rong>
    </dt>
    <dd>
    Call a function when the properties of the proxy object change.


    <h3>Parameters:</h3>
    <ul>
        <li><span class="parameter">callback</span>
            <span class="types"><span class="type">function</span></span>
         a function that will be called when the
 properties change. The callback will receive the proxy object itself and two
 tables: <code>changed_properties</code> (a table where the keys are the properties that
 changed and the values the new values) a