Code Quality Rank: L5
Programming language: Swift
License: MIT License
Tags: Concurrency    
Latest version: v2.0.5

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Lightweight full-featured Promises, Async & Await Library in Swift

What's this?

Hydra is full-featured lightweight library which allows you to write better async code in Swift 3.x/4.x. It's partially based on JavaScript A+ specs and also implements modern construct like await (as seen in Async/Await specification in ES8 (ECMAScript 2017) or C#) which allows you to write async code in sync manner. Hydra supports all sexiest operators like always, validate, timeout, retry, all, any, pass, recover, map, zip, defer and retry.
Starts writing better async code with Hydra!

A more detailed look at how Hydra works can be found in ARCHITECTURE file or on Medium.

❤️ Your Support

Hi fellow developer!
You know, maintaing and developing tools consumes resources and time. While I enjoy making them your support is foundamental to allow me continue its development.

If you are using SwiftLocation or any other of my creations please consider the following options:


What's a Promise?

A Promise is a way to represent a value that will exists, or will fail with an error, at some point in the future. You can think about it as a Swift's Optional: it may or may not be a value. A more detailed article which explain how Hydra was implemented can be found here.

Each Promise is strong-typed: this mean you create it with the value's type you are expecting for and you will be sure to receive it when Promise will be resolved (the exact term is fulfilled).

A Promise is, in fact, a proxy object; due to the fact the system knows what success value look like, composing asynchronous operation is a trivial task; with Hydra you can:

  • create a chain of dependent async operation with a single completion task and a single error handler.
  • resolve many independent async operations simultaneously and get all values at the end
  • retry or recover failed async operations
  • write async code as you may write standard sync code
  • resolve dependent async operations by passing the result of each value to the next operation, then get the final result
  • avoid callbacks, pyramid of dooms and make your code cleaner!

Updating to >=0.9.7

Since 0.9.7 Hydra implements Cancellable Promises. In order to support this new feature we have slightly modified the Body signature of the Promise; in order to make your source code compatible you just need to add the third parameter along with resolve,reject: operation. operation encapsulate the logic to support Invalidation Token. It's just and object of type PromiseStatus you can query to see if a Promise is marked to be cancelled from the outside. If you are not interested in using it in your Promise declaration just mark it as _.

To sum up your code:

return Promise<Int>(in: .main, token: token, { resolve, reject in ...

needs to be:

return Promise<Int>(in: .main, token: token, { resolve, reject, operation in // or resolve, reject, _

Create a Promise

Creating a Promise is trivial; you need to specify the context (a GCD Queue) in which your async operations will be executed in and add your own async code as body of the Promise.

This is a simple async image downloader:

func getImage(url: String) -> Promise<UIImage> {
    return Promise<UIImage>(in: .background, { resolve, reject, _ in
        self.dataTask(with: request, completionHandler: { data, response, error in
            if let error = error {
            } else if let data = data, let response = response as? HTTPURLResponse {
                resolve((data, response))
            } else {
                reject("Image cannot be decoded")

You need to remember only few things:

  • a Promise is created with a type: this is the object's type you are expecting from it once fulfilled. In our case we are expecting an UIImage so our Promise is Promise<UIImage> (if a promise fail returned error must be conform to Swift's Error protocol)
  • your async code (defined into the Promise's body) must alert the promise about its completion; if you have the fulfill value you will call resolve(yourValue); if an error has occurred you can call reject(occurredError) or throw it using Swift's throw occurredError.
  • the context of a Promise define the Grand Central Dispatch's queue in which the async code will be executed in; you can use one of the defined queues (.background,.userInitiated etc. Here you can found a nice tutorial about this topic)

How to use a Promise

Using a Promise is even easier.
You can get the result of a promise by using then function; it will be called automatically when your Promise fullfill with expected value. So:

getImage(url).then(.main, { image in
    myImageView.image = image

As you can see even then may specify a context (by default - if not specified - is the main thread): this represent the GCD queue in which the code of the then's block will be executed (in our case we want to update an UI control so we will need to execute it in .main thread).

