What are the event streams in javascript?-Front-end Q&A-php.cn

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1. Events

2. Event flow

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What are the event streams in javascript?

青灯夜游

Jan 26, 2022 pm 01:02 PM

javascript

The event streams of JavaScript are: 1. Bubbling event stream, the propagation of events is from the most specific event target to the least specific event target; 2. Capture type event stream, the propagation of events is from From the least specific event target to the most specific event target; 3. DOM event flow.

What are the event streams in javascript?

The operating environment of this tutorial: windows7 system, javascript version 1.8.5, Dell G3 computer.

1. Events

An event is a specific interaction moment that occurs in a document or browser window.

An event is a certain action performed by the user or the browser itself, such as click, load and mouseover are the names of the events.

Events are the bridge between javaScript and DOM.

If you trigger it, I will execute it - when the event occurs, its handler function is called to execute the corresponding JavaScript code to give a response.

Typical examples are: the load event is triggered when the page is loaded; the click event is triggered when the user clicks on an element.

2. Event flow

The event flow describes the order in which events are received from the page.

1. Event awareness

Question: Click on a page element, what kind of element Can you sense such an event?

Answer: When you click on an element, you also click on the container element of the element, or even the entire page.

Example: There are three concentric circles. Add the corresponding event processing function to each circle and pop up the corresponding text. When you click the innermost circle, you also click the outer circle, so the click event of the outer circle will also be triggered.

<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
    <title></title>
</head>
<style>
    #outer{
        position: absolute;
        width: 400px;
        height: 400px;
        top:0;
        left: 0;
        bottom:0;
        right: 0;
        margin: auto;
        background-color: deeppink;
    }
    #middle{
        position: absolute;
        width: 300px;
        height:300px;
        top:50%;
        left: 50%;
        margin-left: -150px;
        margin-top: -150px;
        background-color: deepskyblue;
    }
    #inner{
        position: absolute;
        width: 100px;
        height:100px;
        top:50%;
        left:50%;
        margin-left: -50px;
        margin-top: -50px;;
        background-color: darkgreen;
        text-align: center;
        line-height: 100px;
        color:white;
    }
    #outer,#middle,#inner{
border-radius:100%;
    }
</style>
<body>
<div id="outer">
   <div id="middle">
        <div id="inner">
            click me!
        </div>
    </div>
</div>
<script>
       var innerCircle= document.getElementById("inner");
        innerCircle.onclick= function () {
            alert("innerCircle");
        };
        var middleCircle= document.getElementById("middle");
        middleCircle.onclick=function(){
            alert("middleCircle");
        }
        var outerCircle= document.getElementById("outer");
        outerCircle.onclick= function () {
            alert("outerCircle");
        }
</script>
</body>
</html>

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The effect is as follows:

2 , event flow

When an event occurs, it will be propagated between the element node and the root node in a specific order. All nodes along the path will receive the event. This propagation process is the DOM event stream.

1), two event flow models

The order of event propagation corresponds to the two event flow models of the browser: capture event flow and bubbling event streams.

Bubble-type event flow: The propagation of events is from the most specific event target to The least specific event target. That is, from the leaves of the DOM tree to the root. [Recommended]

Captured event stream: The propagation of events is from the least specific's event target to the most specific event target . That is, from the root of the DOM tree to the leaves.

The idea of event capture is that less specific nodes should receive events earlier, and the most specific nodes should receive events last.

<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
    <title></title>
</head>
<body>
<div id="myDiv">Click me!</div>
</body>
</html>

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In the above html code, the

element in the page is clicked.

The order of click event propagation in the bubbling event stream is—》—》—》document

click in the capture event stream The order of event propagation is document—》—》—》

