public ApnsServiceBuilder withDelegate(ApnsDelegate delegate) {
        this.delegate = delegate == null ? ApnsDelegate.EMPTY : delegate;
        return this;
    }
Sets the delegate of the service, that gets notified of the
status of message delivery.

Note: This option has no effect when using non-blocking
connections.
src/main/java/com/notnoop/apns/ApnsServiceBuilder.java