span class="type-annotation"><a href="dart-async/RegisterCallbackHandler.html">RegisterCallbackHandler</a><span class="signature">&lt;R&gt;</span></span> <span class="parameter-name">registerCallback</span>: <span class="default-value">null</span>, </span> <span class="parameter" id="from-param-registerUnaryCallback"><span class="type-annotation"><a href="dart-async/RegisterUnaryCallbackHandler.html">RegisterUnaryCallbackHandler</a><span class="signature">&lt;R, T&gt;</span></span> <span class="parameter-na