class TweenProperty {

 final DisplayObject _displayObject;
 final int _propertyIndex;
 num _startValue = double.NAN;
 num _targetValue = double.NAN;
 num _deltaValue =  double.NAN;

 TweenProperty._internal(this._displayObject, this._propertyIndex);

 /// Animate the property from the current value to a given target value.
 void to(num targetValue) {
   _targetValue = targetValue.toDouble();
 }

 /// Animate the property from the current value by a given delta value.
 void by(num deltaValue) {
   _deltaValue = deltaValue.toDouble();
 }

 void _init() {
   switch(_propertyIndex) {
     case 0: _startValue = _displayObject.x; break;
     case 1: _startValue = _displayObject.y;  break;
     case 2: _startValue = _displayObject.pivotX; break;
     case 3: _startValue = _displayObject.pivotY; break;
     case 4: _startValue = _displayObject.scaleX; break;
     case 5: _startValue = _displayObject.scaleY; break;
     case 6: _startValue = _displayObject.skewX; break;
     case 7: _startValue = _displayObject.skewY; break;
     case 8: _startValue = _displayObject.rotation; break;
     case 9: _startValue = _displayObject.alpha; break;
     default: _startValue = 0.0;
   }

   if (_deltaValue.isNaN && _targetValue.isFinite) {
     _deltaValue = _targetValue - _startValue;
   }
   if (_targetValue.isNaN && _deltaValue.isFinite) {
     _targetValue = _startValue + _deltaValue;
   }
 }

 void _update(num transition, bool roundToInt) {
   if (_startValue.isFinite && _targetValue.isFinite) {
     var value = _startValue + transition * (_targetValue - _startValue);
     value = roundToInt ? value.roundToDouble() : value;

     switch(_propertyIndex) {
       case 0: _displayObject.x = value; break;
       case 1: _displayObject.y = value;  break;
       case 2: _displayObject.pivotX = value; break;
       case 3: _displayObject.pivotY = value; break;
       case 4: _displayObject.scaleX = value; break;
       case 5: _displayObject.scaleY = value; break;
       case 6: _displayObject.skewX = value; break;
       case 7: _displayObject.skewY = value; break;
       case 8: _displayObject.rotation = value; break;
       case 9: _displayObject.alpha = value; break;
     }
   }
 }
}