Working with the Curve – Advanced Animation with Dojo

If you use animations, you probably use them to animate CSS properties. But this post is about some real bareknuckle animation – using the dojo toolkit. We’ll have a look into dojo.Animation and talk about the curve, the line, easing and rate, and we’ll check out (the somehow undocumented) dojox.fx._core – and see how to work with multidimensional lines.

The Curve

Every dojo.Animation needs a curve. If you call animateProperty or it’s shorthand anim, you don’t have to provide the curve for yourself, dojo will do it for you – if you build an animation using new dojo.Animation(), you have to provide your own curve. You can think of the curve as of a line that the animation walks on. The line is already present before the animation starts to walk it, and it can always take a break and pause in the middle and eventually continue walking where it had stopped. To create a curve, you need to call new dojo._Line(start, end). In return, you get an object with three properties: end, start and a getValue() method.

var anim = new dojo.Animation({
	curve: new dojo._Line(0,100)
});

Easing

The curve will then be modified by a easing function. By default, this is the linear dojo._defaultEasing. Easing breathes a bit of life into the curve, you can say. Other properties of dojo.Animation that affect the curve are rate and duration. rate is the timespan between two frames. To calculate the targeted fps, use (1000/rate). By default, dojo aims at 50 fps.

var anim = new dojo.Animation({
	curve: new dojo._Line(0,100),
	easing: dojo.fx.easing.quadInOut
});

Waking Life

To get some numbers out of dojo.Animation, let’s let it go wild and play(). When using animateProperty or the likes, you just fire and forget. In our case, we need to get the numbers. To achieve this, we connect to our animation’s onAnimate event. This is fired on every frame of the animation. When your connected function is called, you get – who would have guessed – the current value of the curve. Now you have a function called at a given rate, for a given period of time and you get get a numeric value that is already calculated for you. Decent!

dojo.connect(anim,'onAnimate',function(curveValue){
	// do something
});

To see some curves in action, go to this test page, open the console and type play(1); and play(2); and check the source code to see what’s happening.

Multidimensional Curves

dojo._Line generates only one-dimensional curves. But, there is dojox.fx._core, which contains an (old) version of dojox.fx._Line, which is capable of creating multi-dimensional curves. To use it, you have to explicitely dojo.require("dojox.fx._core"). It’s a bit odd, but the _core file contains only the _Line, and it is not loaded when requiring “dojox.fx”. Well, whatever, you can pass arrays of numbers into dojox.fx._Line:

new dojox.fx._Line([0,2],[100,6]);

Now you have a multi-dimensional curve. Or, if you could say, you have two curves in one. You can (technically) pass an unlimited amount of numbers into dojox.fx._Line. When you hand this curve to a dojo.Animation, it will animate all dimensions of the curve. And it will modify all dimensions of the curve according to easing, rate and duration. If you connect a function to onAnimate, it will now recieve an array of numbers.

var anim = new dojo.Animation({
	curve: new dojox.fx._Line([0,2],[100,6]),
	easing: dojo.fx.easing.quadInOut
});

Head over to the test page again, and type play(3); into the console. Now, the width of the bars is also animated – by the same animation function that calculates the height of the bars.

What now?

If your mind is still not blown away with ideas about how to use this, let me give you a start:

animate position, width and height of a browser window and use it as a screensaver for some unknowing, yet thankful visitor…
take some CSS 2D and 3D transisitions/transforms and animate away…
canvas! No need to say another word…
Still too banal? Go WebGL! Grab some 3DContext and animate an evil ball of jelly jumping around!

In short: dojo’s animation system allows you to not only animate CSS properties, but gives you a good tool for animations in general. You get the curves on the one hand, but you also get the “player” for these curves, including play/pause/stop/gotoPercent methods. And dojo.fx.easing has a lot of easing functions in stock.