通过例子来理解 React 事件系统

说明:本文结论均基于 React 16.13.1 得出,若有出入请参考对应版本源码

几个题目

我们先来看几个题目,如果你都能很确定的说出结果,那么这篇文章就不用看了。

点击 BUTTON 打印的结果是:

题目一:

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export default class App extends React.Component {
innerClick = () => {
console.log('A: react inner click.')
}

outerClick = () => {
console.log('B: react outer click.')
}

componentDidMount() {
document.getElementById('outer').addEventListener('click', () => {
console.log('C: native outer click')
})
document.getElementById('inner').addEventListener('click', () => {
console.log('D: native inner click')
})
}

render() {
return (
<div id='outer' onClick={this.outerClick}>
<button id='inner' onClick={this.innerClick}>
BUTTON
</button>
</div>
)
}
}

答案:D C A B

题目二:

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export default class App extends React.Component {
innerClick = (e) => {
console.log('A: react inner click.')
e.stopPropagation()
}

outerClick = () => {
console.log('B: react outer click.')
}

componentDidMount() {
document.getElementById('outer').addEventListener('click', () => {
console.log('C: native outer click')
})
document.getElementById('inner').addEventListener('click', () => {
console.log('D: native inner click')
})
}

render() {
return (
<div id='outer' onClickCapture={this.outerClick}>
<button id='inner' onClick={this.innerClick}>
BUTTON
</button>
</div>
)
}
}

答案:D C A

题目三:

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export default class extends React.Component {
constructor(props) {
super(props)
document.addEventListener('click', () => {
console.log('C: native document click')
})
}

innerClick = () => {
console.log('A: react inner click.')
}

outerClick = () => {
console.log('B: react outer click.')
}

render() {
return (
<div id='outer' onClick={this.outerClick}>
<button id='inner' onClick={this.innerClick}>
BUTTON
</button>
</div>
)
}
}

答案:C A B

题目四:

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export default class extends React.Component {
constructor(props) {
super(props)
document.addEventListener('click', () => {
console.log('C: native document click')
})
}

innerClick = (e) => {
console.log('A: react inner click.')
e.nativeEvent.stopImmediatePropagation()
}

outerClick = () => {
console.log('B: react outer click.')
}

componentDidMount() {
document.addEventListener('click', () => {
console.log('D: native document click')
})
}

render() {
return (
<div id='outer' onClick={this.outerClick}>
<button id='inner' onClick={this.innerClick}>
BUTTON
</button>
</div>
)
}
}

答案:C A B

你全都答对了吗?

DOM 事件

首先,我们先简单地复习下 DOM 事件的相关知识点:

  1. 事件委托。React 利用了事件委托,将事件都绑定在 document 之上。
  2. DOM 事件模型。分成捕获、目标、冒泡阶段。

事件委托

如下所示,我们想监听 li 标签上的点击事件,但是我们不把事件绑定在 li 上,而是绑定在它的父元素上,通过 e.target 来获取当前点击的目标元素,这种做法就是事件委托。通过事件委托我们可以减少页面中的事件监听函数,提升性能。

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<ul>
<li>1</li>
<li>2</li>
<li>3</li>
</ul>
<script>
const $ul = document.querySelector('ul')
$ul.addEventListener('click', (e) => {
console.log(e.target.innerText)
})
</script>

DOM 事件模型

我们知道 DOM 事件分为三个阶段:捕获、目标、冒泡。我们通过几个例子来说明其工作流程:

例一:

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<div id="id">
<button id="btn">Button</button>
</div>
<script>
const $div = document.querySelector('#id')
const $btn = document.querySelector('#btn')

document.addEventListener('click', () => {
console.log('document click')
})

$div.addEventListener('click', (e) => {
console.log('div click 1')
})

$div.addEventListener('click', (e) => {
console.log('div click 2')
})

$div.addEventListener('click', (e) => {
console.log('div click 3')
})

$btn.addEventListener('click', () => {
console.log('button click')
})
</script>

我们知道, addEventListener 第三个参数是指定是否在捕获阶段触发事件相应函数,默认 false,所以上面的事件均在冒泡阶段触发。事件触发的顺序是从下至上,同一个元素上的事件按照绑定的顺序执行,如下图:

所以结果是:

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button click
div click 1
div click 2
div click 3
document click

例二:

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<div id="id">
<button id="btn">Button</button>
</div>
<script>
const $div = document.querySelector('#id')
const $btn = document.querySelector('#btn')

document.addEventListener('click', () => {
console.log('document click')
})

