Runtime model

Streams are similar to futures, but instead of yielding a single value, they asynchronously yield one or more values. They can be thought of as asynchronous iterators.

Just like futures, streams are able to represent a wide range of things as long as those things produce discrete values at different points sometime in the future. For instance:

  • UI Events caused by the user interacting with a GUI in different ways. When an event happens the stream yields a different message to your app over time.
  • Push Notifications from a server. Sometimes a request/response model is not what you need. A client can establish a notification stream with a server to be able to receive messages from the server without specifically being requested.
  • Incoming socket connections. As different clients connect to a server, the connections stream will yield socket connections.

The Stream trait

Just like Future, implementing Stream is common when using Tokio. Streams yield many values, so lets start with a stream that generates the fibonacci sequence.

The Stream trait is as follows:

trait Stream {
    /// The type of the value yielded by the stream.
    type Item;

    /// The type representing errors that occurred while processing the computation.
    type Error;

    /// The function that will be repeatedly called to see if the stream has
    /// another value it can yield
    fn poll(&mut self) -> Poll<Option<Self::Item>, Self::Error>;
}

The Item associated type is the type yielded by the stream. The Error associated type is the type of the error yielded when something unexpected happens. The poll function is very similar to Future’s poll function. The only difference is that, this time, Option<Self::Item> is returned.

Stream implementations have the poll function called many times. When the next value is ready, Ok(Async::Ready(Some(value))) is returned. When the stream is not ready to yield a value, Ok(Async::NotReady) is returned. When the stream is exhausted and will yield no further values, Ok(Async::Ready(None)) is returned.

Next up: I/O Overview