Why should I use PromiseKit over X-Promises-Foo?

  • PromiseKit has a heavy focus on developer experience. You’re a developer; do you care about your experience? Yes? Then pick PromiseKit.
  • Do you care about having any bugs you find fixed? Then pick PromiseKit.
  • Do you care about having your input heard and reacted to in a fast fashion? Then pick PromiseKit.
  • Do you want a library that has been maintained continuously and passionately for 6 years? Then pick PromiseKit.
  • Do you want a library that the community has chosen to be their №1 Promises/Futures library? Then pick PromiseKit.
  • Do you want to be able to use Promises with Apple’s SDKs rather than having to do all the work of writing the Promise implementations yourself? Then pick PromiseKit.
  • Do you want to be able to use Promises with Swift 3.x, Swift 4.x, ObjC, iOS, tvOS, watchOS, macOS, Android & Linux? Then pick PromiseKit.
  • PromiseKit verifies its correctness by testing against the entire Promises/A+ test suite.

How do I create a fulfilled Void promise?

let foo = Promise()

// or:

let bar = Promise.value(())

How do I “early return”?

func foo() -> Promise<Void> {
   guard thingy else {
       return Promise()


func bar() -> Promise<SomethingNotVoid> {
    guard thingy else {
        return .value(instanceOfSomethingNotVoid)


Do I need to worry about retain cycles?

Generally, no. Once a promise completes, all handlers are released and so any references to self are also released.

However, if your chain contains side effects that you would typically not want to happen after, say, a view controller is popped, then you should still use weak self (and check for self == nil) to prevent any such side effects.

However, in our experience most things that developers consider side effects that should be protected against are in fact not side effects.

Side effects include changes to global application state. They do not include changing the display state of a viewController. So, protect against setting UserDefaults or modifying the application database, and don’t bother protecting against changing the text in a UILabel.

This stackoverflow question has some good discussion on this topic.

Do I need to retain my promises?

No. Every promise handler retains its promise until the handler is executed. Once all handlers have been executed, the promise is deallocated. So you only need to retain the promise if you need to refer to its final value after its chain has completed.

Where should I put my catch?

catch deliberately terminates the chain. You should put it low in your promise hierarchy at a point as close to the root as possible. Typically, this would be somewhere such as a view controller, where your catch can then display a message to the user.

This means you should be writing one catch for many thens and returning promises that do not have internal catch handlers of their own.

This is obviously a guideline; do what is necessary.

How do branched chains work?

Suppose you have a promise:

let promise = foo()

And you call then twice:

promise.then {
    // branch A

promise.then {
    // branch B

You now have a branched chain. When promise resolves, both chains receive its value. However, the two chains are entirely separate and Swift will prompt you to ensure that both have catch handlers.

You can most likely ignore the catch for one of these branches, but be careful: in these situations, Swift cannot help you ensure that your chains are error-handled.

promise.then {
    // branch A
}.catch { error in

_ = promise.then {

    // ignoring errors here as print cannot error and we handle errors above

It may be safer to recombine the two branches into a single chain again:

let p1 = promise.then {
    // branch A

let p2 = promise.then {
    // branch B

when(fulfilled: p1, p2).catch { error in

It’s worth noting that you can add multiple catch handlers to a promise, too. And indeed, both will be called if the chain is rejected.

Is PromiseKit “heavy”?

No. PromiseKit contains hardly any source code. In fact, it is quite lightweight. Any “weight” relative to other promise implementations derives from 6 years of bug fixes and tuning, from the fact that we have stellar Objective-C-to-Swift bridging and from important things such as Zalgo prevention that hobby-project implementations don’t consider.

Why is debugging hard?

Because promises always execute via dispatch, the backtrace you see at the point of an error has less information than is usually required to trace the path of execution.

One solution is to turn off dispatch during debugging:

// Swift
DispatchQueue.default = zalgo


Don’t leave this on. In normal use, we always dispatch to avoid you accidentally writing a common bug pattern. See this blog post.

Where is all()?

Some promise libraries provide all for awaiting multiple results. We call this function when, but it is the same thing. We chose when because it’s the more common term and because we think it reads better in code.

How can I test APIs that return promises?

You need to use XCTestExpectation.

We also define wait() and hang(). Use them if you must, but be careful because they block the current thread!

Is PromiseKit thread-safe?

Yes, entirely.

However the code you write in your thens might not be!

Just make sure you don’t access state outside the chain from concurrent queues. By default, PromiseKit handlers run on the main thread, which is serial, so you typically won’t have to worry about this.

Why are there separate classes for Objective-C and Swift?

Promise<T> is generic and and so cannot be represented by Objective-C.

Does PromiseKit conform to Promises/A+?

Yes. We have tests that prove this.

How do PromiseKit and RxSwift/ReactiveSwift differ?

PromiseKit is a lot simpler.

The top-level difference between PromiseKit and RxSwift is that RxSwift Observables (roughly analogous to PromiseKit Promises) do not necessarily return a single result: they may emit zero, one, or an infinite stream of values. This small conceptual change leads to an API that’s both surprisingly powerful and surprisingly complex.

RxSwift requires commitment to a paradigm shift in how you program. It proposes that you restructure your code as a matrix of interacting value pipelines. When applied properly to a suitable problem, RxSwift can yield great benefits in robustness and simplicity. But not all applications are suitable for RxSwift.

