I don’t know why people are so enamoured with ASP.NET AJAX. Sure it works and is developed by Microsoft, but what’s wrong with the dozen other alternatives out there? Many developers I talk to will say “yeah I used Ajax on this” (with a capital A) and what they are referring to is ASP.NET AJAX. While I don’t think it is a horrible framework (I use it on occasion), I think it is too complicated and requires too much to get a simple example started. When people say they have ASP.NET AJAX experience, it means to me that they have experience with declarative controls and <ScriptManager> tags. It doesn’t necessarily mean they know how ajax works and it definitely doesn’t mean they know anything about javascript. This worries me.
I often talk about how awesome prototype is (and lately I’ve been digging jquery) and inevitably someone tells me it’s just eaiser to wrap your sections in an UpdatePanel and be done with it. It is sometimes hard to argue with that, even though it is the most inefficient way of doing ajax. In addition, this can cause side-effects in WebForms because the entire lifecycle is being executed, even though only part of the page is rendered. This method is actually called “partial html replacement” or “partial page rendering” and can be achieved easily without needing ASP.NET AJAX.
Now that ASP.NET MVC is out, which doesn’t work with ASP.NET AJAX (not the full stack) people are foaming at the mouth for their UpdatePanel equivalent. (I think we’ll see an update for this in the coming weeks that will show ASP.NET AJAX working specifically in the MVC environment, but this point still stands. You don’t have to use it.)
I’m going to use jquery in my sample, but the technique will apply to prototype, mochikit, and others. In this sample we will take a functioning page and turn it into an ajax enabled page, while preserving the non-ajax scenario (if javascript is disabled).
For our example, I will borrow from an impressive rails tutorial [Quicktime] and implement the same thing in ASP.NET MVC. The basic concept is this: Search for flickr photos and display them on the page using ajax.
We have our page:
if you search for “flowers”, you are taken to another action (physically) that shows you the results:
Now let’s assume that you want this to happen with ajax. We’d like the results to be displayed directly on the same page as the query.
Here’s the form from that first action:
<h1>Photo Search</h1>
<div class="box">
<form id="search-form" method="get" action="/photos/find">
<label for="query">Query:</label>
<input type="text" id="query" name="query" />
<input type="submit" id="search" value="search" />
<img id="indicator" style="display:none" src="../../content/load.gif" alt="loading...." />
</form>
</div>
And the action that the form submits to:
[ControllerAction]
public void Find(string query, string format)
{
var flickr = new Flickr(ConfigurationManager.AppSettings["flickr.api.key"]);
var photos = flickr.PhotosSearch(query, TagMode.AnyTag, query, 40, 1);
ViewData["query"] = query;
ViewData["photos"] = photos;
RenderView("results");
}
(This uses the FlickrNet library available on CodePlex)
In order to translate this into an ajax-enabled page (while preserving no-javascript behavior) we have to do 2 things.
- Prevent the form from posting, so that the browser doesn’t take us off the page
- Make the Find() action render only the partial html for the images (in the case of an ajax call
Let’s solve the first problem first. Using jquery, we can capture the form submit by doing this:
function init()
{
$('#search-form').submit(
function() {
$('#photos').hide();
$('#indicator').show();
hijack(this, loadPhotos, "html"),
return false;
});
}
$(document).ready(init);
When the document has been loaded, init() is called. This finds our form and adds an onsubmit handler to it. Next, we hide the photos div (in case there were pictures there already) and then we start the ajax indicator. For the love of all that is good and holy, let the user know that you’re doing something! Ok next up is this strange hijack() function. This guy takes the form and submits it via ajax instead. It accepts the form, the method to call when the result comes back (callback), and the data type of the return value. Finally, to cancel the browser from actually submitting the form, we return false.
Here is that “photos” div I mentioned a second ago: (I put this directly under our form)
<div id="photos" class="box" style="display: none">
</div>
It’s initially hidden because it will be empty when the page physically loads.
hijack() is a generic javascript function, and is defined as:
function hijack(form, callback, params, format)
{
try {
$.ajax({
url: form.action,
type: form.method,
data: $(form).serialize() + "&ajax=true",
dataType: format,
success: callback,
failure: function(e) { alert("error! " + e); }
});
} catch(e) {
alert(e);
}
}
This is pretty basic ajax stuff in jquery. The only thing different that I am doing here is notifying the controller that this is an ajax call, so that we may make it behave a little differently. After all, we don’t want it to return a full html document… we only want the images!
That’s the essense of issue #2. If we revisit our controller action, we can make the following change:
[ControllerAction]
public void Find(string query, bool? ajax)
{
var flickr = new Flickr(ConfigurationManager.AppSettings["flickr.api.key"]);
var photos = flickr.PhotosSearch(query, TagMode.AnyTag, query, 40, 1);
ViewData["query"] = query;
ViewData["photos"] = photos;
if(ajax.HasValue && ajax.Value)
{
RenderView("_images", photos);
}
else
{
RenderView("results");
}
}
We’ve added an optional parameter for the action to check for. There are a number of other ways you might accomplish the same thing by embedding this into the controller itself. You could make a convention that allows a base Controller class set an IsAjax boolean property to true if it detects this variable in form variables or querystring. Whatever you find the cleanest.
And, Here is our new form!
You couldn’t see it in that still image, but there was an animated ajax indicator, and a nice slide down effect on the photo div. If you turn off javascript, the whole thing still works.
This entire process is basically what you get with UpdatePanel. The same techniques are used to accomplish partial page rendering. Granted, UpdatePanel works slightly different (it’s parts of the same page that get updated) but you could easily return a full html document from the action and snip out the html you want to update. Or you might have a specific ViewFactory that knows to only serialize HTML starting with an element root.
With some conventions and a tiny bit of javascript, you could easily achieve partial page rendering with out any fuss. Plus you get to work with jquery, which is much mor enjoyable than ASP.NET AJAX.
UPDATE: Download the code below. You’ll need your own flickr api key (you can get one instantly from flickr.com). Place it in the web.config.
PhotoSearch.zip (124 KB).