DOM事件流三阶段详解

事件流模型基础

DOM事件流分为三个阶段：捕获阶段、目标阶段和冒泡阶段。当事件触发时，浏览器会从文档根节点开始向下传播（捕获阶段），到达目标元素（目标阶段），再向上回溯到根节点（冒泡阶段）。

// 事件监听函数基本语法
element.addEventListener(eventType, handler, useCapture);

useCapture参数默认为false表示在冒泡阶段处理，设为true则在捕获阶段处理。

捕获阶段工作机制

事件从window对象开始向下传播至目标元素父级。捕获阶段的事件监听器会按照DOM树从上到下的顺序执行。适合用于需要优先处理全局事件或事件拦截的场景。

<div id="grandparent">
  <div id="parent">
    <div id="child"></div>
  </div>
</div>

<script>
document.getElementById('grandparent').addEventListener('click', () => {
  console.log('Grandparent captured');
}, true);

document.getElementById('parent').addEventListener('click', () => {
  console.log('Parent captured');
}, true);
</script>

点击child元素时控制台输出顺序为：Grandparent → Parent。

目标阶段特性

当事件到达目标元素时，无论useCapture如何设置，注册在目标元素上的事件处理程序都会执行。目标阶段没有单独的API控制，执行顺序取决于代码注册顺序。

const child = document.getElementById('child');
child.addEventListener('click', () => console.log('Child bubble'));
child.addEventListener('click', () => console.log('Child capture'), true);

两个处理程序按注册顺序执行，与useCapture参数无关。

冒泡阶段运行原理

事件从目标元素向上传播至文档根节点。大多数事件都会冒泡（focus/blur等除外），这是默认的事件处理阶段。

document.getElementById('parent').addEventListener('click', () => {
  console.log('Parent bubbled');
});

document.getElementById('grandparent').addEventListener('click', () => {
  console.log('Grandparent bubbled');
});

点击child元素时输出顺序为：Parent → Grandparent。

事件传播控制方法

event.stopPropagation()可阻止事件继续传播，但不会阻止同阶段的其他监听器执行。event.stopImmediatePropagation()会立即停止所有后续监听器。

element.addEventListener('click', (e) => {
  e.stopPropagation();
  console.log('This will execute');
});

element.addEventListener('click', () => {
  console.log('This won\'t execute if stopImmediatePropagation was called');
});

实际应用场景

表单验证适合在冒泡阶段处理，可以统一管理提交事件。组件库中的事件代理通常在捕获阶段实现，便于优先处理系统级行为。自定义事件系统可能需要同时利用两个阶段实现复杂交互逻辑。

// 事件委托示例
document.addEventListener('click', (e) => {
  if(e.target.matches('.dynamic-element')) {
    handleDynamicElementClick();
  }
}, false); // 冒泡阶段处理

性能优化建议

避免在大量元素上单独绑定事件，使用事件委托可以减少内存占用。谨慎使用事件阻止传播，可能影响其他插件功能。合理选择处理阶段可以减少不必要的事件处理调用。

// 性能较差的做法
document.querySelectorAll('.items').forEach(item => {
  item.addEventListener('click', handler);
});

// 更好的做法
document.body.addEventListener('click', (e) => {
  if(e.target.closest('.items')) {
    handler(e);
  }
});

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