No Values

Two extremely simple operators manage situations involving no values.

.ignoreOutput (Publishers.IgnoreOutput) ensures that the downstream has no values. It is the ultimate blocker; it just swallows all the incoming values and doesn’t output anything. Its output type is the same as the output type of the upstream, but in fact it will never emit a value of that type. It merely passes along the completion it receives from upstream.

.replaceEmpty (Publishers.ReplaceEmpty) deals with an upstream that has no values. It is the opposite of .ignoreOutput: it transforms nothingness into somethingness. This operator is initialized with a single value of the upstream Output type, which it stores. If a value or an error comes down the pipeline, this operator passes it on down the pipeline, and never does anything else. But if a .finished completion comes down the pipeline, without a value or an error having come down the pipeline previously, this operator passes its stored value down the pipeline, and then sends a .finished completion.

In this way, a pipeline that does nothing is made to do something. For example, suppose we’re subscribed to a stream of Ints, but the upstream might finish without ever providing us with an Int. And suppose we’d like to signal that as a value. We might decide to convert all incoming Ints into Optional Ints; that way, we can signal nil to mean there were no values:

myPublisher
    .map(Optional.init)
    .replaceEmpty(with:nil)