Signal

Returns a stream of the current value and subsequent updates to this signal.

Even if you are pulling as fast as possible, updates that are very close together may result in only the last update appearing in the stream. In general, when you pull from this stream you may be notified of only the latest update since your last pull. If you want to be notified about every single update, use a Queue or Channel instead.

Returns when the condition becomes true, semantically blocking in the meantime.

This method is particularly useful to transform naive, recursive polling algorithms on the content of a Signal/ SignallingRef into semantically blocking ones. For example, here's how to encode a very simple cache with expiry, pay attention to the definition of view:

trait Refresh[F[_], A] {
 def get: F[A]
}
object Refresh {
 def create[F[_]: Temporal, A](
   action: F[A],
   refreshAfter: A => FiniteDuration,
   defaultExpiry: FiniteDuration
 ): Resource[F, Refresh[F, A]] =
   Resource
     .eval(SignallingRef[F, Option[Either[Throwable, A]]](None))
     .flatMap { state =>
       def refresh: F[Unit] =
         state.set(None) >> action.attempt.flatMap { res =>
           val t = res.map(refreshAfter).getOrElse(defaultExpiry)
           state.set(res.some) >> Temporal[F].sleep(t) >> refresh
         }

       def view = new Refresh[F, A] {
         def get: F[A] = state.get.flatMap {
           case Some(res) => Temporal[F].fromEither(res)
           case None => state.waitUntil(_.isDefined) >> get
         }
       }

       refresh.background.as(view)
     }
}

Note that because Signal prioritizes the latest update when its state is updating very quickly, completion of the F[Unit] might not trigger if the condition becomes true and then false immediately after.

Therefore, natural use cases of waitUntil tend to fall into two categories: - Scenarios where conditions don't change instantly, such as periodic timed processes updating the Signal/SignallingRef. - Scenarios where conditions might change instantly, but the p predicate is monotonic, i.e. if it tests true for an event, it will test true for the following events as well. Examples include waiting for a unique ID stored in a Signal to change, or waiting for the value of the Signal of an ordered Stream[IO, Int] to be greater than a certain number.