Jan 1 2010

jQuery event delegation - Adding/removing markup dynamically

Category: NovoGeek @ 20:12

First of all, my Happy New Year wishes to all netizens! As you journey through 2010, leave a trail of hardwork, innovation, passion and excellence...

Coming to the topic, in web apps, most of the times, we would require adding and removing controls to a web page dynamically. E.g.,adding "Browse" buttons for email attachments(live mail, gmail etc). In a traditional ASP.NET app, we used to build a string containing markup and append it to some control. However, this requires postback. Here is the end result which we would want to achieve:

With the help of jQuery, we can achieve this functionality with ease. Let us see the various ways of implementing this functionality.

1. Using jQuery “clone(true)” function:

Many of us know about jQuery clone() function, which will clone any jQuery selector. But the hidden feature is, it accepts a boolean parameter. When the param is set to true, it will clone DOM elements along with their event handlers.

Though this method can help us achieve the end result, the disadvantage is, it will fail to initialize complex controls like date pickers, auto suggest textboxes etc.

2. Using jQuery “live” event:

jQuery live looks like a perfect choice for this requirement. Live events are added for current as well as future elements in the DOM and hence events for “+”, “-“ buttons can be written only once using Live.

Though this works as expected in simple screens, even this is not a perfect approach! If suppose you are loading pages via AJAX, the events would be bound multiple times. So clicking on “+” will add more than 1 row, as multiple click events are bound. This is because, Live adds events to the document and not to the element. Check this article to see how live works.

3. Using jQuery Event Delegation:

Yup! This will solve the problems caused by above methods. I don’t want to explain about event delegation as there are very good articles already available. Check these:

The concept is very simple. Suppose you have a html table with 100 cells and you need to fire an event when any cell is clicked, instead of writing the below code:

$('#myTable TD').click(function(){
    $(this).css('background', 'blue');

you can write code like this:

$('#myTable').click(function(e) {
    var clickedElement = $(e.target);
    clickedElement.css('background', 'blue');

So instead of binding 100 events as in case 1, you can achieve the same output by binding only a single event, as in case 2.

This will solve the problem of Live, as the events are not added at document level, but are added to parent container. This will also solve the problem of Clone as date pickers will be initialized properly.

DEMO: Here is a demo for the above functionality using LIVE (Use it only when you are not loading pages via AJAX). I shall very soon come up with a demo using event delegation. This is a real time problem which I faced in a large scale project. So wanted to share immediately, so that others might not fall in the traps in which I fell!

Happy Coding :)

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