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JavaScript系列（77）-- Web Components 深入解析

article 2025/2/23 4:35:35

JavaScript Web Components 深入解析 🧩

Web Components 是一套用于创建可重用用户界面组件的技术标准集合。今天让我们深入探讨这项强大的原生技术，学习如何创建真正封装的、可重用的组件。

Web Components 概述 🌟

💡 小知识：Web Components 由三大核心技术组成：Custom Elements（自定义元素）、Shadow DOM（影子DOM）和HTML Templates（HTML模板）。这些技术让我们能够创建独立的、可重用的组件，而不依赖任何框架。

核心技术详解 📊

// 1. Custom Elements API
class CustomButton extends HTMLElement {
    constructor() {
        super();
        
        // 创建Shadow DOM
        this.attachShadow({ mode: 'open' });
        
        // 设置初始样式和结构
        this.shadowRoot.innerHTML = `
            <style>
                :host {
                    display: inline-block;
                }
                button {
                    padding: 10px 20px;
                    border: none;
                    border-radius: 4px;
                    background: var(--button-bg, #007bff);
                    color: white;
                    cursor: pointer;
                    transition: all 0.3s;
                }
                button:hover {
                    opacity: 0.9;
                }
            </style>
            <button><slot></slot></button>
        `;
        
        this._button = this.shadowRoot.querySelector('button');
    }
    
    // 生命周期回调
    connectedCallback() {
        this._button.addEventListener('click', this._handleClick.bind(this));
    }
    
    disconnectedCallback() {
        this._button.removeEventListener('click', this._handleClick.bind(this));
    }
    
    // 私有方法
    _handleClick(e) {
        this.dispatchEvent(new CustomEvent('custom-click', {
            bubbles: true,
            composed: true,
            detail: { timestamp: Date.now() }
        }));
    }
}

// 注册自定义元素
customElements.define('custom-button', CustomButton);

Shadow DOM 和样式封装 🎨

// 1. Shadow DOM 基础
class StyledComponent extends HTMLElement {
    constructor() {
        super();
        
        // 创建封装的Shadow DOM
        const shadow = this.attachShadow({ mode: 'open' });
        
        // 定义组件样式
        const style = document.createElement('style');
        style.textContent = `
            :host {
                display: block;
                padding: 20px;
                border: 1px solid #ddd;
            }
            
            :host([theme="dark"]) {
                background: #333;
                color: white;
            }
            
            ::slotted(*) {
                margin: 0;
                font-family: sans-serif;
            }
        `;
        
        // 创建内容结构
        const wrapper = document.createElement('div');
        wrapper.innerHTML = `
            <slot name="title">默认标题</slot>
            <slot>默认内容</slot>
        `;
        
        // 添加到Shadow DOM
        shadow.appendChild(style);
        shadow.appendChild(wrapper);
    }
}

customElements.define('styled-component', StyledComponent);

HTML Templates 应用 📝

// 1. 模板定义和使用
const template = document.createElement('template');
template.innerHTML = `
    <style>
        .card {
            border: 1px solid #ddd;
            border-radius: 8px;
            padding: 16px;
            margin: 8px;
        }
        .card-title {
            font-size: 1.2em;
            margin-bottom: 8px;
        }
        .card-content {
            color: #666;
        }
    </style>
    <div class="card">
        <div class="card-title">
            <slot name="title">Card Title</slot>
        </div>
        <div class="card-content">
            <slot>Card Content</slot>
        </div>
    </div>
`;

class CardComponent extends HTMLElement {
    constructor() {
        super();
        this.attachShadow({ mode: 'open' });
        this.shadowRoot.appendChild(template.content.cloneNode(true));
    }
}

customElements.define('card-component', CardComponent);

组件生命周期 ⚡

class LifecycleComponent extends HTMLElement {
    // 观察的属性
    static get observedAttributes() {
        return ['color', 'size'];
    }
    
    constructor() {
        super();
        console.log('1. Constructor called');
    }
    
    connectedCallback() {
        console.log('2. Component added to DOM');
    }
    
    disconnectedCallback() {
        console.log('3. Component removed from DOM');
    }
    
    adoptedCallback() {
        console.log('4. Component moved to new document');
    }
    
    attributeChangedCallback(name, oldValue, newValue) {
        console.log(`5. Attribute ${name} changed from ${oldValue} to ${newValue}`);
    }
}

customElements.define('lifecycle-component', LifecycleComponent);

