:clapper: Tame async code with battle-tested promises

What's New

Swift 5.9




Language: Swift 5 Platform: iOS 8+/macOS10.11 SPM compatible Build Status License: MIT Release version

Reason - Example - Documentation - Installation

fetchUserId().then { id in
    print("UserID : \(id)")
}.onError { e in
    print("An error occured : \(e)")
}.finally {
    print("Everything is Done :)")
  let userId = try! awaitPromise(fetchUserId())

Because async code is hard to write, hard to read, hard to reason about. A pain to maintain

Try it

then is part of freshOS iOS toolset. Try it in an example App! Download Starter Project


By using a then keyword that enables you to write aSync code that reads like an English sentence
Async code is now concise, flexible and maintainable ❤️


  • Based on the popular Promise / Future concept
  • Async / Await
  • progress race recover validate retry bridgeError chain noMatterWhat ...
  • Strongly Typed
  • Pure Swift & Lightweight



fetchUserId({ id in
    fetchUserNameFromId(id, success: { name in
        fetchUserFollowStatusFromName(name, success: { isFollowed in
            // The three calls in a row succeeded YAY!
        }, failure: { error in
            // Fetching user ID failed
    }, failure: { error in
        // Fetching user name failed
}) {  error in
    // Fetching user follow status failed




Going further 🤓

fetchUserId().then { id in
    print("UserID : \(id)")
}.onError { e in
    print("An error occured : \(e)")
}.finally {
    print("Everything is Done :)")

If we want this to be maintainable, it should read like an English sentence
We can do this by extracting our blocks into separate functions:


This is now concise, flexible, maintainable, and it reads like an English sentence <3
Mental sanity saved // #goodbyeCallbackHell


  1. Writing your own Promise
  2. Progress
  3. Registering a block for later
  4. Returning a rejecting promise
  5. Common Helpers
  6. race
  7. recover
  8. validate
  9. retry
  10. bridgeError
  11. whenAll
  12. chain
  13. noMatterWhat
  14. unwrap
  15. AsyncTask
  16. Async/Await

Writing your own Promise 💪

Wondering what fetchUserId() is?
It is a simple function that returns a strongly typed promise :

func fetchUserId() -> Promise<Int> {
    return Promise { resolve, reject in
        print("fetching user Id ...")
        wait { resolve(1234) }

Here you would typically replace the dummy wait function by your network request <3


As for then and onError, you can also call a progress block for things like uploading an avatar for example.

uploadAvatar().progress { p in
  // Here update progressView for example

Registering a block for later

Our implementation slightly differs from the original javascript Promises. Indeed, they do not start right away, on purpose. Calling then, onError, or finally will start them automatically.

Calling then starts a promise if it is not already started. In some cases, we only want to register some code for later. For instance, in the case of JSON to Swift model parsing, we often want to attach parsing blocks to JSON promises, but without starting them.

In order to do that we need to use registerThen instead. It's the exact same thing as then without starting the promise right away.

let fetchUsers:Promise<[User]> = fetchUsersJSON().registerThen(parseUsersJSON)

// Here promise is not launched yet \o/

// later...
fetchUsers.then { users in
    // YAY

Note that onError and finally also have their non-starting counterparts : registerOnError and registerFinally.

Returning a rejecting promise

Oftetimes we need to return a rejecting promise as such :

return Promise { _, reject in

This can be written with the following shortcut :

return Promise.reject(error:anError)

Common Helpers


With race, you can send multiple tasks and get the result of the first one coming back :

race(task1, task2, task3).then { work in
  // The first result !


With .recover, you can provide a fallback value for a failed Promise.
You can :

  • Recover with a value
  • Recover with a value for a specific Error type
  • Return a value from a block, enabling you to test the type of error and return distinct values.
  • Recover with another Promise with the same Type
.recover(with: 12)
.recover(MyError.defaultError, with: 12)
.recover { e in
  if e == x { return 32 }
  if e == y { return 143 }
  throw MyError.defaultError
.recover { e -> Promise<Int> in
  // Deal with the error then
  return Promise<Int>.resolve(56)
  // Or
  return Promise<Int>.reject(e)
.recover(with: Promise<Int>.resolve(56))

Note that in the block version you can also throw your own error \o/


With .validate, you can break the promise chain with an assertion block.