$div.addEventListener('click', (e) => {
console.log('div click 1')
})

$div.addEventListener('click', (e) => {
e.stopPropagation()
console.log('div click 2')
})

$div.addEventListener('click', (e) => {
console.log('div click 3')
})

$btn.addEventListener('click', () => {
console.log('button click')
})
</script>

这里新加了一句 e.stopPropagation(),其作用是阻止事件扩散,所以 document 上的事件监听函数就不会执行了。

例三:

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<div id="id">
<button id="btn">Button</button>
</div>
<script>
const $div = document.querySelector('#id')
const $btn = document.querySelector('#btn')

document.addEventListener('click', () => {
console.log('document click')
})

$div.addEventListener('click', (e) => {
console.log('div click 1')
})

$div.addEventListener(
'click',
(e) => {
console.log('div click 2')
},
true
)

$div.addEventListener(
'click',
(e) => {
console.log('div click 3')
},
true
)

$btn.addEventListener('click', () => {
console.log('button click')
})
</script>

这里把 div 的两个事件监听函数绑定在捕获阶段。当事件触发的时候会先执行捕获阶段的监听函数,执行顺序是从上而下,相同元素上仍然按照绑定顺序执行。

所以结果是:

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div click 2
div click 3
button click
div click 1
document click

例四:

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<div id="id">
<button id="btn">Button</button>
</div>
<script>
const $div = document.querySelector('#id')
const $btn = document.querySelector('#btn')

document.addEventListener('click', () => {
console.log('document click')
})

$div.addEventListener('click', (e) => {
console.log('div click 1')
})

$div.addEventListener(
'click',
(e) => {
e.stopImmediatePropagation()
console.log('div click 2')
},
true
)

$div.addEventListener(
'click',
() => {
console.log('div click 3')
},
true
)

$btn.addEventListener('click', () => {
console.log('button click')
})
</script>

这里新增了 e.stopImmediatePropagation(),该方法是加强版的 stopPropagation,不仅可以阻止向其他元素扩散,也可以在本元素内部阻止扩散。

React 事件系统

回顾了下 DOM 事件的知识点后我们进入正题,首先我们看 React 事件绑定是怎么做的。

React 事件绑定

首先,我们知道 React 利用了事件委托机制,将所有事件绑定到了 document 之上(17 版本有变动)。
具体到代码,可以查看 react-reconciler/src/ReactFiberCompleteWork.old.js 文件:

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...
// 通过 FiberNode 创建真实 DOM
// 这里已经执行过类组件的 constructor 方法,但是还没有执行 componentDidMount
const instance = createInstance(
type,
newProps,
rootContainerInstance,
currentHostContext,
workInProgress
)

...

if (
// 该方法最终会进行事件绑定
finalizeInitialChildren(
instance,
type,
newProps,
rootContainerInstance,
currentHostContext
)
) {
...
}

其中 finalizeInitialChildren 最终会调用 react-dom/src/events/EventListener.js 文件中的 addEventBubbleListener

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export function addEventBubbleListener(
target: EventTarget,
eventType: string,
listener: Function
): Function {
target.addEventListener(eventType, listener, false)
return listener
}

注意, constructor 函数在事件绑定前就执行了,而 componentDidMount 则在事件绑定之后才执行。

事件触发

我们用下面的例子来体会事件触发的流程:

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export default class App extends React.Component {
innerClick = () => {
console.log('A: react inner click.')
}

outerClick = () => {
console.log('B: react outer click.')
}

render() {
return (
<div id='outer' onClickCapture={this.outerClick}>
<button id='inner' onClick={this.innerClick}>
BUTTON
</button>
</div>
)
}
}

当事件在 document 上触发的时候,我们可以拿到原生事件对象 NativeEvent,通过 target 可以访问到当前点击的 DOM 元素 button,通过其属性 __reactFiber$******表示随机数)可以获取 button 所对应的 FiberNode

同时,React 还会利用 NativeEvent 来生成 SyntheticEvent,其中 SyntheticEvent 有几个重要的属性值得关注下:

  1. nativeEvent,指向 NativeEvent
  2. _dispatchListeners,存储要执行的事件监听函数。
  3. _dispatchInstances,存储要执行的事件监听函数所属的 FiberNode 对象。

接下来就会分捕获和冒泡两个阶段来收集要执行的事件监听函数:

最后,按照顺序执行 _dispatchListeners 中的方法,并通过 _dispatchInstances 中的 FiberNode 来得到 currentTarget