最佳实践与性能优化 💪

// 1. 性能优化示例
class OptimizedComponent extends HTMLElement {
    constructor() {
        super();
        
        // 使用DocumentFragment优化DOM操作
        const fragment = document.createDocumentFragment();
        const shadow = this.attachShadow({ mode: 'open' });
        
        // 延迟加载非关键资源
        requestIdleCallback(() => {
            this._loadNonCriticalResources();
        });
        
        // 使用CSS containment优化渲染
        const style = document.createElement('style');
        style.textContent = `
            :host {
                contain: content;
                display: block;
            }
        `;
        
        fragment.appendChild(style);
        shadow.appendChild(fragment);
    }
    
    // 私有方法
    _loadNonCriticalResources() {
        // 加载非关键资源
    }
}

// 2. 组件通信最佳实践
class CommunicationComponent extends HTMLElement {
    constructor() {
        super();
        
        // 使用CustomEvent进行组件通信
        this.addEventListener('custom-event', this._handleCustomEvent.bind(this));
    }
    
    // 事件处理
    _handleCustomEvent(e) {
        // 处理事件
        console.log(e.detail);
    }
    
    // 公共API
    publicMethod() {
        // 提供公共API
    }
}

实战应用示例 🔨

// 1. 可复用的表单组件
class CustomForm extends HTMLElement {
    constructor() {
        super();
        
        this.attachShadow({ mode: 'open' });
        
        this.shadowRoot.innerHTML = `
            <style>
                :host {
                    display: block;
                    font-family: system-ui;
                }
                
                form {
                    display: grid;
                    gap: 16px;
                    padding: 20px;
                }
                
                label {
                    display: block;
                    margin-bottom: 4px;
                }
                
                input {
                    width: 100%;
                    padding: 8px;
                    border: 1px solid #ddd;
                    border-radius: 4px;
                }
                
                button {
                    padding: 10px 20px;
                    background: #007bff;
                    color: white;
                    border: none;
                    border-radius: 4px;
                    cursor: pointer;
                }
            </style>
            
            <form id="form">
                <div>
                    <label for="name">Name</label>
                    <input type="text" id="name" required>
                </div>
                <div>
                    <label for="email">Email</label>
                    <input type="email" id="email" required>
                </div>
                <button type="submit">Submit</button>
            </form>
        `;
        
        this._form = this.shadowRoot.getElementById('form');
        this._form.addEventListener('submit', this._handleSubmit.bind(this));
    }
    
    _handleSubmit(e) {
        e.preventDefault();
        
        const formData = new FormData(this._form);
        const data = Object.fromEntries(formData.entries());
        
        this.dispatchEvent(new CustomEvent('form-submit', {
            bubbles: true,
            composed: true,
            detail: data
        }));
    }
}

customElements.define('custom-form', CustomForm);

// 2. 响应式数据组件
class DataComponent extends HTMLElement {
    constructor() {
        super();
        
        this._data = new Proxy({}, {
            set: (target, property, value) => {
                target[property] = value;
                this._render();
                return true;
            }
        });
        
        this.attachShadow({ mode: 'open' });
    }
    
    set data(value) {
        Object.assign(this._data, value);
    }
    
    _render() {
        // 实现渲染逻辑
    }
}

调试与测试 🔍

// 1. 组件调试工具
class DebugComponent extends HTMLElement {
    constructor() {
        super();
        
        // 开发模式检测
        if (process.env.NODE_ENV === 'development') {
            this._enableDebugMode();
        }
    }
    
    _enableDebugMode() {
        // 添加调试信息
        this.setAttribute('debug', '');
        
        // 监控生命周期
        const lifecycleMethods = [
            'connectedCallback',
            'disconnectedCallback',
            'attributeChangedCallback'
        ];
        
        lifecycleMethods.forEach(method => {
            const original = this[method];
            this[method] = function(...args) {
                console.log(`Debug: ${method} called`, args);
                return original.apply(this, args);
            };
        });
    }
}

// 2. 测试辅助函数
function createTestComponent(ComponentClass) {
    const element = new ComponentClass();
    document.body.appendChild(element);
    
    return {
        element,
        cleanup() {
            element.remove();
        },
        triggerEvent(eventName, detail) {
            element.dispatchEvent(new CustomEvent(eventName, { detail }));
        }
    };
}