You can:

  • Insert assertion in Promise chain
  • Insert assertion and return you own Error

For instance checking if a user is allowed to drink alcohol :

.validate { $0 > 18 }
.then { age in
  // Offer a drink

.validate(withError: MyError.defaultError, { $0 > 18 })`

A failed validation will retrun a PromiseError.validationFailed by default.


With retry, you can restart a failed Promise X number of times.

  .then { v in
   // YAY!
  }.onError { e in
    // Failed 10 times in a row


With .bridgeError, you can intercept a low-level Error and return your own high level error. The classic use-case is when you receive an api error and you bridge it to your own domain error.

You can:

  • Catch all errors and use your own Error type
  • Catch only a specific error
.bridgeError(to: MyError.defaultError)
.bridgeError(SomeError, to: MyError.defaultError)


With .whenAll, you can combine multiple calls and get all the results when all the promises are fulfilled :

whenAll(fetchUsersA(),fetchUsersB(), fetchUsersC()).then { allUsers in
  // All the promises came back


With chain, you can add behaviours without changing the chain of Promises.

A common use-case is for adding Analytics tracking like so:

extension Photo {
    public func post() -> Async<Photo> {
        return { _ in


With noMatterWhat you can add code to be executed in the middle of a promise chain, no matter what happens.

func fetchNext() -> Promise<[T]> {
    isLoading = true
    call.params["page"] = page + 1
    return call.fetch()
        .noMatterWhat {
            self.isLoading = false


With unwrap you can transform an optional into a promise :

func fetch(userId: String?) -> Promise<Void> {
   return unwrap(userId).then {

Unwrap will fail the promise chain with unwrappingFailed error in case of a nil value :)


AsyncTask and Async<T> typealisases are provided for those of us who think that Async can be clearer than Promise. Feel free to replace Promise<Void> by AsyncTask and Promise<T> by Async<T> wherever needed.
This is purely for the eyes :)


awaitPromise waits for a promise to complete synchronously and yields the result :

let photos = try! awaitPromise(getPhotos())

async takes a block and wraps it in a background Promise.

async {
  let photos = try awaitPromise(getPhotos())

Notice how we don't need the ! anymore because async will catch the errors.

Together, async/awaitPromise enable us to write asynchronous code in a synchronous manner :

async {
  let userId = try awaitPromise(fetchUserId())
  let userName = try awaitPromise(fetchUserNameFromId(userId))
  let isFollowed = try awaitPromise(fetchUserFollowStatusFromName(userName))
  return isFollowed
}.then { isFollowed in
}.onError { e in
  // handle errors

Await operators

Await comes with .. shorthand operator. The ..? will fallback to a nil value instead of throwing.

let userId = try awaitPromise(fetchUserId())

Can be written like this:

let userId = try ..fetchUserId()


The Swift Package Manager (SPM) is now the official way to install Then. The other package managers are now deprecated as of 5.1.3 and won't be supported in future versions.

Swift Package Manager

Xcode > File > Swift Packages > Add Package Dependency... > Paste

Cocoapods - Deprecated

target 'MyApp'
pod 'thenPromise'

Carthage - Deprecated

github "freshOS/then"


S4cha, Max Konovalov, YannickDot, Damien, piterlouis

Swift Version

  • Swift 2 -> version 1.4.2
  • Swift 3 -> version 2.2.5
  • Swift 4 -> version 3.1.0
  • Swift 4.1 -> version 4.1.1
  • Swift 4.2 -> version 4.2.0
  • Swift 4.2.1 -> version 4.2.0
  • Swift 5.0 -> version 5.0.0
  • Swift 5.1 -> version 5.1.0
  • Swift 5.1.3 -> version 5.1.2


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Last updated: Thu May 09 2024 22:27:05 GMT-0900 (Hawaii-Aleutian Daylight Time)