Applying animation in Objective-C

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Applying animation
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We re going to start with a caveat: Canvas isn t really intended for animation. You can do it (depending on the good graces of your user s CPU), but unlike with Flash, animation is not a core purpose of the programming language. This means that whenever you do animation with Canvas, it s going to be a lot awkward and a little kludgy and thus, it s generally something that you should use as spice in your graphical web app, and not as the main course. You ll have to overcome two main obstacles when animating using Canvas. First, you have to deal with the fact that Canvas doesn t have any animation methods. Worse, once things are drawn on the canvas, they stay on the canvas. That means that whenever you animate, you have to clear your canvas (using clearRect), draw your background, and draw your figure, and then clear your canvas, draw your background, and draw your somewhat changed or moved figure, and so on. You repeat this exercise throughout the animation. Second, you should note that whenever you re writing to the canvas with a function, nothing gets written to the canvas until the function is completed. Thus, you can t use anything obvious like a for loop to do an animation. Instead, you have to draw, end your function, then later call it again with setInterval, setTimeout, or an event handler. Listing 6.6 shows how to do a simple animation using our old friend the rhombus function.
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Listing 6.6 An animation of a changing rhombus
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function initCanvas() { var canvas = document.getElementById('mycanvas'); if (canvas.getContext) { setInterval(animateRhombus,500);
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function animateRhombus() { var context = document.getElementById('mycanvas').getContext('2d'); context.clearRect(0,0,320,356); context.save(); context.fillStyle = "#dddddd"; context.fillRect(0,0,320,356); context.restore();
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context.save(); var gradient = context.createLinearGradient(0,125,0,300); gradient.addColorStop(0,'rgba(255,0,0,0)'); gradient.addColorStop(1,'rgba(255,0,0,1)'); context.fillStyle = gradient; var time = new Date(); rhombus(context,200,200,50, (time.getSeconds()%10 + 1)/20*Math.PI+.5*Math.PI, 'fill'); context.restore();
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As suggested, this programs kicks off your animation with a setInterval statement B. Every 500 milliseconds, you call the animateRhombus function. That routine starts off by clearing your canvas C, and then it fills in your background E as required. Note that you make use of the save function D, along with restore G, so that you don t have to keep track of which styles you ve set at which time. Your actual animation is done with a call to the rhombus routine F. You use the system clock to control the animation. This recognizes ten different frames and loads a new one each second. Each frame uses a different value for the rhombus angle, with the result being an animated rhombus that starts off as a square (angle = radians) and then collapses down toward a line (angle = .5 radians). Clearly, other animations are possible using shapes, lines, or even images. It s easy to imagine an animated version of figure 6.9 where the heat lines above the bowl fluctuate, thanks to changing Bezier curves, or where the smiley face changes by pulling up different image files. Global variables or other means could be used to control the animation if a clock doesn t do the job. For our purposes, however, the time has come to close the book on Canvas, and to move on to the last aspect of iPhone web development: tools.
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When you start using the SDK, you re going to have access to a few different drawing libraries. One of them, Quartz, will look a lot like Canvas. Quartz is similarly built around paths, and you ll similarly have to decide whether to stroke or fill paths when you re done with them. You ll also see a lot of other terminology in common, because Apple created both libraries. There are differences, in part because Quartz is built around Objective-C s unique methods of object-orientation. There are also just enough practical differences to sometimes get you into trouble, such as the fact that the Quartz coordinate system starts at the bottom left, not the top left (though this is partially corrected for on the iPhone). But there s enough in common that if you learn Canvas, you ll have a big leg-up on Quartz, and vice versa.
6.10 Summary
In the previous two chapters, our advice on preparing web apps for the iPhone had a distinctly textual focus. Fortunately, Apple provides a tool that you can use to add great-looking graphics to your iPhone web apps: Canvas. Though its animation functionality is somewhat limited, it does a great job of creating vector-based graphics. One of the best things about learning Canvas is that its penetration of the browser market is only going to grow. You can already view your iPhone Canvas code on Safari, Opera, and Firefox. As the HTML 5 standard moves toward completion, even Internet Explorer will doubtless catch up with the rest of the field. Something as simple as a scale command, Canvas s version of the viewport in a way, could allow you to quickly port your iPhone graphics to those other browsers. For now, however, Canvas is the last major tool in your iPhone web app toolkit. With graphics, text, and a handful of iPhone UI fundamentals in your back pocket, you should now be able to create whatever great-looking iPhone web apps you imagine. That doesn t mean that we re entirely done looking at web apps, however. We ve finished talking about the great libraries available to you, but there are also some interesting programs available for creating iPhone web pages. The first one we re going to cover is Dashcode, which can offer an interesting (if orthogonal) way to create web apps.
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