Latest 0.4.3
License MIT
Platforms ios 9.0, osx 10.10, watchos 2.0, tvos 9.0

Carthage compatible

Tomorrowland is an implementation of Promises for Swift and Objective-C. A Promise is a wrapper around an
asynchronous task that provides a standard way of subscribing to task resolution as well as chaining promises together.

UIApplication.shared.isNetworkActivityIndicatorVisible = true
MyAPI.requestFeed(for: user).then { (feedItems) in
    self.refreshUI(with: feedItems)
}.catch { (error) in
}.always { _ in
    UIApplication.shared.isNetworkActivityIndicatorVisible = false

It is loosely based on both PromiseKit and Hydra, with a few key distinctions:

  • It uses atomics internally instead of creating a separate DispatchQueue for each promise. This means it’s faster and uses fewer resources.
  • It provides full support for cancellable promises. PromiseKit supports detection of "cancelled" errors but has no way to request cancellation of a promise. Hydra
    supports cancelling a promise, but it can’t actually stop any work being done by the promise unless the promise body itself polls for the cancellation status (so e.g.
    a promise wrapping a network task can’t reasonably cancel the network task). Tomorrowland improves on this by allowing the promise body to observe the
    cancelled state, and allows linking cancellation of a child promise to its parent.
  • Its Obj-C support makes use of generics for improved type safety and better documentation.
  • Like Hydra but unlike PromiseKit, it provides a way to suppress a registered callback (e.g. because you don’t care about the result anymore and don’t want stale
    data affecting your UI). This is distinct from promise cancellation.
  • Tomorrowland promises are fully generic over the error type, whereas both PromiseKit and Hydra only support using Error as the error type. This may result in
    more typing to construct a promise but it allows for much more powerful error handling. Tomorrowland also has some affordances for working with promises that
    use Error as the error type.
  • Tomorrowland is fully thread-safe. I have no reason to believe PromiseKit isn’t, but (at the time of this writing) there are parts of Hydra that are incorrectly
    implemented in a non-thread-safe manner.



You can add Tomorrowland to your workspace manually like any other project and add the resulting Tomorrowland.framework to your application’s frameworks.


github "lilyball/Tomorrowland" ~> 0.4.0


pod 'Tomorrowland', '~> 0.4.0'


Tomorrowland currently relies on a private Obj-C module for its atomics. This arrangement means it is not compatible with Swift Package Manager (as adding
compatibility would necessitate publicly exposing the private Obj-C module).

Quick Start

Creating Promises

Promises can be created using code like the following:

let promise = Promise<String,Error>(on: .utility, { (resolver) in
    let value = try expensiveCalculation()
    resolver.fulfill(with: value)

The body of this promise runs on the specified PromiseContext, which in this case is .utility (which means .utility)).
Unlike callbacks, all created promises must specify a context, so as to avoid accidentally running expensive computations on the main thread. The available contexts
include .main, every Dispatch QoS, a specific DispatchQueue, a specific OperationQueue, or the value .immediate which means to run the block
synchronously. There’s also the special context .auto, which evaluates to .main on the main thread and .default otherwise. This special context is the default
context for all callbacks that don’t otherwise specify one.

Note: The .immediate context can be dangerous to use for callback handlers and should be avoided in most cases. It’s primarily intended for creating
promises, and whenever it’s used with a callback handler the handler must be prepared to execute on any thread. For callbacks it’s usually only useful for short
thread-agnostic callbacks, such as an .onRequestCancel that does nothing more than cancelling a URLSessionTask.

The body of a Promise receives a "resolver", which it must use to fulfill, reject, or cancel the promise. If the resolver goes out of scope without being used, the
promise is automatically cancelled. If the promise’s error type is Error, the promise body may also throw an error (as seen above), which is then used to reject the
promise. This resolver can also be used to observe cancellation requests using resolver.onRequestCancel, as seen here:

let promise = Promise<Data,Error>(on: .immediate, { (resolver) in
    let task = urlSession.dataTask(with: url, completionHandler: { (data, response, error) in
        if let data = data {
            resolver.fulfill(with: data)
        } else if case URLError.cancelled? = error {
        } else {
            resolver.reject(with: error!)
    resolver.onRequestCancel(on: .immediate, { _ in

Resolvers also have a convenience method handleCallback() that is intended to make it easy to wrap framework callbacks in promises. This method returns a
closure that can be used as a callback directly. It also takes an optional isCancelError parameter that can be used to indicate when an error represents
cancellation. For example:

geocoder.reverseGeocodeLocation(location, completionHandler: resolver.handleCallback(isCancelError: { CLError.geocodeCanceled ~= $0 }))

Using Promises

Once you have a promise, you can register callbacks to be executed when the promise is resolved. As mentioned above, you can specify the context for the
callback, but if you don’t specify it then it defaults to .auto, which means the main thread if the callback is registered from the main thread, otherwise the dispatch
queue with QoS .default.

