CommonPatterns.md

Common Patterns

One feature of promises that makes them so useful is that they are composable; enabling complex, yet safe asynchronous patterns that would otherwise be quite intimidating with traditional methods.

Chaining

The most common pattern with promises is chaining:

firstly {
    fetch()
}.then {
    map($0)
}.then {
    set($0)
    return animate()
}.ensure {
    cleanup()
}.catch {
    handle(error: $0)
}

If you return a promise in a then the next then waits on that promise before continuing. This is the essence of promises.

Composing promises is easy, and they thus encourage you to develop great apps without fear for the typical spaghetti (and associated refactoring pains) of asynchronous systems that use completion handlers.

APIs That Use Promises

Promises are composable, return them instead of providing completion blocks:

class MyRestAPI {
    func user() -> Promise<User> {
        return firstly {
            URLSession.shared.dataTask(.promise, with: url)
        }.compactMap {
            try JSONSerialization.jsonObject(with: $0.data) as? [String: Any]
        }.map { dict in
            User(dict: dict)
        }
    }

    func avatar() -> Promise<UIImage> {
        return user().then { user in
            URLSession.shared.dataTask(.promise, with: user.imageUrl)
        }.compactMap {
            UIImage(data: $0.data)
        }
    }
}

This way, your asynchronous systems can easily be engaged in chains all over your apps.

Note we provide promises for Alamofire too!

Background Work

class MyRestAPI {
    func avatar() -> Promise<UIImage> {
        let bgq = DispatchQueue.global(qos: .userInitiated)

        return firstly {
            user()
        }.then(on: bgq) { user in
            URLSession.shared.dataTask(.promise, with: user.imageUrl)
        }.compactMap(on: bgq) {
            UIImage(data: $0)
        }
    }
}

All PromiseKit handlers take an on parameter allowing you to choose the queue the handler executes upon. The default is always the main queue.

PromiseKit is entirely thread safe.

Tip with caution you can have all then, map, compactMap, etc. run on a background queue, see PromiseKit.conf. Note that we suggest only changing the queue for the map suite of functions, thus done and catch will continue to run on the main queue which is usually what you want.

Failing Chains

If an error occurs mid chain, simply throw:

firstly {
    foo()
}.then { baz in
    bar(baz)
}.then { result in
    guard !result.isBad else { throw MyError.myIssue }
    //…
    return doOtherThing()
}

The error will surface at the next catch handler.

Thus if you call a throwing function, you don't have to wrap it in a do:

foo().then { baz in
    bar(baz)
}.then { result in
    try doOtherThing()
}.catch { error in
    // if doOtherThing() throws, we end up here
}

Tip with Swift you can define inline enum Error inside the function you are working at. This isn’t great coding practice, but it is better than avoiding throwing an error because you can’t be bothered to define a good global Error enum.

Abstracting Away Asychronicity

var fetch = API.fetch()

override func viewDidAppear() {
    fetch.then { items in
        //…
    }
}

func buttonPressed() {
    fetch.then { items in
        //…
    }
}

func refresh() {
    // ensure only one fetch operation happens at a time

    if fetch.isResolved {
        startSpinner()
        fetch = API.fetch().ensure {
            stopSpinner()
        }
    }
    return fetch
}

With promises you don’t need to worry about when your asynchronous operation finishes: act like it already has.

Above we can see that you can call then as many times on a promise as you like, they will all be executed in the order they were added.

Chaining Sequences

When you have a series of tasks to perform on an array of data:

// fade all visible table cells one by one in a “cascading” effect

let fade = Guarantee()
for cell in tableView.visibleCells {
    fade = fade.then {
        UIView.animate(.promise, duration: 0.1) {
            cell.alpha = 0
        }
    }
}
fade.done {
    // finish
}

Note usually you want when() since when executes all the promises in parallel and thus is much faster to complete. Use the above pattern in situations where tasks must be done sequentially; animation is a good example.

We also provide when(concurrently:) which allows you to schedule more than one promise at a time if required.

Timeout

let fetches: [Promise<T>] = makeFetches()
let timeout = after(seconds: 4)

race(when(fulfilled: fetches).asVoid(), timeout).then {
    //…
}

race continues as soon as one of the promises it watches finishes.

Common pitfalls: ensure the promises you pass to race are the same type. The easiest way to ensure this is using asVoid().

Please note if any promise you pass rejects, then race will be rejected.

Minimum Duration

Sometimes you need something to take at least a certain amount of time (eg. you want to show a spinner for something, but if it shows for less than 0.3 seconds the UI appears broken to the user).

let waitAtLeast = after(seconds: 0.3)

firstly {
    foo()
}.then {
    waitAtLeast
}.done {
    //…
}

The above works because we create the delay before we do work in foo(), thus it will have either already timed-out or we wait whatever amount of the 0.3 seconds remains before the chain continues.

