'Send a change event notification if the property is set to a
value is not equal to ``old``.
Args:
obj (HasProps)
The object the property is being set on.
old (obj) :
The previous value of the property to compare
Returns:
None'

def trigger_if_changed(self, obj, old):

    new_value = self.__get__(obj, obj.__class__)
    if (not self.property.matches(old, new_value)):
        self._trigger(obj, old, new_value)
