Future and Deferred

A Future is a publisher that generalizes the sort of thing a data task publisher does: it launches some asynchronous task, and when it is subscribed to and the task has completed (or there is an error), it publishes.

Anything that takes a completion function (also known as a completion handler) is a good candidate for encapsulation into a Future. When you initialize the Future, you hand it a function. That function takes one parameter, called a promise. The promise function itself is, in fact, a completion function. When your completion function is called, your job is to call the promise completion function, thus signalling to the Future that it is time to publish.

The promise function takes one parameter: a Result. As you know, a Result is an enum with a .success case and a .failure case, each of which can have an associated value:

• If you call the promise function with a .success Result, the Future emits that case’s associated value, publishing it on down to the downstream subscriber. It then sends a .finished completion.

• If you call the promise function with a .failure Result, the associated value is an Error; the Future treats that as a failure, and it sends that Error down the pipeline as a failure in the usual way.

An example will clarify instantly. I can’t use a data task as my example, because there’s already a built-in publisher for that; so I’ll use Core Location geocoding. You’ve probably written code along these lines:

let s = "Nordhoff High School, Ojai, California"
CLGeocoder().geocodeAddressString(s) { placemarks, error in
    guard let placemarks = placemarks else { return }
    let p = placemarks[0]
    let mp = MKPlacemark(placemark:p)
    if let coord = mp.location?.coordinate {
        // latitude: 34.4418801, longitude: -119.2671168
        // ...
    }
}

That’s an asynchronous task; everything starting with placemarks, error in is the completion function, which is not called until the CLGeocoder has gone out on the network and tried to obtain location information from an online database. Let’s express that as a Future.

Our Future will either publish a CLLocationCoordinate2D, if the geocoding succeeds, or it will emit an Error if the geocoding fails. The important thing is that in our completion function we must call the promise function with a Result, no matter what. The elegant way to ensure that is to wrap all of our code in a Result initializer and then just call the promise function once with that Result — because the compiler will ensure that every path of execution inside the Result initializer will either throw an error or return a coordinate:

enum MyError : Error { case oops }
let future = Future<CLLocationCoordinate2D, Error> { promise in
    let s = "Nordhoff High School, Ojai, California"
    CLGeocoder().geocodeAddressString(s) { placemarks, error in
        let result = Result<CLLocationCoordinate2D, Error> {
            if let error = error { throw error }
            guard let placemarks = placemarks else { throw MyError.oops }
            let p = placemarks[0]
            let mp = MKPlacemark(placemark:p)
            if let coord = mp.location?.coordinate { return coord }
            throw MyError.oops
        }
        promise(result)
    }
}

We have now formed a Future publisher called future. If we attach a Sink to future, we find that the Sink receives the CLLocationCoordinate2D followed by a .finished completion. If any of the throw statements had executed, it would have received a .failure completion instead.

However, there’s a potential problem. What happens if we don’t attach a Sink to future? The answer is that the Future’s function runs anyway. We go out on the network with our geocoder and try to get a coordinate for our address, even though we have no subscriber. That is not the way a data task publisher behaves; it waits to start operating until it actually has a subscriber. If we want our Future to behave like that, we can wrap it in a Deferred publisher.

A Deferred publisher is extremely simple: it is initialized with a function that returns another publisher, but it doesn’t run that function, and thus doesn’t create that publisher, until it is subscribed to. To turn any publisher into a deferred publisher, simply write

Deferred {
    // publisher goes here
}

So let’s turn our Future into a Deferred Future:

let deferredFuture = Deferred {
    Future<CLLocationCoordinate2D, Error> { promise in
        let s = "Nordhoff High School, Ojai, California"
        CLGeocoder().geocodeAddressString(s) { placemarks, error in
            let result = Result<CLLocationCoordinate2D, Error> {
                if let error = error { throw error }
                guard let placemarks = placemarks else { throw MyError.oops }
                let p = placemarks[0]
                let mp = MKPlacemark(placemark:p)
                if let coord = mp.location?.coordinate { return coord }
                throw MyError.oops
            }
            promise(result)
        }
    }
}

If we run that code, nothing happens. And that’s exactly what we want! It isn’t until we actually attach a subscriber to deferredFuture that the geocoder springs into action and we start networking.