Take you to quickly understand the event model in JavaScript

This article mainly introduces the event model in javascript, which includes DOM0-level event model and DOM2-level event model (event capture and event bubbling and DOM2-level registration events and deactivation events) Friends who are familiar with JavaScript can refer to this article

There are two event models in JavaScript: DOM0 and DOM2. As for these two time models, I have never been very clear about them. Now I finally understand a little more by looking up information online.

1. DOM0-level event model

The DOM0-level event model is an early event model and is supported by all browsers. Its implementation is also relatively simple. The code is as follows:

click me

This event model is to register the event name directly on the dom object. This code registers an onclick event on the p tag and outputs it inside the event function. Click target. Dismissing the event is even simpler, just copy null to the event function, as follows:

document.getElementById('click'_).onclick = null;
From this we can We know that in dom0, a dom object can only register one function of the same type, because if multiple functions of the same type are registered, overwriting will occur and the previously registered functions will be invalid.

var click = document.getElementById('click');
click.onclick = function(){
 alert('you click the first function');
};
click.onclick = function(){
 alert('you click the second function')
}

In this code, we registered two onclick functions for the DOM object, but as a result, only the second registered function was executed, and the previously registered function was overwritten.

2. DOM2-level event model

　1. Event capture and event bubbling (capture, bubble)

First of all, IE8 and below do not support this event model. The mechanism of event capture and event bubbling is as follows:

As shown in the figure above, 123 represents event capture and 4567 represents event bubbling. First we use the following code:

Suppose we click on p with the ID inner, then the event flow at this time is to first execute the capture phase: document-html-body-p(outer) . Then execute the bubbling phase: p(inner)-p(outer)-body-html-document.

　 2. DOM2-level registration events and deactivation events

Use addEventListener and removeEventListener in DOM2 level to register and deactivate events (IE8 and Not supported in previous versions). The advantage of this function compared to the previous method is that a DOM object can register multiple events of the same type, and event overwriting will not occur, and each event function will be executed in sequence.

　addEventListener('event name','event callback','capture/bubble'). The example is as follows:

First we need to know that the first parameter of addEventListenr is the event name. Different from DOM level 0, there is no "on". In addition, the third parameter represents capture or bubbling, and true represents the capture event. , false represents a bubbling event.

In this code, we registered two click event functions for inner p. The result is that the browser will execute these two functions in sequence.
Next we demonstrate how to use the event stream generation mechanism.

In this code, we use the capture event. Since inner is nested in outer, we know that when using capture, outer should capture the event first, and then inner can capture it. this incident. Then the result is that outer is executed first, followed by inner.

So what if I change the execution timing of outer to the bubbling stage?

alickouter.addEventListener('click',function(){
 alert('outer show'); 
},false);

In this case, inner is executed first and then outer. In the same way, if we change the execution timing of both events to the bubbling stage, the inner will still be executed first and then the outer. Then there is another problem, that is, if we register two click events in inner, one is in the capturing phase and the other is in the bubbling phase, that is to say, the third parameter of addEventListenter is set to false and true respectively, then execute What is the order of .

In this case, the first thing to do is the capture show, followed by the bubble show. However, this result is related to the order of registration. The first to register will be executed first. Because we are looking at the diagram of event capture and event bubbling, we find that there is only one specific DOM object in the end.

So what if we register click events for both outer and inner but I don’t want outer to execute? At this time, we need to use the stopPropagation function. This function is used to prevent bubbling. The implication is that the event will no longer continue to bubble, so that the DOM object that registers the same type of event will not be executed.

For example, when making a homemade drop-down box, we click elsewhere in the browser and we need the options of the drop-down box to be hidden. In this case, we need to use stopPropagation. as follows:

　　正常的情况下，我们在不添加stopPropagation函数时，首先应该执行inner，然后执行outer，但是当我们在inner的事件函数中添加了stopPropagation函数之后，执行完inner的事件函数之后，就不会在执行outer的事件函数了，也可以理解为事件冒泡到inner之后就消失了，因此也就不会在执行接下来的事件函数了。

　　由于事件捕获阶段没有可以阻止事件的函数，所以一般都是设置为事件冒泡。

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