note:
1）、所有现代浏览器都支持事件冒泡，但在具体实现中略有差别：
IE5.5及更早版本中事件冒泡会跳过元素(从body直接跳到document)。
IE9、Firefox、Chrome、和Safari则将事件一直冒泡到window对象。
2）、IE9、Firefox、Chrome、Opera、和Safari都支持事件捕获。尽管DOM标准要求事件应该从document对象开始传播，但这些浏览器都是从window对象开始捕获事件的。
3）、由于老版本浏览器不支持，很少有人使用事件捕获。建议使用事件冒泡。
2）、DOM事件流
DOM标准采用捕获+冒泡。两种事件流都会触发DOM的所有对象，从document对象开始，也在document对象结束。
DOM标准规定事件流包括三个阶段：事件捕获阶段、处于目标阶段和事件冒泡阶段。
事件捕获阶段：实际目标（
）在捕获阶段不会接收事件。也就是在捕获阶段，事件从document到再到就停止了。上图中为1~3.
处于目标阶段：事件在
上发生并处理。但是事件处理会被看成是冒泡阶段的一部分。
冒泡阶段：事件又传播回文档。
note:
1）、尽管“DOM2级事件”标准规范明确规定事件捕获阶段不会涉及事件目标，但是在IE9、Safari、Chrome、Firefox和Opera9.5及更高版本都会在捕获阶段触发事件对象上的事件。结果，就是有两次机会在目标对象上面操作事件。
2）、并非所有的事件都会经过冒泡阶段。所有的事件都要经过捕获阶段和处于目标阶段，但是有些事件会跳过冒泡阶段：如，获得输入焦点的focus事件和失去输入焦点的blur事件。
两次机会在目标对象上面操作事件例子：
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<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
    <title></title>
</head>
<style>
    #outer{
        position: absolute;
        width: 400px;
        height: 400px;
        top:0;
        left: 0;
        bottom:0;
        right: 0;
        margin: auto;
        background-color: deeppink;
    }
    #middle{
        position: absolute;
        width: 300px;
        height:300px;
        top:50%;
        left: 50%;
        margin-left: -150px;
        margin-top: -150px;
        background-color: deepskyblue;
    }
    #inner{
        position: absolute;
        width: 100px;
        height:100px;
        top:50%;
        left:50%;
        margin-left: -50px;
        margin-top: -50px;;
        background-color: darkgreen;
        text-align: center;
        line-height: 100px;
        color:white;
    }
    #outer,#middle,#inner{
        border-radius:100%;
    }
</style>
<body>
<div id="outer">
    <div id="middle">
        <div id="inner">
            click me!
        </div>
    </div>
</div>
<script>
    var innerCircle= document.getElementById("inner");
    innerCircle.addEventListener("click", function () {
        alert("innerCircle的click事件在捕获阶段被触发");
    },true);
    innerCircle.addEventListener("click", function () {
        alert("innerCircle的click事件在冒泡阶段被触发");
    },false);
    var middleCircle= document.getElementById("middle");
    middleCircle.addEventListener("click", function () {
        alert("middleCircle的click事件在捕获阶段被触发");
    },true);
    middleCircle.addEventListener("click", function () {
        alert("middleCircle的click事件在冒泡阶段被触发");
    },false);
    var outerCircle= document.getElementById("outer");
    outerCircle.addEventListener("click", function () {
        alert("outerCircle的click事件在捕获阶段被触发");
    },true);
    outerCircle.addEventListener("click", function () {
        alert("outerCircle的click事件在冒泡阶段被触发");
    },false);
</script>
</body>
</html>
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运行效果就是会陆续弹出6个框，为说明原理我整合成了一个图：
3、事件流的典型应用事件代理
传统的事件处理中，需要为每个元素添加事件处理器。js事件代理则是一种简单有效的技巧，通过它可以把事件处理器添加到一个父级元素上，从而避免把事件处理器添加到多个子级元素上。
1）、事件代理
事件代理的原理用到的就是事件冒泡和目标元素，把事件处理器添加到父元素，等待子元素事件冒泡，并且父元素能够通过target（IE为srcElement）判断是哪个子元素，从而做相应处理。关于target更多内容请参考javaScript事件（四）event的公共成员（属性和方法）下面举例来说明。
传统事件处理，为每个元素添加事件处理器，代码如下：
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<body>
<ul id="color-list">
<li>red</li>
<li>orange</li>
<li>yellow</li>
<li>green</li>
<li>blue</li>
<li>indigo</li>
<li>purple</li>
</ul>
<script>
(function(){
    var colorList=document.getElementById("color-list");
    var colors=colorList.getElementsByTagName("li");
    for(var i=0;i<colors.length;i++)
    {
        colors[i].addEventListener('click',showColor,false);
    };
    function showColor(e)
    {
        e=e||window.event;
        var targetElement=e.target||e.srcElement;
        alert(targetElement.innerHTML);
    }
})();
</script>
</body>
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事件代理的处理方式，代码如下：
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<body>
<ul id="color-list">
<li>red</li>
<li>orange</li>
<li>yellow</li>
<li>green</li>
<li>blue</li>
<li>indigo</li>
<li>purple</li>
</ul>
<script>
(function(){
    var colorList=document.getElementById("color-list");
    colorList.addEventListener('click',showColor,false);
    function showColor(e)
    {
        e=e||window.event;
        var targetElement=e.target||e.srcElement;
        if(targetElement.nodeName.toLowerCase()==="li"){
        alert(targetElement.innerHTML);
        }
    }
})();
</script>
</body>
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2）、事件代理的好处
总结一下事件代理的好处：
将多个事件处理器减少到一个，因为事件处理器要驻留内存，这样就提高了性能。想象如果有一个100行的表格，对比传统的为每个单元格绑定事件处理器的方式和事件代理（即table上添加一个事件处理器），不难得出结论，事件代理确实避免了一些潜在的风险，提高了性能。
DOM更新无需重新绑定事件处理器，因为事件代理对不同子元素可采用不同处理方法。如果新增其他子元素（a,span,p等），直接修改事件代理的事件处理函数即可，不需要重新绑定处理器，不需要再次循环遍历。
3）、事件代理的问题：【update20170502】
代码如下：事件代理同时绑定了li和span，当点击span的时候，li和span都会冒泡。
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<li><span>li中的span的内容</span></li>

<script>
    $(document).on('click', 'li', function(e){
        alert('li li');
    });

    $(document).on('click', 'span', function(e){
        alert('li span');
    })
</script>
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解决办法：
方法一：span的事件处理程序中阻止冒泡
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$(document).on('click', 'span', function(e){
       alert('li span');
       e.stopPropagation();
   })
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方法二：li的事件处理程序中检测target元素
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$(document).on('click', 'li', function (e) {
        if (e.target.nodeName == 'SPAN') {
            e.stopPropagation();
            return;
        }
        alert('li li');
    });
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4）、事件代理的一个有趣应用【update20170502】
点击一个列表时，输出对应的索引
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<script>
    var ul=document.querySelector('ul');
    var lis=ul.querySelectorAll('ul li');
    ul.addEventListener('click', function (e) {
        var target= e.target;
        if(target.nodeName.toUpperCase()==='LI'){
            alert([].indexOf.call(lis,target));
        }
    },false)
</script>
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【相关推荐：javascript学习教程】
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