When you register a callback, the method also returns a Promise. All callback registration methods return a new Promise even if the callback doesn’t affect the
value of the promise. The reason for this is so chained callbacks always guarantee that the previous callback finished executing before the new one starts, even
when using concurrent contexts (e.g. .utility).

Most callback registration methods also have versions that allow you to return a Promise from your callback. In this event, the resulting Promise waits for the
promise you returned to resolve before adopting its value. This allows for easy composition of promises.

fetchUserCredentials().then { (credentials) in
    // This returns a new promise
    return MyAPI.login(name:, password: credentials.password)
}.then { [weak self] (apiKey) in
    // this is invoked when the promise returned by MyAPI.login fulfills.
    MyAPI.apiKey = apiKey
}.always { [weak self] _ in
    // This is always invoked regardless of whether the previous chain was
    // fulfilled, rejected, or cancelled.
}.catch { [weak self] (error) in
    // this handles any error returned from the previous chain, meaning any error
    // from `fetchUserCredentials()` or from `MyAPI.login(name:password:)`.

When composing callbacks that return promises, you may run into issues with incompatible error types. There are convenience methods for working with promises
whose errors are compatible with Error, but they don’t cover all cases. If you find yourself hitting one of these cases, any Promise whose error type conforms to
Error has a property .upcast that will convert that error into an Error to allow for easier composition of promises.

Tomorrowland also offers a typealias StdPromise<Value> as shorthand for Promise<T,Error>. This is frequently useful to avoid having to repeat the types,
such as with StdPromise(fulfilled: someValue) instead of Promise<SomeValue,Error>(fulfilled: someValue).

Cancelling and Invalidation

All promises expose a method .requestCancel(). It is named such because this doesn’t actually guarantee that the promise will be cancelled. If the promise
supports cancellation, this method will trigger a callback that the promise can use to cancel its work. But promises that don’t support cancellation will ignore this
and will eventually fulfill or reject as normal. Naturally, requesting cancellation of a promise that has already been resolved does nothing, even if the callbacks have
not yet been invoked.

In order to handle the issue of a promise being resolved after you no longer care about it, there is a separate mechanism called a PromiseInvalidationToken
that can be used to suppress callbacks. All callback methods have an optional token parameter that accepts a PromiseInvalidationToken. If provided,
calling invalidate() on the token prior to the callback being executed guarantees the callback will not fire. If the callback returns a value that is required in order
to resolve the Promise returned from the callback registration method, the resulting Promise is cancelled instead. PromiseInvalidationTokens can be used
with multiple callbacks at once, and a single token can be re-used as much as desired. It is recommended that you take advantage of both invalidation tokens and
cancellation. This may look like

class URLImageView: UIImageView {
    private var promise: StdPromise<Void>?
    private let invalidationToken = PromiseInvalidationToken()

    enum LoadError: Error {
        case dataIsNotImage

    /// Loads an image from the URL and displays it in the image view.
    func loadImage(from url: URL) {
        // Note: dataTaskAsPromise does not actually exist
        promise = URLSession.shared.dataTaskAsPromise(with: url)
        // Use `_ =` to avoid having to handle errors with `.catch`.
        _ = promise?.then(on: .utility, { (data) in
            if let image = UIImage(data: data) {
                return image
            } else {
                throw LoadError.dataIsNotImage
        }).then(token: invalidationToken, { [weak self] (image) in
            self?.image = image

PromiseInvalidationToken also has a method .requestCancelOnInvalidate(_:) that can register any number of Promises to be automatically
requested to cancel (using .requestCancel()) the next time the token is invalidated. Promise also has the same method (except it takes a token as the
argument) as a convenience for calling .requestCancelOnInvalidate(_:) on the token. This can be used to terminate a promise chain without ever assigning
the promise to a local variable. PromiseInvalidationToken also has a method .cancelWithoutInvalidating() which cancels any associated promises
without invalidating the token.

By default PromiseInvalidationTokens will invalidate themselves automatically when deinitialized. This is primarily useful in conjunction with
requestCancelOnInvalidate(_:) as it allows you to automatically cancel your promises when object that owns the token deinits. This behavior can be
disabled with an optional parameter to init.