But what happened if your Promise fail due to a network error or if the image is not decodable? catch func allows you to handle Promise's errors (with multiple promises you may also have a single errors entry point and reduce the complexity).

getImage(url).then(.main, { image in
    myImageView.image = image
}).catch(.main, { error in
    print("Something bad occurred: \(error)")

Chaining Multiple Promises

Chaining Promises is the next step thought mastering Hydra. Suppose you have defined some Promises:

func loginUser(_ name:String, _ pwd: String)->Promise<User>
func getFollowers(user: User)->Promise<[Follower]>
func unfollow(followers: [Follower])->Promise<Int>

Each promise need to use the fulfilled value of the previous; plus an error in one of these should interrupt the entire chain.
Doing it with Hydra is pretty straightforward:

loginUser(username,pass).then(getFollowers).then(unfollow).then { count in
    print("Unfollowed \(count) users")
}.catch { err in
    // Something bad occurred during these calls

Easy uh? (Please note: in this example context is not specified so the default .main is used instead).

Cancellable Promises

Cancellable Promises are a very sensitive task; by default Promises are not cancellable. Hydra allows you to cancel a promise from the outside by implementing the InvalidationToken. InvalidationToken is a concrete open class which is conform to the InvalidatableProtocol protocol. It must implement at least one Bool property called isCancelled.

When isCancelled is set to true it means someone outside the promise want to cancel the task.

Its your responsibility to check from inside the Promise's body the status of this variable by asking to operation.isCancelled. If true you can do all your best to cancel the operation; at the end of your operations just call cancel() and stop the workflow.

Your promise must be also initialized using this token instance.

This is a concrete example with UITableViewCell: working with table cells, often the result of a promise needs to be ignored. To do this, each cell can hold on to an InvalidationToken. An InvalidationToken is an execution context that can be invalidated. If the context is invalidated, then the block that is passed to it will be discarded and not executed.

To use this with table cells, the queue should be invalidated and reset on prepareForReuse().

class SomeTableViewCell: UITableViewCell {
    var token = InvalidationToken()

    func setImage(atURL url: URL) {
        downloadImage(url).then(in: .main, { image in
            self.imageView.image = image

    override func prepareForReuse() {
        token.invalidate() // stop current task and ignore result
        token = InvalidationToken() // new token

    func downloadImage(url: URL) -> Promise<UIImage> {
        return Promise<Something>(in: .background, token: token, { (resolve, reject, operation) in
        // ... your async operation

        // somewhere in your Promise's body, for example in download progression
        // you should check for the status of the operation.
        if operation.isCancelled {
            // operation should be cancelled
            // do your best to cancel the promise's task
            operation.cancel() // request to mark the Promise as cancelled
            return // stop the workflow! it's important
        // ... your async operation

Await & Async: async code in sync manner

Have you ever dream to write asynchronous code like its synchronous counterpart? Hydra was heavily inspired by Async/Await specification in ES8 (ECMAScript 2017) which provides a powerful way to write async doe in a sequential manner.

Using async and await is pretty simple.

NOTE: Since Hydra 2.0.6 the await function is available under Hydra.await() function in order to supress the Xcode 12.5+ warning (await will become a Swift standard function soon!)

For example the code above can be rewritten directly as:

// With `async` we have just defined a Promise which will be executed in a given
// context (if omitted `background` thread is used) and return an Int value.
let asyncFunc = async({ _ -> Int in // you must specify the return of the Promise, here an Int
    // With `await` the async code is resolved in a sync manner
    let loggedUser = try Hydra.await(loginUser(username,pass))
    // one promise...
    let followersList = try Hydra.await(getFollowers(loggedUser))
    // after another...
    let countUnfollowed = try Hydra.await(unfollow(followersList))
    // ... linearly
    // Then our async promise will be resolved with the end value
    return countUnfollowed
}).then({ value in // ... and, like a promise, the value is returned
    print("Unfollowed \(value) users")

Like magic! Your code will run in .background thread and you will get the result of each call only when it will be fulfilled. Async code in sync sauce!

Important Note: await is a blocking/synchronous function implemented using semaphore. Therefore, it should never be called in main thread; this is the reason we have used async to encapsulate it. Doing it in main thread will also block the UI.

async func can be used in two different options:

  • it can create and return a promise (as you have seen above)
  • it can be used to simply execute a block of code (as you will see below)

As we said we can also use async with your own block (without using promises); async accepts the context (a GCD queue) and optionally a start delay interval. Below an example of the async function which will be executed without delay in background:

    print("And now some intensive task...")
    let result = try! Hydra.await(.background, { resolve,reject, _ in
        delay(10, context: .background, closure: { // jut a trick for our example
    print("The result is \(result)")

There is also an await operator:

  • await with throw: .. followed by a Promise instance: this operator must be prefixed by try and should use do/catch statement in order to handle rejection of the Promise.
  • await without throw: ..! followed by a Promise instance: this operator does not throw exceptions; in case of promise's rejection result is nil instead.


    do {
        let result_1 = try ..asyncOperation1()
        let result_2 = try ..asyncOperation2(result_1) // result_1 is always valid
    } catch {
        // something goes bad with one of these async operations

    let result_1 = ..!asyncOperation1() // may return nil if promise fail. does not throw!
    let result_2 = ..!asyncOperation2(result_1) // you must handle nil case manually

When you use these methods and you are doing asynchronous, be careful to do nothing in the main thread, otherwise you risk to enter in a deadlock situation.

