Friday, September 11, 2009

How To Track Changes in the Location Hash

As the web becomes more "2.0" we're collapsing collections of pages into fewer and even single, more dynamic pages. Instead of ten pages of event listings, we might have page that loads further items dynamically as you scroll. The state that was once static to a page is now loose and can alter in the lifetime of a page, which grows longer every day. Parameters of the page state have always sat in either the path, in URLs like http://myblog.us/post/how-are-you or the querystring in cases like http://www.coolstore.com/view.html?product=4356.

Neither approach works when those parameters are changing for the life of the page, and where a single URL needs to be able to represent these multiple parameter values at any time. In most uses so far, the bullet is simply bitten. The user can browse to your site and click around, and if they bookmark or send the link to a friend, they'll always come back to the front page, because the state of the page is no longer held in that URL. This wasn't acceptable for long and a few projects, including GMail, have taken to tossing some state information into the hash or anchor of the URL, after the # symbol. This has traditionally told a browser to scroll to a <a> tag with that name, but if none exists it becomes, essentially, a no-op. We have a place in URL we can stare state, now, without causing page loads and persisting the state in links and bookmarks. There still haven't been great or standard ways to deal with this, yet.

A couple years ago I started my own attempt to make this easier, when I found existing libraries outdated or just not really doing what I hoped for. They either seemed to depend on obsolete versions of other libraries, or only give a little trigger when the hash changed. I thought we needed something more than that, because this is really replacing everything we used to use querystrings for. Sure, I could toss #2 or #43 at the end of the URL depending on what page of results you saw, but what if the state was more than a single number? Querystrings can store lots of variables. This is what I wanted within the hash.

Born was hashtrack.js!

The API is pretty simple. You can check and set variables in the hash with hashtrack.getVar() and hashtrack.setVar(). Changes to the hash or to specific variables in it can be registered with callbacks via hashtrack.onhashchange() and hashtrack.onhashvarchange(). You can view the full documentation, included embedded interactive examples, at the github pages hosting it.

1 comment:

schmichael said...

Thanks for the code! I've been using jQuery's History plugin, but it doesn't offer per-variable callbacks (and it seems to be unmaintained).

I removed the jQuery.each dependency in a fork on GitHub. I also fixed up the whitespace and tried to make jslint happy, but you can ignore those changesets if you want. :-)

@schmichael

I write here about programming, how to program better, things I think are neat and are related to programming. I might write other things at my personal website.

I am happily employed by the excellent Caktus Group, located in beautiful and friendly Carrboro, NC, where I work with Python, Django, and Javascript.

Blog Archive