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⚠️ BrightFutures has reached end-of-life. After a long period of limited development activity, Swift's Async/Await has made the library obsolete. Please consider migrating from BrightFutures to async/await. When you do so, the asyncget() method will prove to be useful:

// in an async context...letuserFuture=User.logIn(username, password)letuser=tryawait userFuture.get()// or simply:letposts=tryawaitPosts.fetchPosts(user).get()

The remainder of the README has not been updated recently, but is preserved for historic reasons.

How do you leverage the power of Swift to write great asynchronous code? BrightFutures is our answer.

BrightFutures implements provenfunctional concepts in Swift to provide a powerful alternative to completion blocks and support typesafe error handling in asynchronous code.

The goal of BrightFutures is to bethe idiomatic Swift implementation of futures and promises.Our Big Hairy Audacious Goal (BHAG) is to be copy-pasted into the Swift standard library.

The stability of BrightFutures has been proven through extensive use in production. It is currently being used in several apps, with a combined total of almost 500k monthly active users. If you use BrightFutures in production, we'd love to hear about it!

BrightFutures 8.0 is now available! This update adds Swift 5 compatibility.

1. Add the following to yourPodfile:

   pod'BrightFutures'

2. Integrate your dependencies using frameworks: adduse_frameworks! to your Podfile.

3. Runpod install.

1. Add the following to yourCartfile:

   github "Thomvis/BrightFutures"

2. Runcarthage update and follow the steps as described in Carthage'sREADME.

• This README covers almost all features of BrightFutures
• Thetests contain (trivial) usage examples for every feature (97% test coverage)
• The primary author, Thomas Visser, gavea talk at the April 2015 CocoaHeadsNL meetup
• TheHighstreet Watch App was an Open Source WatchKit app that made extensive use of an earlier version of BrightFutures

We write a lot of asynchronous code. Whether we're waiting for something to come in from the network or want to perform an expensive calculation off the main thread and then update the UI, we often do the 'fire and callback' dance. Here's a typical snippet of asynchronous code:

User.logIn(username, password){ user, errorinif !error{Posts.fetchPosts(user, success:{ postsin            // do something with the user's posts}, failure: handleError)}else{handleError(error) // handeError is a custom function to handle errors}}

Now let's see what BrightFutures can do for you:

User.logIn(username, password).flatMap{ userinPosts.fetchPosts(user)}.onSuccess{ postsin    // do something with the user's posts}.onFailure{ errorin    // either logging in or fetching posts failed}

BothUser.logIn andPosts.fetchPosts now immediately return aFuture. A future can either fail with an error or succeed with a value, which can be anything from an Int to your custom struct, class or tuple. You can keep a future around and register for callbacks for when the future succeeds or fails at your convenience.

When the future returned fromUser.logIn fails, e.g. the username and password did not match,flatMap andonSuccess are skipped andonFailure is called with the error that occurred while logging in. If the login attempt succeeded, the resulting user object is passed toflatMap, which 'turns' the user into an array of his or her posts. If the posts could not be fetched,onSuccess is skipped andonFailure is called with the error that occurred when fetching the posts. If the posts could be fetched successfully,onSuccess is called with the user's posts.

This is just the tip of the proverbial iceberg. A lot more examples and techniques can be found in this readme, by browsing through the tests or by checking out the official companion frameworkFutureProofing.

If you already have a function (or really any expression) that you just want to execute asynchronously and have a Future to represent its result, you can easily wrap it in anasyncValue block:

DispatchQueue.global().asyncValue{fibonacci(50)}.onSuccess{ numin    // value is 12586269025}

asyncValue is defined in an extension on GCD'sDispatchQueue. While this is really short and simple, it is equally limited. In many cases, you will need a way to indicate that the task failed. To do this, instead of returning the value, you can return a Result. Results can indicate either a success or a failure:

enumReadmeError:Error{case RequestFailed, TimeServiceError}letf=DispatchQueue.global().asyncResult{()->Result<Date,ReadmeError>iniflet now=serverTime(){return.success(now)}return.failure(ReadmeError.TimeServiceError)}f.onSuccess{ valuein    // value will the NSDate from the server}

The future block needs an explicit type because the Swift compiler is not able to deduce the type of multi-statement blocks.

Instead of wrapping existing expressions, it is often a better idea to use a Future as the return type of a method so all call sites can benefit. This is explained in the next section.

Now let's assume the role of an API author who wants to use BrightFutures. A Future is designed to be read-only, except for the site where the Future is created. This is achieved via an initialiser on Future that takes a closure, the completion scope, in which you can complete the Future. The completion scope has one parameter that is also a closure which is invoked to set the value (or error) in the Future.

func asyncCalculation()->Future<String,Never>{returnFuture{ completeinDispatchQueue.global().async{            // do a complicated task and then hand the result to the promise:complete(.success("forty-two"))}}}

Never indicates that theFuture cannot fail. This is guaranteed by the type system, sinceNever has no initializers. As an alternative to the completion scope, you could also create aPromise, which is the writeable equivalent of a Future, and store it somewhere for later use.

You can be informed of the result of aFuture by registering callbacks:onComplete,onSuccess andonFailure. The order in which the callbacks are executed upon completion of the future is not guaranteed, but it is guaranteed that the callbacks are executed serially. It is not safe to add a new callback from within a callback of the same future.