The last example show how to use cancellable async:

func test_invalidationTokenWithAsyncOperator() {

// create an invalidation token
let invalidator: InvalidationToken = InvalidationToken()

async(token: invalidator, { status -> String in
    Thread.sleep(forTimeInterval: 2.0)
    if status.isCancelled {
        print("Promise cancelled")
    } else {
        print("Promise resolved")
    return "" // read result
}).then { _ in
    // read result

// Anytime you can send a cancel message to invalidate the promise

Await an zip operator to resolve all promises

Await can be also used in conjuction with zip to resolve all promises from a list:

let (resultA,resultB) = Hydra.await(zip(promiseA,promiseB))

All Features

Because promises formalize how success and failure blocks look, it's possible to build behaviors on top of them. Hydra supports:

  • always: allows you to specify a block which will be always executed both for fulfill and reject of the Promise
  • validate: allows you to specify a predica block; if predicate return false the Promise fails.
  • timeout: add a timeout timer to the Promise; if it does not fulfill or reject after given interval it will be marked as rejected.
  • all: create a Promise that resolved when the list of passed Promises resolves (promises are resolved in parallel). Promise also reject as soon as a promise reject for any reason.
  • any: create a Promise that resolves as soon as one passed from list resolves. It also reject as soon as a promise reject for any reason.
  • pass: Perform an operation in the middle of a chain that does not affect the resolved value but may reject the chain.
  • recover: Allows recovery of a Promise by returning another Promise if it fails.
  • map: Transform items to Promises and resolve them (in paralle or in series)
  • zip: Create a Promise tuple of a two promises
  • defer: defer the execution of a Promise by a given time interval.
  • cancel: cancel is called when a promise is marked as cancelled using operation.cancel()


always func is very useful if you want to execute code when the promise fulfills — regardless of whether it succeeds or fails.

showLoadingHUD("Logging in...")
loginUser(username,pass).then { user in
    print("Welcome \(user.username)")
}.catch { err in
    print("Cannot login \(err)")
}.always {


validate is a func that takes a predicate, and rejects the promise chain if that predicate fails.

getAllUsersResponse().validate { httpResponse in
    guard let httpResponse.statusCode == 200 else {
        return false
    return true
}.then { usersList in
    // do something
}.catch { error in
    // request failed, or the status code was != 200


timeout allows you to attach a timeout timer to a Promise; if it does not resolve before elapsed interval it will be rejected with .timeoutError.

loginUser(username,pass).timeout(.main, 10, .MyCustomTimeoutError).then { user in
    // logged in
}.catch { err in
    // an error has occurred, may be `MyCustomTimeoutError


all is a static method that waits for all the promises you give it to fulfill, and once they have, it fulfills itself with the array of all fulfilled values (in order).

If one Promise fail the chain fail with the same error.

Execution of all promises is done in parallel.

let promises = usernameList.map { return getAvatar(username: $0) }
all(promises).then { usersAvatars in
    // you will get an array of UIImage with the avatars of input
    // usernames, all in the same order of the input.
    // Download of the avatar is done in parallel in background!
}.catch { err in
    // something bad has occurred

If you add promise execution concurrency restriction to all operator to avoid many usage of resource, concurrency option is it.

let promises = usernameList.map { return getAvatar(username: $0) }
all(promises, concurrency: 4).then { usersAvatars in
    // results of usersAvatars is same as `all` without concurrency.
}.catch { err in
    // something bad has occurred


any easily handle race conditions: as soon as one Promise of the input list resolves the handler is called and will never be called again.

let mirror_1 = "https://mirror1.mycompany.com/file"
let mirror_2 = "https://mirror2.mycompany.com/file"

any(getFile(mirror_1), getFile(mirror_2)).then { data in
    // the first fulfilled promise also resolve the any Promise
    // handler is called exactly one time!


pass is useful for performing an operation in the middle of a promise chain without changing the type of the Promise. You may also reject the entire chain. You can also return a Promise from the tap handler and the chain will wait for that promise to resolve (see the second then in the example below).

loginUser(user,pass).pass { userObj in 
    print("Fullname is \(user.fullname)")
}.then { userObj in
}.then { userObj in
    print("Login succeded!")


recover allows you to recover a failed Promise by returning another.

let promise = Promise<Int>(in: .background, { fulfill, reject in
}).recover({ error in
    return Promise(in: .background, { (fulfill, reject) in


Map is used to transform a list of items into promises and resolve them in parallel or serially.

