Promise

public struct Promise<Value>

A promise to provide a result later.

This is the provider API for Future<Value>. If you want to return a Future<Value>, you can use the global functions promise(), or create a new Promise<Value> to fulfill in an asynchronous fashion. To create a new promise, returning a Future<Value>, follow this pattern:

promise {
    "Hello World!"
}

The above function will return a Future<String> eventually carrying the stirng Hello World!.

You can also provide a future using a callback, like so:

promise(String.self) { completion in
    ... if success ...
    completion(.fulfilled("Hello World!"))
    ... if error ...
    completion(.rejected(error))
}

If you want to provide a Future<Value> in a completely custom manner, you can create a pending promise, resolve it when convenient, and flatMap return its Future:

func someAsynOperation(args) -> Future<ResultType> {
    let promise = Promise<ResultType>()
    dispatchQueue.async {
        ... if success ...
        promise.fulfill(value)
        ... if error ...
        promise.reject(error)
    }
    return promise.future
}