"IO is an HTTP utility that enables HTTP requests while maintaining a persistent client UI, for the purpose of data retrieval and content update. IO uses the XMLHttpRequest object for making "same-domain" requests. IO can make "cross-domain" requests, when instructed to do so, using an alternate HTTP transport."
What does this mean really? This means we have a single, consistent, interface to various data sources we may point our UI at. This data may be from our site, or another data provider on a different site. When the data resides on our site IO will use XHR (XMLHttpRequest ) to request the data, standard "AJax". When we are making our request cross-domain, the IO utility will use a flash swf to make the call, along with a crossdomain.xml (to let the flash know which sites it has access to). The crossdomain.xml is fairly important on a https: site as IE will complain to no end that you are trying to access non secure data, or mixing secure with non-secure. The crossdomain.xml resides in your document root.
So, let's see what the YUI 3 team has given us. The basic example provided at the IO Utility page sets up the use of io-base, defines a completed function, and subscribes to the event. The key here seems to be subscribing to the event :
// Subscribe to event "io:complete", and pass an array
// as an argument to the event handler "complete". Since
// "complete" is global.
// At this point in the transaction lifecycle, success or
// failure is not yet know.
Y.on('io:complete', complete, this, ['lorem', 'ipsum']);
When the Y.io(uri) event "complete" finishes, we want to run "complete" function with "this" as the context of execution, and pass also an array
This may be quite different and confusing to just about every other Framework out there. But so much better! There are a few posts on nabble where people are asking why the IO isn't "seperate" for each request, and the answer lies above. IO is a presistant connection, and it has a context that it runs in. If you are specifying IO in the global context, then all your requests are going to be in the global context, which is specified by "this".
In other words, Event handlers run in thier own context, so you could have multiple listeners on the io:complete event, but only some would run depending on context: Therefore, you could register a GlobalEvents object and set that context, just as in the examples and this would run for every IO request. If however, you have objects with different scopes, the global would fire as well as the callbacks in different scopes that are set to listen for io:complete.
The new IO utility also offers a "queue" which you can start, stop, promote, purge, and set the size. This is a big step in ajax frameworks as most like prototype do not come with these capabilities built in. With a queue you can send transactions and gracefully monitor and manage them with the io events and queue commands.
One way I can see this being very useful is if you had created a queue on page onload to load various items not immediatly visible to the user. But say the user then clicks on one of the items, we can immediatly stop the queue, and promote that request to the top of the queue, and when that completes, continue with the loading of the rest of the items! This is, to say the lease, very useful.