Event delegation in javascript (picture and text tutorial)

This article mainly introduces relevant information on the detailed explanation of event delegation in javascript. Friends who need it can refer to it

I have seen an interview question these days, which is probably, asking you to give 1,000 li How should I add a click event? Most people's first impression may be to add one to each li. If this is the case, it is estimated that they will be GG during the interview. Here we have withdrawn our Event bubbling and capturing mechanisms, as well as event delegation mechanisms, let’s take a look at the above.

First let’s talk about event bubbling and event capturing mechanisms. Event bubbling was proposed by Microsoft. Event capture was proposed by Netscape. At that time, the two companies were at loggerheads. Later, W3C had no choice but to adopt a compromise method. When an event occurs, it is captured first and then bubbles up.

Usually , there are three ways to listen to events in js, namely:

　ele.addEventListener(type,listener,[useCapture]);//IE6~8 does not support

　　ele.attachEvent('on' type,listener);//Supported by IE6~10, not supported by IE11

##　　ele.onClick=function(){}; //All browsers support

The w3c specification defines three event stages, which are the capture stage, the target stage, and the bubbling stage. In the dom2 level regulations specified by w3c, the addEventListener is used to listen for events. So we will use addEventListener to explain. First of all, bubbles are just like when you throw a stone into the water, the bubbles in the water will pop up from below, which means that after the event is triggered, it will move from the direction of the child element to the parent element. Trigger, while the capture mechanism is just the opposite. The capture mechanism triggers events from the parent element to the child element, and the third parameter in the addEventListener function is used to determine whether to use the capture mechanism or the bubbling mechanism. When useCapture is true It is a capturing mechanism. When useCapture is false, it is a bubbling mechanism. Let’s take a look at the example:

Copy code

<p class="parent">
  <p class="child">

  </p>
</p>
<script>
  var parent = document.getElementsByClassName(&#39;parent&#39;)[0];
  var child = document.getElementsByClassName(&#39;child&#39;)[0];

  parent.addEventListener(&#39;click&#39;,function(){
    console.log("这里是父元素");
  },false);
  child.addEventListener(&#39;click&#39;,function(){
    console.log("这里是子元素");
  },false);
</script>

When we click on the child element it is Show the picture above. When we change false to true, we will find that the execution order will be reversed. This is the difference between event bubbling and capturing. They are just the opposite,

Then the disadvantage of using this binding mechanism is that it is very troublesome to bind events to each object. When we want to delete an event or change an event, it will be particularly cumbersome and more important. What's more, we have increased the association between JavaScript and DOM nodes, and if there is a circular reference, it is very likely to cause memory leaks. These are its drawbacks,

So one way to solve this drawback It is event delegation. This method allows you to avoid adding events to each node one by one. What it does is bind these listening events to the parent elements of these nodes. On the parent element, this The listening function automatically determines which child element triggered the event, so that it can operate on the child element that triggered the event. The example we give here is an example given by davidwalsh:

Now we have a The parent element ul and several li child elements,

<ul id="parent-list">
  <li id="post-1">Item 1</li>
  <li id="post-2">Item 2</li>
  <li id="post-3">Item 3</li>
  <li id="post-4">Item 4</li>
  <li id="post-5">Item 5</li>
  <li id="post-6">Item 6</li>
</ul>

What we want to achieve now is that when we click on each li node, the content in the li node will be output. According to the above writing, you can select After these li, add these methods to them, and then remove them when they are no longer needed. If there are 100 li or 1000 li, this will become your nightmare. A better solution is to give it to your parents. Add a listening event to the element. The next question is how to determine which li was clicked? We can judge the target of the current event in the listening event to determine whether it is the node we are looking for. Here we have a simple Example:

// 找到父元素,绑定一个监听事件
document.getElementById("parent-list").addEventListener("click", function(e) {
  // e.target是点击的元素
  // 如果它是li元素
  if(e.target && e.target.nodeName == "LI") {
    //
    console.log("List item ", e.target.id.replace("post-", ""), " was clicked!");
  }
});

When a click event occurs in ul, because addEventListener is a bubbling event by default, the listening event will be executed when the underlying event bubbles over. After the event is triggered, we will detect whether it is us. If the target element we are looking for is not the target element, it will be ignored. Then we can not only check whether the tag of the target element is the target element we need, but we can also detect it based on the attributes or class name of the target element, using ele. maeches API is used for processing.

document.getElementById("myp").addEventListener("click",function(e) {
  // e.target 就是当前被点击的元素
 if (e.target && e.target.matches("a.classA")) {
  console.log("Anchor element clicked!");
  }
});

The above is what I compiled for everyone. I hope it will be helpful to everyone in the future.

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