Mobile devices present many challenges to web application application
developers. Storage, screen size, user input, processing power and
bandwidth are all constrained when compared even a basic broadband
connected desktop PC with a modern web browser. Mobile browsers are now
entering the market that begin to close the capability gap between the
desktop and mobile device. Existing non-browser technologies such as
Java, Flash, and Brew have all fallen short of providing a portable,
quick and open development environment for creating rich web
applications.  The open web technologies that exist on the desktop are
now beginning to be made available to the mobile application developer.
 With the launch of the iPhone in June, Apple has significantly raised
the bar for everyone in the mobile industry to build better products
which support open web technologies. At SitePen, we are dedicated to
both the Open Web and to Mobile development. We have built many desktop
Ajax applications and we are now beginning to develop mobile Ajax
applications.  We have built a demonstration Chat application that
allows users to communicate in real time with low latency on the iPhone
using Dojo, the open-source JavaScript toolkit that was co-founded by
Alex Russell and Dylan Schiemann, both of SitePen.

Chat is a good example of an Ajax application that needs to quickly
update the user with data from the server. Ajax Chat apps traditionally
rely on polling the server every X seconds with a JavaScript timer.
This approach can be problematic as the number of requests per second
handled by the server can be very high. Our chat application uses Comet
instead of polling.  Comet is an extension of existing Ajax technologies
that allows the user to push data to the user via a long-lasting http
connection. Comet has many advantages over Ajax polling.  Data pushed by
the server is instantly sent to the client for processing without the
user waiting for the next polling interval.

The chat web app we created demonstrates that a stable Comet connection
can be maintained using a mobile browser. This application was built
using an unmodified version of the Dojo Toolkit's Cometd library. The
complete application including the Dojo libraries is around 50K over the
wire when compressed. Even over the relatively slow EDGE connection, the
application loads quickly and is very responsive.

The user interface of our chat app has been optimized for the iPhone to
minimize scrolling and to eliminate zooming. Those developing web
applications for the iPhone have found that Apple has taken considerable
creative license when it comes to JavaScript DOM events. For example,
tapping an element on the screen will fire both mousedown and mouseup
events at the same time instead of once when the user touches the screen
then again when the user lifts their finger. The virtual keyboard also
provides new challenges. For example, Safari fires an onblur event when
the user clicks done from the virtual keyboard.  We used this feature to
streamline the user experience by sending the chat message when the
onblur event is fired for the chat message box.  We were able to
eliminate the send button normally present in chat applications.

The iPhone chat app that we have built is a good proof of concept, but
there are many areas that still need to be explored:

1) What other mobile Ajax browsers will support the Dojo Toolkit and
will they require a modified Dojo to run correctly?
2) What impact to frequent and or long lasting http connections have on
battery life?
3) Under what conditions will the network connection drop, and what can
we do to recover?

Collaboration with hardware manufactures, browser venders, network
operators, and standards bodies will be critical in answering these
questions and to ensure that  open web technologies will be come the
best way to build applications for mobile devices.