Understanding Event Delegation

Event delegation refers to the use of a single event listener on a parent object to listen for events happening on its children (or deeper descendants). Event delegation allows developers to be sparse in their application of event listeners while still reacting to events as they happen on highly specific targets. This proves to be a key strategy for maintaining high performance in event-rich web projects, where the creation of hundreds of event listeners can quickly degrade performance.

This example illustrates the use of event delegation (via the Event Utility) on click, mouseover, and mouseout events. The problem we will solve here involves reacting to click, mouseover, and mouseout events on list items. As there can be many list items, we want to be frugal in applying event listeners. At the same time, we want to know exactly which <li> was involved in a click, mouseover, and mouseout event.

To do this, we'll rely on DOM event bubbling — the process by which an event progresses from its direct target up through the target's node ancestry until it reaches the window object. The graphic below may help to illustrate the flow: In this case, we'll count on the fact that a click or a mouseover on (A) the text node within an <li> progresses to the <li> element itself (B), then to the <ul> element (C), and then to the list's parent <div> (D) and so on up the document:

Getting Started

Because events flow this way, we can count on events happening to our <li>s bubbling up to our <div> whose ID attribute is "container."

We'll start with some structural markup — a <div> containing a <ul> with 5 <li> children.

<div id="container"> <ul id="list"> <li id="li-1">List Item 1</li> <li id="li-2">List Item 2</li> <li id="li-3">List Item 3</li> <li id="li-4">List Item 4</li> <li id="li-5">List Item 5</li> </ul> </div>

Event Delegation Made Easy

The Event Utility makes using event delegation easy by providing a delegate method that enables the use of CSS selector syntax to define the descendants of the delegation container for which the event listener should be called. Note: Event's delegation functionality is packaged separately in an event-delegate module, which itself requires the Selector Utility.

To use the delegate method, pass the id of the element that is the delegation container as the first argument (in this case <div id="container">). The second argument is the type of event you want to delegate, and the third is the event listener. The forth argument is a CSS selector that defines the descendant elements that must match the target of the event in order for the listener to be called.

First we'll add the event listener used to highlight each item when it is clicked. As we're only really interested in clicks on <li>s, we'll specify a CSS selector of "li" as the fourth argument to the delegate method. The element that matched the CSS selector will be passed as the second argument to the event listener. The element that is the delegation container for the listener is passed as the third argument:

(function() { var Dom = YAHOO.util.Dom, Event = YAHOO.util.Event; var onLIClick = function (event, matchedEl, container) { if (Dom.hasClass(matchedEl, "selected")) { Dom.removeClass(matchedEl, "selected"); } else { Dom.addClass(matchedEl, "selected"); } }; // Use the "delegate" method to attach a "click" event listener to the // container (<div id="container">). The listener will only be called if the // target of the click event matches the element specified via the CSS // selector passed as the fourth argument to the delegate method. Event.delegate("container", "click", onLIClick, "li"); })();

The same technique works for the mouseover and mouseout events as well.

(function() { var Dom = YAHOO.util.Dom, Event = YAHOO.util.Event; var onLIMouseOver = function (event, matchedEl, container) { Dom.addClass(matchedEl, "hover"); }; var onLIMouseOut = function (event, matchedEl, container) { Dom.removeClass(matchedEl, "hover"); }; // Use the "delegate" method to attach a mouseover and mouseout event listener // to the container (<div id="container">). The listener will only be called // if the target of the click event matches the element specified via the CSS // selector passed as the fourth argument to the delegate method. Event.delegate("container", "mouseover", onLIMouseOver, "li"); Event.delegate("container", "mouseout", onLIMouseOut, "li"); })();

In this example, we've used three event listeners with their associated memory and performance load to serve as delegates for the same three events on each item in the list. Instead of 3 event listeners per item, we now have three on the container. And the code works regardless of the number of li's added to the list — if the list was 100 items long, we'd be saving ourselves 297 event listeners, which is enough to make a noticable difference in how your page responds. In a complicated page context, it's easy to see how this technique can dramatically reduce the number of event listeners required and thereby improve the performance of your application.