Wednesday, Feb 1, 2012

Missing jQuery events while rendering

While fixing some bugs within my Backbone.LayoutManager plugin, I found an odd behavior in the way I encouraged users to attach a rendered layout into the DOM. Since you can reuse a layout and it's simply a DIV node created by Backbone, I figured it was sufficient to simply have a user call jQuery's html method to inject it into the correct container.

An example of what that code looks like:

// A callback function would fire with a DOM Node callback(function(domNode) { $("body").html(domNode); });

This code looks completely valid and not likely to cause any immediate problems, however this is not the case. The issue is that all events would be removed upon a second render. So the first call to $("body").html(el) would work exactly as expected with all events functioning, but the second call to $("body").html(el) would result in no events firing.

I brought this issue to the attention of jQuery committer Dave Methvin who insisted that this was not a bug and that inserting a DOM element into the DOM using the jQuery html function was not a supported signature in the API.

This was confusing to me as I have always seen and used the html function in this way. After learning from Dave that I should be using $("body").empty().append(el), I went back to my code and implemented swapping using empty.

Unfortunately, no dice.

The events were still disappearing, so I made a reduced test case that looked something like this:

var el = $("<div>lol</div>"); { alert("hi"); }); $("body").empty().append(el); $("body").empty().append(el);

Since the following code yields missing events, this proved to me that something was happening inside the empty method. Sure enough digging into that function yields the following code:

// Remove element nodes and prevent memory leaks if ( elem.nodeType === 1 ) { jQuery.cleanData( elem.getElementsByTagName("*") ); }

The call to jQuery.cleanData removes all events from the element, in this case the "body" element. Since this is a shared reference to the exact same DOM node, that means when you re-attach to the DOM a second time it's not going to get it's events back.

Problem detected and understood, but how to fix?

Luckily the fix is really simple and is now implemented inside of the LayoutManager plugin. Since the call to empty or html only remove events from elements inside the DOM we can easily "detach" using the jQuery detach method in between renders. The updated working code from the reduced test case looks like this:

var el = $("<div>lol</div>"); { alert("hi"); }); $("body").empty().append(el); el.detach(); $("body").empty().append(el);

This may be a very obvious problem and solution to many developers, but it bit me and I've talked to many other developers lately who have been having this same issue while using Backbone.js. It may be that many client side developers do not reuse the same DOM node, avoiding this issue.