<p> Callback invoked when the operation fails. </p>
@return True if the callback is known to never block the caller
<p> Callback invoked when the operation completes. </p>
Instance of Adapter that can be used when the callback methods need an empty implementation without incurring in the cost of allocating a new Adapter object.
<p> A callback abstraction that handles completed/failed events of asynchronous operations. </p> <p> <p> Semantically this is equivalent to an optimise Promise<Void>, but callback is a more meaningful name than EmptyPromise </p>