A representation of a single change in a scalar value.

class Change {
 dynamic oldValue;
 dynamic newValue;

 get isEmpty {
   return oldValue == unset && newValue == unset;
 }

 /**
  * Creates new [Change] from information about the value before change
  * [oldValue] and after the change [newValue].
  */
 Change([this.oldValue = unset, this.newValue = unset]){
   assert(oldValue is! DataReference);
   assert(newValue is! DataReference);
 }

 bool equals(dynamic other) {
   if (other is Change){
     return oldValue == other.oldValue &&
            newValue == other.newValue;
   } else {
     return false;
   }
 }

 /**
  * Applies another [change] to get representation of whole change.
  */
 void mergeIn(Change change) {
   if (change.isEmpty) {
     return;
   }
   assert(isEmpty || change.oldValue == this.newValue);
   if (isEmpty) {
     oldValue = change.oldValue;
   }
   newValue = change.newValue;
 }

 /**
  * Clones the [change].
  */
 Change clone() {
   return new Change(oldValue, newValue);
 }

 String toString() => "Change($oldValue->$newValue)";

}