Swift Future

A Future is used to retrieve the result of a concurrent, asynchronous operation. It is a key building block in asynchronous, non-blocking code.

Nomenclature: Note how the terms Future and Promise are often used interchangeably. In the present context, the future is read-only, while the promise is what the value is set on (i.e. it's writeable). The future is what we'll get the evaluated value from, the promise is what the value will be set on.

In it's simplest form, we can use a Future for time consuming CPU work:

let future = Future<Double>() { return estimateπ() }
future.whenSuccess() { print("π ≅ \($0)") }

This trivial example shows how a Future splits the creation of the value and processing its result into two seperate parts that are both asynchronous.

A common application of a Future is to wrap asynchronous API, such as network API. For this we use a Future-Promise pair:

let (future, promise) = Future<Int>.createPromise()
getNumberOfNetworkHops("www.swift.org") { (hops: Int) -> () in
  promise.completeWithValue(hops)
}
future.whenSuccess() { print("number of network hops: \($0)") }