);
        }
    }

    createItem(index) {
        const item = document.createElement('div');
        item.className = 'list-item';
        item.innerHTML = `
            <div class="avatar">${index + 1}</div>
            <div class="content">
                <div class="name">用户 ${index + 1}</div>
                <div class="email">user${index + 1}@example.com</div>
            </div>
            <div class="status ${index % 2 === 0 ? 'online' : 'offline'}">
                ${index % 2 === 0 ? '在线' : '离线'}
            </div>
        `;
        return item;
    }

    setData(totalItems) {
        this.totalItems = totalItems;
        this.content.style.height = (this.totalItems * this.itemHeight) + 'px';
        this.render();
    }

    destroy() {
        this.container.removeEventListener('scroll', this.render);
        this.content.remove();
    }
}

// 使用示例
const virtualScroll = new VirtualScroll(document.getElementById('container'), {
    itemHeight: 70,
    totalItems: 10000
});</code></pre>
        </div>

        <div class="tips">
            <h4>虚拟滚动的优势</h4>
            <ul>
                <li><strong>大幅减少DOM节点</strong>：只渲染可视区域的元素，DOM节点数量大幅减少</li>
                <li><strong>提升渲染性能</strong>：减少重排重绘，提升页面流畅度</li>
                <li><strong>降低内存占用</strong>：DOM节点减少，内存占用显著降低</li>
                <li><strong>支持海量数据</strong>：可以轻松处理数万甚至数十万条数据</li>
                <li><strong>滚动流畅</strong>：滚动性能不受数据量影响</li>
            </ul>
            <h4>适用场景</h4>
            <ul>
                <li>长列表展示（用户列表、订单列表等）</li>
                <li>数据表格（大量行数据）</li>
                <li>聊天记录（历史消息）</li>
                <li>时间轴（大量时间节点）</li>
                <li>任何需要展示大量数据的场景</li>
            </ul>
            <h4>实现要点</h4>
            <ul>
                <li><strong>计算可视区域</strong>：根据容器高度和项目高度计算可见数量</li>
                <li><strong>设置总高度</strong>：使用占位元素设置正确的滚动高度</li>
                <li><strong>动态渲染</strong>：根据滚动位置动态渲染可见元素</li>
                <li><strong>性能优化</strong>：使用passive事件监听器，避免阻塞滚动</li>
                <li><strong>预加载</strong>：多渲染几个元素，避免滚动时出现空白</li>
            </ul>
        </div>
    </div>

    <script>
        let normalDomCount = 0;
        let virtualDomCount = 0;
        let totalDataCount = 0;

        // 普通滚动 - 渲染所有元素
        function loadNormalList() {
            const container = document.getElementById('normalContainer');
            clearNormal();

            const startTime = performance.now();
            const fragment = document.createDocumentFragment();

            for (let i = 0; i < 10000; i++) {
                const item = document.createElement('div');
                item.className = 'list-item';
                item.innerHTML = `
                    <div class="avatar">${i + 1}</div>
                    <div class="content">
                        <div class="name">用户 ${i + 1}</div>
                        <div class="email">user${i + 1}@example.com</div>
                    </div>
                    <div class="status ${i % 2 === 0 ? 'online' : 'offline'}">
                        ${i % 2 === 0 ? '在线' : '离线'}
                    </div>
                `;
                fragment.appendChild(item);
            }

            container.appendChild(fragment);

            const endTime = performance.now();
            normalDomCount = 10000;
            totalDataCount = 10000;

            document.getElementById('normalDomCount').textContent = normalDomCount;
            document.getElementById('totalData').textContent = totalDataCount;
            updateStats();
        }

        // 虚拟滚动 - 只渲染可见元素
        function loadVirtualList() {
            const container = document.getElementById('virtualContainer');
            clearVirtual();

            const itemHeight = 70;
            const totalItems = 10000;
            const containerHeight = container.offsetHeight;
            const visibleCount = Math.ceil(containerHeight / itemHeight) + 2;

            // 创建虚拟内容容器
            const content = document.createElement('div');
            content.className = 'virtual-content';
            content.style.height = (totalItems * itemHeight) + 'px';
            container.appendChild(content);

            // 渲染函数
            function render() {
                const scrollTop = container.scrollTop;
                const startIndex = Math.floor(scrollTop / itemHeight);
                const endIndex = Math.min(startIndex + visibleCount, totalItems - 1);

                content.innerHTML = '';

                for (let i = startIndex; i <= endIndex; i++) {
                    const item = document.createElement('div');
                    item.className = 'list-item';
                    item.style.position = 'absolute';
                    item.style.top = (i * itemHeight) + 'px';
                    item.style.left = '0';
                    item.style.right = '0';
                    item.innerHTML = `
                        <div class="avatar">${i + 1}</div>
                        <div class="content">
                            <div class="name">用户 ${i + 1}</div>
                            <div class="email">user${i + 1}@example.com</div>
                        </div>
                        <div class="status ${i % 2 === 0 ? 'online' : 'offline'}">
                            ${i % 2 === 0 ? '在线' : '离线'}
                        </div>
                    `;
                    content.appendChild(item);
                }

                virtualDomCount = endIndex - startIndex + 1;
                document.getElementById('virtualDomCount').textContent = virtualDomCount;
                updateStats();
            }

            // 监听滚动事件
            container.addEventListener('scroll', render, { passive: true });

            // 初始渲染
            render();

            totalDataCount = 10000;
            document.getElementById('totalData').textContent = totalDataCount;
        }

        function clearNormal() {
            document.getElementById('normalContainer').innerHTML = '';
            normalDomCount = 0;
            document.getElementById('normalDomCount').textContent = 0;
            updateStats();
        }

        function clearVirtual() {
            const container = document.getElementById('virtualContainer');
            container.innerHTML = '';
            container.removeEventListener('scroll', () => {});
            virtualDomCount = 0;
            document.getElementById('virtualDomCount').textContent = 0;
            updateStats();
        }

        function updateStats() {
            if (normalDomCount > 0 && virtualDomCount > 0) {
                const reduction = ((normalDomCount - virtualDomCount) / normalDomCount * 100).toFixed(1);
                document.getElementById('domReduction').textContent = reduction + '%';
            }
        }
    </script>
</body>
</html>