An event emitted by a PropertyObservable if the value of a property changes.

class PropertyChangeEvent {
 /// the source object where the property was changed
 final Object source;
 /// the property name
 final String name;
 /// the old value
 final oldValue;
 ///  the new value
 final newValue;
 PropertyChangeEvent(this.source, this.name, this.oldValue, this.newValue);

 String toString() =>
     "{PropertyChangeEvent: source=$source name=$name, oldValue=$oldValue,"
     " newValue=$newValue";
}