How to use polling?

Learn how to consume items from pollable sources.

There are many poll-based API around us. Sometimes you need to use these APIs to generate a stream from the polled values. To do this, use the repeat() feature:

PollableDataSource source = new PollableDataSource();
// First creates a uni that emit the polled item.
// Because `poll` blocks, let's use a specific executor
Uni<String> pollItemFromSource = Uni.createFrom().item(source::poll)
        .runSubscriptionOn(executor);
// To get the stream of items, just repeat the uni indefinitely
Multi<String> stream = pollItemFromSource.repeat().indefinitely();

Cancellable cancellable = stream.subscribe().with(item -> System.out.println("Polled item: " + item));
// ... later ..
// when you don't want the items anymore, cancel the subscription and close the source if needed.
cancellable.cancel();
source.close();

You can also stop the repetition using the repeat().until() method which will continue the repetition until the given predicate returns true, and/or directly create a Multi using Multi.createBy().repeating():

PollableDataSource source = new PollableDataSource();
Multi<String> stream = Multi.createBy().repeating()
            .supplier(source::poll)
            .until(s -> s == null)
        .runSubscriptionOn(executor);

stream.subscribe().with(item -> System.out.println("Polled item: " + item));