s which plugin sent the message. The <code>data</code>
parameter is the payload and is typically UTF-8 encoded JSON but can be
arbitrary data.</p>
<p>Message handlers must call the function given in the <code>callback</code> parameter.
If the handler does not need to respond, the handler should pass null to
the callback.</p>
<p>The framework invokes this callback in the same zone in which the
callback was set.</p>
        </section>
                <section class="summary source-code" id="source">
         