A preliminary study on event proxy in javascript_javascript skills-JS Tutorial-php.cn

A preliminary study on event proxy in javascript_javascript skills

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Release： 2016-05-16 16:56:23

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Events have always been one of the most powerful objects in JavaScript. JavaScript provides two methods, addEventListener and attachEvent, to bind events to DOM nodes. jquery further encapsulates them and provides a bind method compatible with various browsers. Looking at it now, this traditional event binding method has the following shortcomings:

1. You may need to bind a lot of EventHanders.

If a table in the page has 100 rows, a click event must be bound to each row. Then 100 EventHandlers must be bound, which is a huge burden on page performance because more memory needs to be created to store these Handlers.

2. Events cannot be added after binding to DOM nodes.

Suppose the code in the page is as follows:

Copy the code The code is as follows:

$( "#dv").bind('click',function(){alert('test');});
$(body).append('

test')

cannot trigger click events.

In order to solve these two problems, JavaScript introduced event proxy. First, let’s understand the bubbling mechanism in js.

Basically all browsers support event bubbling. When an event is triggered on a DOM node, the event will be propagated all the way up to the root node of the document. Since all node events will eventually be delivered to the document root node, if we directly bind the event to the document root node (document node), and then use event.target to determine which node triggered the event, will it reduce a lot of EventHandlers? What about the binding?

The live method in jquery is officially implemented based on this principle. Let’s implement a simple version of live:

Copy code The code is as follows:

$.fn.mylive=function(eventType,fn){
var that=this.selector;
$(document).bind(eventType,function( event){
var match=$(event.target).closest(that)
if(match.length !== 0){
fn.apply($(event.target),[event ]);
}
})
}

$("#tb td").mylive('click',function(event){
alert(event.target.innerHTML);
});

var tb='

the first column

the second column

the third column

';
$("body").append(tb);

In the live method, the event is bound to the document node, and $(event.target).closest(that) matches the element that actually triggered the event. In the demo, we bound click events for each TD added later. When we click on different TDs, we find that their corresponding Text prompt boxes will pop up.

The live method makes up for the two shortcomings of the traditional event binding method mentioned earlier. But the live method still has its shortcomings. Look at this code:

Copy the code The code is as follows:

$("#tb td" ).mylive('click',function(event){
 alert(event.target.innerHTML);
});

It will first traverse the entire document based on the jquery selector , find all #tb td elements and store them as objects. However, in the live implementation method, these objects are not used, but only "#td td" is used as a string to match the event source. This greatly increases a lot of unnecessary consumption.

So is there any way to improve this situation? The delegate proxy method is provided in jQuery, which supports binding events to specified elements, not just documents. Understand its principle, let's implement a simple version of delegate:

Copy the code The code is as follows:

$(body).append('

');

$.fn.mydelegate=function(selector,eventType,fn){
$(this).bind(eventType,function(event){
var match=$(event.target).closest( selector);
if(match.length !== 0){
fn.apply($(event.target),[event]);
}
});
}

$("#dv").mydelegate('td','click',function(event){
alert(event.target.innerHTML);
});

var tb='

the first column

the second column

the third column

';
$("dv").append(tb);

The mydeletage method does not need to obtain all td objects, but only needs to obtain the div object to which the event is bound. This is better than the live method in terms of execution efficiency.

This is just an introduction to let everyone understand the principle of event proxy. The implementation of live and delegate in jquery is much more complicated.