Animation is a set of action to execute onBegin, onTick, onEnd.

It can be used to playe some action based on time :

• animate visual Entity (eg: onTick rotate an 3D object)
• countdown to trigger some action onEnd, or update value onTick
• run periodic action on the idem (without using a dedicated EntitySystem)

class Animation {
 /// set by System_Animator when it start playing
 double _t0 = -1.0;

 /// Callback before first call of [onTick] (same tick)
 OnStart onBegin = onNoop;

 /// Callback each tick of the [System_Animator],
 /// the animation is ended when onUpdate return false
 OnUpdate onTick = onNoop;

 /// Callback when animation is ended (after last [onTick], same tick)
 OnComplete onEnd = onNoop;

 /// [Animation] to chain (to animate when this is completed)
 Animation next = null;
}