Cancellation

Promises don’t have a cancel function, but they do support cancellation via a special error type that conforms to the CancellableError protocol.

func foo() -> (Promise<Void>, cancel: () -> Void) {
    let task = Task(…)
    var cancelme = false

    let promise = Promise<Void> { seal in
        task.completion = { value in
            guard !cancelme else { return reject(PMKError.cancelled) }
            seal.fulfill(value)
        }
        task.start()
    }

    let cancel = {
        cancelme = true
        task.cancel()
    }

    return (promise, cancel)
}

Promises don’t have a cancel function because you don’t want code outside of your control to be able to cancel your operations unless you explicitly want that. In cases where you want it, then it varies how it should work depending on how the underlying task supports cancellation. Thus we have provided primitives but not concrete API.

Cancelled chains do not call a catch handler by default. However you can intercept cancellation if you like:

foo.then {
    //…
}.catch(policy: .allErrorsIncludingCancellation) {
    // cancelled errors are handled *as well*
}

Important, canceling the chain is not the same as canceling the underlying asynchronous task. Promises are a wrapper around asynchronicity but they have no control over the underlying tasks. If you need to cancel the underlying task you need to cancel the underlying task!

The library CancellablePromiseKit extends the concept of Promises to fully cover cancellable tasks.

Retry / Polling

func attempt<T>(maximumRetryCount: Int = 3, delayBeforeRetry: DispatchTimeInterval = .seconds(2), _ body: @escaping () -> Promise<T>) -> Promise<T> {
    var attempts = 0
    func attempt() -> Promise<T> {
        attempts += 1
        return body().recover { error -> Promise<T> in
            guard attempts < maximumRetryCount else { throw error }
            return after(delayBeforeRetry).then(on: nil, attempt)
        }
    }
    return attempt()
}

attempt(maximumRetryCount: 3) {
    flakeyTask(parameters: foo)
}.then {
    //…
}.catch { _ in
    // we attempted three times but still failed
}

Probably you should supplement the above so that you only re-attempt for specific error conditions.

Wrapping Delegate Systems

Be careful with Promises and delegate systems as they are not always suited. Promises complete once where most delegate systems call their callbacks many times. This is why, for example, there is no PromiseKit extension for a UIButton.

A good example of an appropriate time to wrap delegation is when you need a single CLLocation lookup:

extension CLLocationManager {
    static func promise() -> Promise<CLLocation> {
        return PMKCLLocationManagerProxy().promise
    }
}

class PMKCLLocationManagerProxy: NSObject, CLLocationManagerDelegate {
    private let (promise, seal) = Promise<[CLLocation]>.pending()
    private var retainCycle: PMKCLLocationManagerProxy?
    private let manager = CLLocationManager()

    init() {
        super.init()
        retainCycle = self
        manager.delegate = self // does not retain hence the `retainCycle` property

        promise.ensure {
            // ensure we break the retain cycle
            self.retainCycle = nil
        }
    }

    @objc fileprivate func locationManager(_: CLLocationManager, didUpdateLocations locations: [CLLocation]) {
        seal.fulfill(locations)
    }

    @objc func locationManager(_: CLLocationManager, didFailWithError error: Error) {
        seal.reject(error)
    }
}

// use:

CLLocationManager.promise().then { locations in
    //…
}.catch { error in
    //…
}

Please note, we provide this promise with our CoreLocation extensions at https://github.com/PromiseKit/CoreLocation

Recovery

Sometimes you don’t want an error to cascade, instead you have a default value:

CLLocationManager.requestLocation().recover { error -> Promise<CLLocation> in
    guard error == MyError.airplaneMode else {
        throw error
    }
    return .value(CLLocation.savannah)
}.done { location in
    //…
}

Be careful not to ignore all errors; recover only those errors that make sense.

Promises for modal view-controllers

class ViewController: UIViewController {

    private let (promise, seal) = Guarantee<…>.pending()  // use Promise if your flow can fail

    func show(in: UIViewController) -> Promise<…> {
        in.show(self, sender: in)
        return promise
    }

    func done() {
        dismiss(animated: true)
        seal.fulfill(…)
    }
}

// use:

ViewController().show(in: self).done {
    //…
}.catch { error in
    //…
}

This is the best approach we have found, which is a pity as it requires the presentee to control the presentation and the presentee to be dismiss itself explicitly.

Nothing seemingly can beat Storyboard segues for decoupling an app's router.

Saving previous results

Let’s say you have:

login().then { username in
    fetch(avatar: username)
}.done { image in
    //…
}

What if you want access to both username and image in your done?

The most obvious way is with nesting:

login().then { username in
    fetch(avatar: username).done { image in
        // we have access to both `image` and `username`
    }
}.done {
    // the chain still continues as you'd expect
}

However this nesting reduces the chain’s clarity; instead we could use Swift tuples:

login().then { username in
    fetch(avatar: username).map { ($0, username) }
}.then { image, username in
    //…
}

The above simply maps Promise<String> into Promise<(UIImage, String)>.

Waiting on multiple promises whatever their result

Use when(resolved:):

when(resolved: a, b).done { (results: [Result<T>]) in
    // `Result` is an enum of `.fulfilled` or `.rejected`
}

// ^^ cannot call `catch` as `when(resolved:)` returns a `Guarantee`

Generally you don't want this, people ask for it a lot, but usually they actually just want to use recover on one of the promises. Usually you don't want to ignore errors. Errors happen, they should be handled.