<p>A callback abstraction that handles completed/failed events of asynchronous operations.</p>
@param <C> the type of the context object
<p>Empty implementation of {@link Promise}.</p>
<p>Callback invoked when the operation fails.</p>
<p>Callback invoked when the operation completes.</p>
See Implementation
<p>A callback abstraction that handles completed/failed events of asynchronous operations.</p>
@param <C> the type of the context object