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export function executeDispatch(event, listener, inst) {
const type = event.type || 'unknown-event'
event.currentTarget = getNodeFromInstance(inst)
invokeGuardedCallbackAndCatchFirstError(type, listener, undefined, event)
event.currentTarget = null
}

/**
* Standard/simple iteration through an event's collected dispatches.
*/
export function executeDispatchesInOrder(event) {
const dispatchListeners = event._dispatchListeners
const dispatchInstances = event._dispatchInstances
if (__DEV__) {
validateEventDispatches(event)
}
if (Array.isArray(dispatchListeners)) {
for (let i = 0; i < dispatchListeners.length; i++) {
if (event.isPropagationStopped()) {
break
}
// Listeners and Instances are two parallel arrays that are always in sync.
executeDispatch(event, dispatchListeners[i], dispatchInstances[i])
}
} else if (dispatchListeners) {
executeDispatch(event, dispatchListeners, dispatchInstances)
}
event._dispatchListeners = null
event._dispatchInstances = null
}

注意到 event.isPropagationStopped(),该方法是检查当前是否要阻止扩散,假设我们在某个事件监听函数中调用 e.stopPropagation(),则会执行下面的代码:

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function functionThatReturnsTrue() {
return true;
}
...
stopPropagation: function() {
const event = this.nativeEvent;
if (!event) {
return;
}

if (event.stopPropagation) {
event.stopPropagation();
} else if (typeof event.cancelBubble !== 'unknown') {
// The ChangeEventPlugin registers a "propertychange" event for
// IE. This event does not support bubbling or cancelling, and
// any references to cancelBubble throw "Member not found". A
// typeof check of "unknown" circumvents this issue (and is also
// IE specific).
event.cancelBubble = true;
}

this.isPropagationStopped = functionThatReturnsTrue;
}
...

这样,_dispatchListeners 数组中后面的函数就都不会执行了,从而实现了阻止事件扩散的功能。

题目解答

最后,让我们来对文章开头的题目做一个解答。

题目一:

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export default class App extends React.Component {
innerClick = () => {
console.log('A: react inner click.')
}

outerClick = (e) => {
console.log('B: react outer click.')
}

componentDidMount() {
document.getElementById('outer').addEventListener('click', () => {
console.log('C: native outer click')
})
document.getElementById('inner').addEventListener('click', () => {
console.log('D: native inner click')
})
}

render() {
return (
<div id='outer' onClickCapture={this.outerClick}>
<button id='inner' onClick={this.innerClick}>
BUTTON
</button>
</div>
)
}
}

事件模型可以简化为上图,其中 A B 在一个框中表示他们属于同一个事件监听函数中的不同子函数。根据事件冒泡机制,答案为:D C A B

题目二:

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export default class App extends React.Component {
innerClick = () => {
console.log('A: react inner click.')
e.stopPropagation()
}

outerClick = (e) => {
console.log('B: react outer click.')
}

componentDidMount() {
document.getElementById('outer').addEventListener('click', () => {
console.log('C: native outer click')
})
document.getElementById('inner').addEventListener('click', () => {
console.log('D: native inner click')
})
}

render() {
return (
<div id='outer' onClickCapture={this.outerClick}>
<button id='inner' onClick={this.innerClick}>
BUTTON
</button>
</div>
)
}
}

调用了 stopPropagation,所以 B 不打印,答案为:D C A

题目三:

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export default class extends React.Component {
constructor(props) {
super(props)
document.addEventListener('click', () => {
console.log('C: native document click')
})
}

innerClick = (e) => {
console.log('A: react inner click.')
}

outerClick = () => {
console.log('B: react outer click.')
}

render() {
return (
<div id='outer' onClick={this.outerClick}>
<button id='inner' onClick={this.innerClick}>
BUTTON
</button>
</div>
)
}
}

constructor 函数先于 React 事件绑定,所以答案为:C A B

题目四:

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export default class extends React.Component {
constructor(props) {
super(props)
document.addEventListener('click', () => {
console.log('C: native document click')
})
}

innerClick = (e) => {
console.log('A: react inner click.')
e.nativeEvent.stopImmediatePropagation()
}

outerClick = () => {
console.log('B: react outer click.')
}

componentDidMount() {
document.addEventListener('click', () => {
console.log('D: native document click')
})
}

render() {
return (
<div id='outer' onClick={this.outerClick}>
<button id='inner' onClick={this.innerClick}>
BUTTON
</button>
</div>
)
}
}

调用原生事件上的 stopImmediatePropagation,会阻止事件在本元素中继续扩散,所以答案为:C A B