Promise also has a convenience method requestCancelOnDeinit(_:) which can be used to request the Promise to be cancelled when a given object
deinits. This is equivalent to adding a PromiseInvalidationToken property to the object (configured to invalidate on deinit) and requesting cancellation when
the token invalidates, but can be used if the token would otherwise not be explicitly invalidated.

Using these methods, the above loadImage(from:) can be rewritten as the following including cancellation:

class URLImageView: UIImageView {
    private let promiseToken = PromiseInvalidationToken()

    enum LoadError: Error {
        case dataIsNotImage

    /// Loads an image from the URL and displays it in the image view.
    func loadImage(from url: URL) {
        // Note: dataTaskAsPromise does not actually exist
        promise = URLSession.shared.dataTaskAsPromise(with: url)
        // Use `_ =` to avoid having to handle errors with `.catch`.
        _ = promise?.then(on: .utility, { (data) in
            if let image = UIImage(data: data) {
                return image
            } else {
                throw LoadError.dataIsNotImage
        }).then(token: promiseToken, { [weak self] (image) in
            self?.image = image


In order to avoid the repetition of passing a PromiseInvalidationToken to multiple Promise methods as well as cancelling the resulting promise, a type
TokenPromise exists that handles this for you. You can create a TokenPromise with the Promise.withToken(_:) method. This allows you to take code like
the following:

func loadModel() {
        .then(token: promiseToken, { [weak self] (model) in
            self?.updateUI(with: model)
        }).catch(token: promiseToken, { [weak self] (error) in

And rewrite it to be less repetitive:

func loadModel() {
        .then({ [weak self] (model) in
            self?.updateUI(with: model)
        }).catch({ [weak self] (error) in

Automatic cancellation propagation

Nearly all callback registration methods will automatically propagate cancellation requests from the child to the parent if the parent has no other observers. If all
observers for a promise request cancellation, the cancellation request will propagate upwards at this time. This means that a promise will not automatically cancel
as long as there’s at least one interested observer. Do note that promises that have no observers do not get automatically cancelled, this only happens if there’s at
least one observer (which then requests cancellation). Automatic cancellation propagation also requires that the promise itself no longer be in scope. For this reason
you should avoid holding onto promises long-term and instead use the .cancellable property or PromiseInvalidationToken‘s
requestCancelOnInvalidate(_:) if you want to be able to cancel the promise later.

Automatic cancellation propagation also works with the utility functions when(fulfilled:) and when(first:) as well as the convenience methods
timeout(on:delay:) and delay(on:_:).

Promises have a couple of methods that do not participate in automatic cancellation propagation. You can use tap(on:token:_:) as an alternative to always in
order to register an observer that won’t interfere with the existing automatic cancellation propagation (this is suitable for inserting into the middle of a promise
chain). You can also use tap() as a more generic version of this.

Note that ignoringCancel() disables automatic cancellation propagation on the receiver. Once you invoke this on a promise, it will never automatically cancel.

Promise Helpers

There are a few helper functions that can be used to deal with multiple promises.


when(fulfilled:) is a global function that takes either an array of promises or 2–6 promises as separate arguments, and returns a single promise that is
eventually fulfilled with the values of all input promises. With the array version all input promises must have the same type and the result is fulfilled with an array.
With the separate argument version the promises may have unique value types (but the same error type) and the result is fulfilled with a tuple.

If any of the input promises is rejected or cancelled, the resulting promise is immediately rejected or cancelled as well. If multiple input promises are rejected or
cancelled, the first such one affects the result.

This function has an optional parameter cancelOnFailure: that, if provided as true, will cancel all input promises if any of them are rejected.


when(first:) is a global function that takes an array of promises of the same type, and returns a single promise that eventually adopts the same value or error as
the first input promise that gets fulfilled or rejected. Cancelled input promises are ignored, unless all input promsies are cancelled, at which point the resulting
promise will be cancelled as well.

This function has an optional parameter cancelRemaining: that, if provided as true, will cancel the remaining input promises as soon as one of them is fulfilled
or rejected.


Promise.timeout(on:delay:) is a method that returns a new promise that adopts the same value as the receiver, or is rejected with an error if the receiver isn’t
resolved within the given interval.


Promise.delay(on:_:) is a method that returns a new promise that adopts the same result as the receiver after the specified delay. It is intended primarily for
testing purposes.


