Although it’s easy to get Javascript to read from an XML file, pulling out the appropriate data can sometimes be tricky. Here are some tips to help you tame the XML beast.

Part 6: Taming XML

Let’s look at a typical XMLHttpRequest again.

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When I first started developing this crossword tutorial, I wanted the end game to access USA Today’s crossword XML feed directly and offer a new game each day. Sadly, their feed is secured. Although I can track it down, view its source in a Web browser, save to a local file and upload to my server, I cannot get my code to access the file directly from their server. Keep that in mind when we reach the end of this tutorial. You’ll be able to play one completed game, but don’t get too attached. There’s not much replayability.

Part 5: Importing From XML

Most programs, including games, require data. More often than not this data must be updated regularly so as to sustain the usefulness of the application. This type of data is usually stored externally (out of the application’s main code) since it makes very little sense to require the application’s user to download a new version of the program every day in order to receive the latest and greatest data. Therefore, with respect to our game, it makes a great deal of sense to store the data for each crossword puzzle in an external XML file. This way each puzzle can easily be loaded individually by referencing a different XML file.

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Unlike desktop applications, or native iPhone apps, whose code is pre-compiled with assets prepped and readily available in nearby bundles, Web apps are burdened with the task of downloading their assets (HTML, CSS, images, scripts, etc) in real time, as a user is attempting to interact with the app. In this context I’m referring to a Web site as an “app” since so many of today’s Web 2.0 sites are robust enough to be considered applications. Also, Apple has often referred to iPhone Web sites as Web apps. HTML, CSS and scripts are generally low weight files, a couple of kilobytes a piece (some are larger, but most aren’t) and can be downloaded quickly. Images, on the other hand, can be very large—upwards of 100K in some instances. Downloading large images can be time consuming, especially on mobile devices, therefore you can improve your application’s user experience by downloading images ahead of time.

This part of the tutorial is actually not going to explain proper procedures for preloading images. There are a number of methods available, all of which can be found by doing a simple Google search for “preload images”. This article will instead concentrate on ways to reduce the necessity of preloading altogether by means of self-contained data and CSS Sprites.

I’ll start off by briefing explaining the importance of CSS Sprites (since Part 4 below will explain how to use self-contained data). An in-depth discussion of CSS Sprites can be read on A List Apart. In a nutshell, a sprite map is a collection of smaller images mapped onto one large image. By combining images, you will reduce server requests and speed up download times. CSS Sprites especially come in handy when dealing with hovers.

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This part of the tutorial makes mention of the canvas tag. When Mac OS X 10.4 was released, Safari began supporting a new feature called the canvas. The canvas (refered to as CSS Canvas on the Safari 4 features page) provides a method for drawing arbitrary content in a Web page. This extension lets you reserve an area of your web page or widget and use rendering calls like those found in Quartz (or ActionScript) to paint complex paths and shapes in that area. CSS Canvas was heavily utilized by the initial offering of Dashboard widgets released by Apple.

Safari, Firefox, Dashboard, and all other WebKit-based applications, including Safari for the iPhone, support CSS Canvas. There’s even rumor that Internet Explorer 9 will use WebKit, allowing it access to CSS Canvas as well. So it really makes a lot of sense to start learning how to use CSS Canvas now, so that you can build really cool Web sites that take advantage of it later.

Part 3: Jumping Into Code

Let’s start programming this game. The canvas tag, supported by Firefox and Safari, is a really great way to draw to (or animate) a Web page. Since the iPhone does not yet support Flash, utilizing the canvas makes a lot of sense. Since more qualified people than ! have already written great tutorials on how to use the canvas, it may behoove you to read up a bit if you become confused with what follows.

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When I originally posted part two of this tutorial last year, I prefaced it with news of an “imminent” release of Adobe’s Flash for the iPhone. Well, a year has passed and still no Flash. Around a month ago, MacRumors announced a collaboration with Adobe and Apple on a Flash solution, but I’ve since read that business negotiations are delaying that initiative. So we’re still a no go.

I mention Flash because it would seem that the inclusion of the plug-in would open the door to many established and seasoned Flash developers who are not savvy with HTML/CSS/JavaScript or Objective-C (for SDK development). But with the introduction of CSS animation, which the iPhone (and now Safari 4 beta) already support, creating visually intriguing web user interfaces (for games or other web apps) is more plausible than ever before, even without Flash.

It may not seem realistic to create iPhone web app games anymore, since the release of the SDK over a year ago, but iPhone Web designers and developers can still strive to create visually impressive web experiences. And some of these tips may help towards that cause.

Part 2: Design Considerations

When designing web app games for the iPhone keep in mind that although the screen’s resolution is 320×480 pixels, some of the vertical pixels are lost to the status bar, address bar and lower toolbar… 64 pixels to be exact. So, if you want your entire game to display “above the fold,” design it to be 320×416 pixels. (Above the fold is a reference web designers throw around a lot to appear cool. It refers to the topmost part of a Web page. The area that is initially viewable. The content that shows up as you scroll down the page is “below the fold.”)

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