Why Deferred?

If you’re a computer, people are really slow. And networks are REALLY slow.

Async Programming with Callbacks Is Bad News

The standard solution to this in Cocoaland is writing async methods with a callback, either to a delegate or a completion block.

Async programming with callbacks rapidly descends into callback hell: you’re transforming your program into what amounts to a series of gotos. This is hard to understand, hard to reason about, and hard to debug.

How Deferred is Different

When you’re writing synchronous code, you call a function, you get a return value, you work with that return value:

let friends = getFriends(forUser: jimbob)
let names = friends.map { $0.name }
dataSource.array = names
tableView.reloadData()

Deferred enables this comfortable linear flow for async programming:

let friends = fetchFriends(forUser: jimbob)
friends.upon(.main) { friends in
    let names = friends.map { $0.name }
    dataSource.array = names
    tableView.reloadData()
}

A Deferred<Value> is a Value whose value is unknown till some future time. The method passed to upon gets run once the value becomes known.

More Than Just a Callback

You might be thinking, Wait! That’s just a callback! You got me: upon is indeed a callback: the closure gets the Value that the Deferred was determined to represent and can work with it.

Where Deferred shines is the situations where callbacks make you want to run screaming. Like, say, chaining async calls, or fork–join of calls. What if getting a friend’s name also required a remote call? We’d want to do this:

• Fetch jimbob’s friends
• For each friend, fetch their name
• Once you have all the friends’ names, now update the data source and reload the table view.

This gets really messy with callbacks. But with Deferred, it’s just:

// Let's use type annotations to make it easier to see what's going on here.
let friends: Deferred<[Friend]> = fetchFriends(for: jimbob)
let names: Future<[Name]> = friends.andThen(upon: .global()) { (friends: [Friend]) -> Future<[Name]> in
    // fork: get an array of not-yet-determined names
    let names: [Deferred<Name>] = friends.map { AsynchronousCall(.Name, friend: $0) }

    // get a not-yet-determined array of now-determined names
    return names.allFilled()
}

names.upon(.main) { (names: [Name]) in
    // names has been determined - use it!
    dataSource.array = names
    tableView.reloadData()
}