Working with Events, part 2
In my last article, I described the common problem of events seemingly ceasing to work for new elements added to a document, whether by some form of ajax or by DOM modification. We examined one way to overcome the problem: Event Delegation. With event delegation, we bind the event handler to a containing element that remains in the DOM and then check for the target of the event.
Cloning Nodes
This time, we'll take a look at re-binding event handlers. But before we do, I should mention that, as of jQuery version 1.2, event handlers can be cloned along with elements. Consider this unordered list:
[html]- plain
- special
- plain
We can make a copy of <li class="special"> and insert it after the original, while at the same time retaining any event handlers that were attached within the original. The plain .clone() method doesn't work that way; instead, it just copies the element:
- plain
- special
- plain
As you can see, the original button is able to keep creating new list items, but the buttons inside the "dynamically generated" list items can't create new ones.
To get the event handlers copied over as well, all we have to do is pass in true to the method's single argument:
Note: I've added .append(' I\'m a clone!') here only as visual reinforcement of what is going on.
Try it now:
- plain
- special
- plain
Using .clone(true) is great when we want to make a copy of existing elements and their event handlers, but there are plenty of other situations that don't involve cloning in which we might want event handlers to persist.
Re-binding Basics
The basic concept behind re-binding is fairly straightforward: We create a function that binds the handlers and then call it whenever new elements are introduced. For example, with our unordered list above, we first create a function called addItem that registers the click handler, which in turn will add a new item:
Next, we call that function when the DOM initially loads:
[js]$(document).ready(function() { addItem(); });[/js]Finally, we can call the function inside the click handler—and inside itself. That way, it will bind the event handlers to the new list item as well.
We'll add one more click handler to the button, but this one will not be re-bound, so that we can see the difference.
Here is what the code for buttons in #list5 looks like, all together:
[js]function addItem() { $('#list5 li.special button').click(function() { var $newLi = $('Try this one out:
- plain
- special
- plain
We can see that "pressed" is appended to the first list item each time it is clicked, but it is not appended to any of the list items created by our clicks. On the other hand, the created buttons are able to generate further list items because that function is being rebound.
However, what we've just done produces unwelcome results if we click on a button more than once. The click handler is bound again with each click of a button, producing a multiplier effect. The first click of a button creates one extra list item; the second creates two; the third, four; and so on.
Unbind, then Bind
To avoid the multiple binding, we can unbind first and then re-bind. So in line 2, instead of this ...
$('#list5 li.special button').click(function() {
... we'll have this ...
$('#list6 li.special button').unbind('click').bind('click', function() {
Note the use of .bind() here. This is the universal event binder that jQuery uses. All the others, such as .click(), .blur(), .resize(), and so on, are shorthand methods for their .bind('event') equivalent.
The complete new code, again with the additional non-rebinding click handler for contrast, looks like this:
[js]function addItemUnbind() { $('#list6 li.special button') .unbind('click') .bind('click', function() { var $newLi = $('See how this one works:
- plain
- special
- plain
Unfortunately, our attempt to unbind the addItemUnbind() function went too far, unbinding the "non-rebinding" click handler as well, before it even had a chance to run once (it's evident because there is no "pressed" text after the "I am special" button here). Clearly, we're going to have to be more careful about what we're unbinding.
Event Namespacing
One way to avoid the over-reaching event unbinding is to apply a "namespace" to the click event for both binding and unbinding. So, instead of .bind('click') and .unbind('click), we'll have, for example, .bind('click.addit') and .unbind('click.addit). Here is one last code sample, which looks identical to the previous, except that it now has the namespaced event (and the list id is list7):
We should now — finally! — have a set of behaviors attached to these buttons that act the way we intend them to:
- plain
- special
- plain
For more information about event namespacing, read Brandon Aaron's article, Namespace Your Events.
Bonus: Unbind by Function Reference
If you've made it this far, then you must have extraordinary patience, in which case I'll reward it with one final method of rebinding. Rather than namespace the events, we can reference the function in the second argument of the .bind() and .unbind() methods. We have to shuffle things around a bit to avoid "too much recursion," but it'll do just fine like so:
Note here that there are no parentheses after "addItemFinal" when it appears inside the bind/unbind, because we are referencing the function, not calling it. So let's test it out one last time:
- plain
- special
- plain
Plugin Options
There are three great plugins that can do a lot of this work for us:
- Live Query by Brandon Aaron
- Listen by Ariel Flesler
- Intercept by Ariel Flesler
If this entry left you bewildered, or if you want a quick, well tested solution, you should definitely try one of them. Each works a little differently, but they're all super plugins.
Update: Event Namespacing with Add and Remove
In response to the comment by Nick Johns below, I'm providing an example that allows for both adding a row and removing a row. The code is based on the "Event Namespacing" example above:
[js]function addRemoveItemNS() { var $newLi = $('I added an "addone" class to the initial button, but otherwise the list is the same as the others. You can try it out here:
- plain
- special
- plain