[urlString1,urlString2,urlString3].map {
    return self.asyncFunc2(value: $0)
}.then(.main, { dataArray in
    // get the list of all downloaded data from urls
    // something bad has occurred


zip allows you to join different promises (2,3 or 4) and return a tuple with the result of them. Promises are resolved in parallel.

zip(a: getUserProfile(user), b: getUserAvatar(user), c: getUserFriends(user))
  .then { profile, avatar, friends in
    // ... let's do something
}.catch {
    // something bad as occurred. at least one of given promises failed


As name said, defer delays the execution of a Promise chain by some number of seconds from current time.

asyncFunc1().defer(.main, 5).then...


retry operator allows you to execute source chained promise if it ends with a rejection. If reached the attempts the promise still rejected chained promise is also rejected along with the same source error.
Retry also support delay parameter which specify the number of seconds to wait before a new attempt (2.0.4+).

// try to execute myAsyncFunc(); if it fails the operator try two other times
// If there is not luck for you the promise itself fails with the last catched error.
myAsyncFunc(param).retry(3).then { value in
    print("Value \(value) got at attempt #\(currentAttempt)")
}.catch { err in
    print("Failed to get a value after \(currentAttempt) attempts with error: \(err)")

Conditional retry allows you to control retryable if it ends with a rejection.

// If myAsyncFunc() fails the operator execute the condition block to check retryable.
// If return false in condition block, promise state rejected with last catched error.
myAsyncFunc(param).retry(3) { (remainAttempts, error) -> Bool in
  return error.isRetryable
}.then { value in
    print("Value \(value) got at attempt #\(currentAttempt)")
}.catch { err in
    print("Failed to get a value after \(currentAttempt) attempts with error: \(err)")


cancel is called when a promise is marked as cancelled from the Promise's body by calling the operation.cancel() function. See the Cancellable Promises for more info.

asyncFunc1().cancel(.main, {
    // promise is cancelled, do something

Chaining Promises with different Value types

Sometimes you may need to chain (using one of the available operators, like all or any) promises which returns different kind of values. Due to the nature of Promise you are not able to create an array of promises with different result types. However thanks to void property you are able to transform promise instances to generic void result type. So, for example, you can execute the following Promises and return final values directly from the Promise's result property.

let op_1: Promise<User> = asyncGetCurrentUserProfile()
let op_2: Promise<UIImage> = asyncGetCurrentUserAvatar()
let op_3: Promise<[User]> = asyncGetCUrrentUserFriends()

all(op_1.void,op_2.void,op_3.void).then { _ in
    let userProfile = op_1.result
    let avatar = op_2.result
    let friends = op_3.result
}.catch { err in
    // do something


You can install Hydra using CocoaPods, Carthage and Swift package manager

  • Swift 3.x: Latest compatible is 1.0.2 pod 'HydraAsync', ~> '1.0.2'
  • Swift 4.x: 1.2.1 or later pod 'HydraAsync'


pod 'HydraAsync'


github 'malcommac/Hydra'

Swift Package Manager

Add Hydra as dependency in your Package.swift

  import PackageDescription

  let package = Package(name: "YourPackage",
    dependencies: [
      .Package(url: "https://github.com/malcommac/Hydra.git", majorVersion: 0),

Consider ❤️ support the development of this library!


Current version is compatible with:

  • Swift 5.x
  • iOS 9.0 or later
  • tvOS 9.0 or later
  • macOS 10.10 or later
  • watchOS 2.0 or later
  • Linux compatible environments


  • If you need help or you'd like to ask a general question, open an issue.
  • If you found a bug, open an issue.
  • If you have a feature request, open an issue.
  • If you want to contribute, submit a pull request.

Copyright & Acknowledgements

SwiftLocation is currently owned and maintained by Daniele Margutti.
You can follow me on Twitter @danielemargutti.
My web site is https://www.danielemargutti.com

This software is licensed under [MIT License](LICENSE.md).

Follow me on:

*Note that all licence references and agreements mentioned in the Hydra README section above are relevant to that project's source code only.