Tomorrowland has Obj-C compatibility in the form of TWLPromise<ValueType,ErrorType>. This is a parallel promise implementation that can be bridged to/from
Promise and supports all of the same functionality. Note that some of the method names are different (due to lack of overloading), and while TWLPromise is
generic over its types, the return values of callback registration methods that return new promises are not parameterized (due to inability to have generic methods).


Requires a minimum of iOS 9, macOS 10.10, watchOS 2.0, or tvOS 9.0.


Licensed under either of


Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you shall be dual licensed as above, without any additional terms or conditions.

Version History


  • Fix compatibility with Xcode 10.2 / Swift 5 compiler (#31, SR-9753).


  • Add new method Promise.Resolver.resolve(with: somePromise) that resolves the receiver using another promise (#30).


  • Mark PromiseCancellable.requestCancel() as public (#29).


  • Improve the behavior of .delay(on:_:) and .timeout(on:delay:) when using PromiseContext.operationQueue. The relevant operation is now added
    to the queue immediately and only becomes ready once the delay/timeout has elapsed.
  • Add -[TWLPromise initCancelled] to construct a pre-cancelled promise.
  • Add Promise.init(on:fulfilled:after:), Promise.init(on:rejected:after:), and Promise.init(on:result:after:). These initializers produce
    something akin to Promise(fulfilled: value).delay(after) except they respond to cancellation immediately. This makes them more suitable for use as
    cancellable timers, as opposed to .delay(_:) which is more intended for debugging (#27).
  • Try to clean up the callback list when calling PromiseInvalidationToken.requestCancelOnInvalidate(_:). Any deallocated promises at the head of the
    callback list will be removed. This will help keep the callback list from growing uncontrollably when a token is used merely to cancel all promises when the owner
    deallocates as opposed to being periodically invalidated during its lifetime (#25).
  • Cancel the .delay(_:) timer if .requestCancel() is invoked and the upstream promise cancelled. This way requested cancels will skip the delay, but
    unexpected cancels will still delay the result (#26).


  • Add PromiseInvalidationToken.cancelWithoutInvalidating(). This method cancels any associated promises without invalidating the token, thus
    allowing for any onCancel and always handlers on the promises to fire (#23).
  • Add missing PromiseObjCPromise bridging methods for the case of Value: AnyObject, Error == Swift.Error (#20).
  • Update some documentation.


  • Add Hashable / Equatable conformance to PromiseInvalidationToken.
  • Add a new type TokenPromise that wraps a Promise and automatically applies a PromiseInvalidationToken. This API is Swift-only.


  • Add a missing Swift->ObjC convenience bridging method.
  • Add Decodable conformance to NoError.
  • Add method Promise.fork(_:).
  • Fix compilation failure when targeting 32-bit iOS 9 simulator in Xcode 9.3.
  • Fix cancellation propagation test cases on iOS 9 simulators.


  • Add Promise.requestCancelOnInvalidate(_:) as a convenience for token.requestCancelOnInvalidate(_:).
  • Add Promise.requestCancelOnDeinit(_:) as a convenience for adding a token property to an object that invalites on deinit.
  • Better support for OperationQueue with delay/timeout. Instead of using the OperationQueue‘s underlying queue, we instead use a .userInitiated
    queue for the timer and hop onto the OperationQueue to resolve the promise.


  • Implement automatic cancellation propagation and remove the .linkCancel option.
  • Remove the cancelOnTimeout: parameter to timeout(on:delay:) in favor of automatic cancellation propagation.
  • Automatically invalidate PromiseInvalidationTokens on deinit. This behavior can be disabled via a parameter to init.


Initial alpha release.

Latest podspec

    "name": "Tomorrowland",
    "version": "0.4.3",
    "summary": "Lightweight Promises for Swift and Obj-C",
    "description": "Tomorrowland is a lightweight Promise implementation for Swift and Obj-C that supports cancellation and strongly-typed errors.",
    "homepage": "",
    "license": {
        "type": "MIT",
        "file": "LICENSE-MIT"
    "authors": "Lily Ballard",
    "social_media_url": "",
    "platforms": {
        "ios": "9.0",
        "osx": "10.10",
        "watchos": "2.0",
        "tvos": "9.0"
    "swift_version": "4.0",
    "source": {
        "git": "",
        "tag": "v0.4.3"
    "source_files": [
    "private_header_files": "Sources/Private/*.h",
    "preserve_paths": [
    "libraries": "c++",
    "module_map": "Sources/tomorrowland.modulemap"

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