By contrast, PromiseKit selectively applies the best parts of reactive programming to the hardest part of pure Swift development, the management of asynchrony. It’s a broadly applicable tool. Most asynchronous code can be clarified, simplified and made more robust just by converting it to use promises. (And the conversion process is easy.)

Promises make for code that is clear to most developers. RxSwift, perhaps not. Take a look at this signup panel implemented in RxSwift and see what you think. (Note that this is one of RxSwift’s own examples.)

Even where PromiseKit and RxSwift are broadly similar, there are many differences in implementation:

  • RxSwift has a separate API for chain-terminating elements (subscribers) versus interior elements. In PromiseKit, all elements of a chain use roughly the same code pattern.

  • The RxSwift API to define an interior element of a chain (an operator) is hair-raisingly complex. So, RxSwift tries hard to supply every operator you might ever want to use right off the shelf. There are hundreds. PromiseKit supplies a few utilities to help with specific scenarios, but because it’s trivial to write your own chain elements, there’s no need for all this extra code in the library.

  • PromiseKit dispatches the execution of every block. RxSwift dispatches only when told to do so. Moreover, the current dispatching state is an attribute of the chain, not the specific block, as it is in PromiseKit. The RxSwift system is more powerful but more complex. PromiseKit is simple, predictable and safe.

  • In PromiseKit, both sides of a branched chain refer back to their shared common ancestors. In RxSwift, branching normally creates a duplicate parallel chain that reruns the code at the head of the chain…except when it doesn’t. The rules for determining what will actually happen are complex, and given a chain created by another chunk of code, you can’t really tell what the behavior will be.

  • Because RxSwift chains don’t necessarily terminate on their own, RxSwift needs you to take on some explicit garbage collection duties to ensure that pipelines that are no longer needed are properly deallocated. All promises yield a single value, terminate and then automatically deallocate themselves.

You can find some additional discussion in this ticket.

Why can’t I return from a catch like I can in Javascript?

Swift demands that functions have one purpose. Thus, we have two error handlers:

  • catch: ends the chain and handles errors
  • recover: attempts to recover from errors in a chain

You want recover.

When do promises “start”?

Often people are confused about when Promises “start”. Is it immediately? Is it later? Is it when you call then?

The answer is: promises do not choose when the underlying task they represent starts. That is up to that task. For example here is the code for a simple promise that wraps Alamofire:

func foo() -> Promise<Any>
    return Promise { seal in
        Alamofire.request(rq).responseJSON { rsp in
            seal.resolve(rsp.value, rsp.error)

Who chooses when this promise starts? The answer is: Alamofire does, and in this case, it “starts” immediately when foo() is called.

What is a good way to use Firebase with PromiseKit

There is no good way to use Firebase with PromiseKit. See the next question for a more detailed rationale.

The best option is to embed your chain in your Firebase handler:

foo.observe(.value) { snapshot in
    firstly {
        bar(with: snapshot)
    }.then {
    }.then {
    }.catch {

I need my then to fire multiple times

Then we’re afraid you cannot use PromiseKit for that event. Promises only resolve once. This is the fundamental nature of promises and it is considered a feature because it gives you guarantees about the flow of your chains.

How do I change the default queues that handlers run on?

You can change the values of PromiseKit.conf.Q. There are two variables that change the default queues that the two kinds of handler run on. A typical pattern is to change all your then-type handlers to run on a background queue and to have all your “finalizers” run on the main queue:

PromiseKit.conf.Q.map = .global()
PromiseKit.conf.Q.return = .main  //NOTE this is the default

Be very careful about setting either of these queues to nil. It has the effect of running immediately, and this is not what you usually want to do in your application. This is, however, useful when you are running specs and want your promises to resolve immediately. (This is basically the same idea as stubbing an HTTP request.)

// in your test suite setup code
PromiseKit.conf.Q.map = nil
PromiseKit.conf.Q.return = nil

How do I use PromiseKit on the server side?

If your server framework requires that the main queue remain unused (e.g., Kitura), then you must use PromiseKit 6 and you must tell PromiseKit not to dispatch to the main queue by default. This is easy enough:

PromiseKit.conf.Q = (map: DispatchQueue.global(), return: DispatchQueue.global())

Note, we recommend using your own queue rather than .global(), we’ve seen better performance this way.

Here’s a more complete example:

import Foundation
import HeliumLogger
import Kitura
import LoggerAPI
import PromiseKit


let pmkQ = DispatchQueue(label: "pmkQ", qos: .default, attributes: .concurrent, autoreleaseFrequency: .workItem)
PromiseKit.conf.Q = (map: pmkQ, return: pmkQ)

let router = Router()
router.get("/") { _, response, next in
    Log.info("Request received")
    after(seconds: 1.0).done {
        Log.info("Sending response")

Log.info("Starting server")
Kitura.addHTTPServer(onPort: 8888, with: router)

How do I control console output?

By default PromiseKit emits console messages when certain events occur. These events include:

  • A promise or guarantee has blocked the main thread
  • A promise has been deallocated without being fulfilled
  • An error which occurred while fulfilling a promise was swallowed using cauterize

You may turn off or redirect this output by setting a thread safe closure in PMKCOnfiguration before processing any promises. For example, to turn off console output:

conf.logHandler = { event in }

My question was not answered

Please open a ticket.