ZTween is a simple tweening engine written in AS3 for Flash Player version 10 and up. A "tweening engine" is something that takes a property and makes it go from an initial value to a target value in a given amount of time and with specific pre-programmed types of transition (linear, exponential, sine, etc). This can be used for animations and other kinds of timed behaviors in a Flash interface.

ZTween is built to be fast, small, have a minimal memory footprint, and be simple. The latter means that, as part of its design, ZTween does not employ any "special" tweening properties - it will only tween existing properties of objects, or variable values. It cannot create "proxy" functions like scale for scaleX and scaleY, auto-invisible set for alpha, auto-updater properties for BitmapFilter instances, etc.

ZTween uses a syntax that is similar to modern AS3 engines. You basically add a new tween by using the line:

ZTween.add(target, properties, parameters);

• target is the target object you want to change a property - any kind of object.
• properties is an object containing the properties you want to tween (as the key), and their new values (as their value).
• parameters is another object, this time containing some options about how the tween should be ran.

Here are some examples:

// Move sprite `myBox` to X position 10, in 1 second
ZTween.add(myBox, {x:10}, {time:1});

// Fade sprite `myCircle` out (to alpha 0) in 2 seconds, while also scaling it to 200% of its original size
ZTween.add(myCircle, {alpha:0, scaleX:2, scaleY:2}, {time:2});

// Set the `position` of an arbritary `slider` instance to 0.5 in 2 seconds, with a 1.5 second delay
ZTween.add(slider, {position:0.5}, {time:2, delay:1.5});

// Moves sprite `myImage` to position X=10, Y=20, in 10 seconds, using an On/Out Cubic transition equation:
ZTween.add(myImage, {x:10, y:20}, {time:10, transition:Equations.cubicInOut});

And so on and so forth. Notice that any object can be used as the target, and any of its properties can be used as part of a tweened property (as long as it's numeric).

These parameter names can be used as part of the parameters for a new tween:

• time: time for the tween to be executed, in seconds or number of frames (see useFrames). Default is 0 (which does an immediate tween).

• delay: delay to wait before the tween is executed, in seconds or number of frames (see useFrames). Default is 0.

• transition: any function that takes one parameter t (0-1) and returns a new transformed t value to create different transition behaviors. This is used for tween update acceleration and deceleration. Common equations for this parameter are provided on the Equations class. Default is Equations.none, which produces a linear tween. See Transitions.

• onStart: a function to be called when the tween starts. Works like a callback/event. See Using signals.

• onStartParams: parameters for the onStart function, as an Array of items. See Using signals.

• onUpdate: a function to be called when the tween update occurs. Works like a callback/event. See Using signals.

• onUpdateParams: parameters for the onUpdate function, as an Array of items. See Using signals.

• onComplete: a function to be called when the tween ends. Works like a callback/event. See Using signals.

• onCompleteParams: parameters for the onComplete function, as an Array of items. See Using signals.

• useFrames: a Boolean value that indicates whether the number used in time and delay represents frames. If set to true, this tween will have frame-based timing. If not, its duration time (and property update value) is based on real time (seconds). This dictates how time is controlled, not when updates are made; updates are always made on frame cycles. It is advised to always have this set as false, save on exceptional cases. Default is false.

• TODO: Note: the above is not working on parameters; it only works when setting it on the tween property

• paused

In ZTween, the transition parameter defines the function used when updating the values. By default, the transition used is a linear function, that is, the property being tweened will go from the initial value to the target value in a uniform fashion. But by using different transition functions - usually called easing functions - you can have different update speeds, usually giving the appearance of acceleration or deceleration of the value being updated.

All the classic easing equations, as introduced by Robert Penner, are bundled with ZTween. They are part of an additional class, Equations, and are as such:

• Equations.none
• Equations.quadIn
• Equations.quadOut
• Equations.quadInOut
• Equations.cubicIn
• Equations.cubicOut
• Equations.cubicInOut
• Equations.quartIn
• Equations.quartOut
• Equations.quartInOut
• Equations.quintIn
• Equations.quintOut
• Equations.quintInOut
• Equations.sineIn
• Equations.sineOut
• Equations.sineInOut
• Equations.expoIn
• Equations.expoOut
• Equations.expoInOut
• Equations.circIn
• Equations.circOut
• Equations.circInOut
• Equations.elasticIn
• Equations.elasticOut
• Equations.elasticInOut
• Equations.backIn
• Equations.backOut
• Equations.backInOut
• Equations.bounceIn
• Equations.bounceOut
• Equations.bounceInOut

For event-like behavior, ZTween uses signals, an approach based on Robert Penner's AS3Signals project. This allows faster, easy-to-use callbacks to functions to be fired when certain events occur with specific tweens.

The signals currently available for tweens are as such:

• onStart: called when a tween starts
• onUpdate: called when a tween update occurs
• onComplete: called when a tween ends

There are two ways to define the signals used by a tween in ZTween. The first method is when creating a new tween:

ZTween.add(this, {x:10}, {time:2, onComplete:myOnCompleteFunction});`

Or when assigning it to an existing ZTween instance:

var myTween:ZTween = ZTween.add(this, {x:10}, {time:2});
myTween.onComplete.add(myOnCompleteFunction);

In both of the cases above, the function myOnCompleteFunction will be called when the tween finishes executing.

The latter method has one advantage: you can attach several different functions to the same signal. For example:

var myTween:ZTween = ZTween.add(this, {x:10}, {time:2});
myTween.onComplete.add(myOnCompleteFunction);
myTween.onComplete.add(myOtherOnCompleteFunction);

In the above example, both myOnCompleteFunction and myOtherOnCompleteFunction will be called when a tween finishes executing.

• Better documentation
• Decide whether to allow new ZTween() constructor or not, or get rid of ZTween.add() in favor of it
• Make sure useFrames work
• Add equationParams
• Properly document Equations.combined()
• Not sure if all equations are working, since I cleaned them up a little bit and rewrote some of them