Using theandThen function on aFuture, the order of callbacks can be explicitly defined. The closure passed toandThen is meant to perform side-effects and does not influence the result.andThen returns a new Future with the same result as this future that completes after the closure has been executed.

varanswer=10let _=Future<Int,Never>(value:4).andThen{ resultinswitch result{case.success(let val):        answer*= valcase.failure(_):break}}.andThen{ resultinif case.success(_)= result{        answer+=2}}// answer will be 42 (not 48)

map returns a new Future that contains the error from this Future if this Future failed, or the return value from the given closure that was applied to the value of this Future.

fibonacciFuture(10).map{ number->Stringinif number>5{return"large"}return"small"}.map{ sizeStringin    sizeString=="large"}.onSuccess{ numberIsLargein    // numberIsLarge is true}

flatMap is used to map the result of a future to the value of a new Future.

fibonacciFuture(10).flatMap{ numberinfibonacciFuture(number)}.onSuccess{ largeNumberin    // largeNumber is 139583862445}

letf=Future<Int,Never>(value:1)letf1=Future<Int,Never>(value:2)f.zip(f1).onSuccess{ a, bin    // a is 1, b is 2}

Future<Int,Never>(value:3).filter{ $0>5}.onComplete{ resultin        // failed with error NoSuchElementError}Future<String,Never>(value:"Swift").filter{ $0.hasPrefix("Sw")}.onComplete{ resultin        // succeeded with value "Swift"}

If aFuture fails, userecover to offer a default or alternative value and continue the callback chain.

// imagine a request failedFuture<Int,ReadmeError>(error:.RequestFailed).recover{ _in // provide an offline defaultreturn5}.onSuccess{ valuein        // value is 5 if the request failed or 10 if the request succeeded}

In addition torecover,recoverWith can be used to provide a Future that will provide the value to recover with.

BrightFutures also comes with a number of utility functions that simplify working with multiple futures. These are implemented as free (i.e. global) functions to work around current limitations of Swift.

The built-infold function allows you to turn a list of values into a single value by performing an operation on every element in the list thatconsumes it as it is added to the resulting value. A trivial usecase for fold would be to calculate the sum of a list of integers.

Folding a list of Futures is not very convenient with the built-infold function, which is why BrightFutures provides one that works especially well for our use case. BrightFutures'fold turns a list of Futures into a single Future that contains the resulting value. This allows us to, for example, calculate the sum of the first 10 Future-wrapped elements of the fibonacci sequence:

letfibonacciSequence=[fibonacciFuture(1),fibonacciFuture(2),...,fibonacciFuture(10)]// 1+1+2+3+5+8+13+21+34+55fibonacciSequence.fold(0, f:{ $0+ $1}).onSuccess{ sumin    // sum is 143}

Withsequence, you can turn a list of Futures into a single Future that contains a list of the results from those futures.

letfibonacciSequence=[fibonacciFuture(1),fibonacciFuture(2),...,fibonacciFuture(10)]    fibonacciSequence.sequence().onSuccess{ fibNumbersin    // fibNumbers is an array of Ints: [1, 1, 2, 3, etc.]}

traverse combinesmap andfold in one convenient function.traverse takes a list of values and a closure that takes a single value from that list and turns it into a Future. The result oftraverse is a single Future containing an array of the values from the Futures returned by the given closure.

(1...10).traverse{    iinfibonacciFuture(i)}.onSuccess{ fibNumbersin    // fibNumbers is an array of Ints: [1, 1, 2, 3, etc.]}

delay returns a new Future that will complete after waiting for the given interval with the result of the previous Future.To simplify working withDispatchTime andDispatchTimeInterval, we recommend to use thisextension.

Future<Int,Never>(value:3).delay(2.seconds).andThen{ resultin    // execute after two additional seconds}

BrightFutures tries its best to provide a simple and sensible default threading model. In theory, all threads are created equally and BrightFutures shouldn't care about which thread it is on. In practice however, the main thread ismore equal than others, because it has a special place in our hearts and because you'll often want to be on it to do UI updates.

A lot of the methods onFuture accept an optionalexecution context and a block, e.g.onSuccess,map,recover and many more. The block is executed (when the future is completed) in the given execution context, which in practice is a GCD queue. When the context is not explicitly provided, the following rules will be followed to determine the execution context that is used:

• if the method is called from the main thread, the block is executed on the main queue
• if the method is not called from the main thread, the block is executed on a global queue

If you want to have custom threading behavior, skip do do not the section. next😉

The default threading behavior can be overridden by providing explicit execution contexts. You can choose from any of the built-in contexts or easily create your own. Default contexts include: any dispatch queue, anyNSOperationQueue and theImmediateExecutionContext for when you don't want to switch threads/queues.

letf=Future<Int,Never>{ completeinDispatchQueue.global().async{complete(.success(fibonacci(10)))}}f.onComplete(DispatchQueue.main.context){ valuein    // update the UI, we're on the main thread}

Even though the future is completed from the global queue, the completion closure will be called on the main queue.

An invalidation token can be used to invalidate a callback, preventing it from being executed upon completion of the future. This is particularly useful in cases where the context in which a callback is executed changes often and quickly, e.g. in reusable views such as table views and collection view cells. An example of the latter:

classMyCell:UICollectionViewCell{vartoken=InvalidationToken()publicoverridefunc prepareForReuse(){        super.prepareForReuse()        token.invalidate()        token=InvalidationToken()}publicfunc setModel(model:Model){ImageLoader.loadImage(model.image).onSuccess(token.validContext){[weak self] UIImageinself?.imageView.image= UIImage}}}

By invalidating the token on every reuse, we prevent that the image of the previous model is set after the next model has been set.

Invalidation tokensdo not cancel the task that the future represents. That is a different problem. With invalidation tokens, the result is merely ignored. Invalidating a token after the original future completed does nothing.

If you are looking for a way to cancel a running task, you could look into usingNSProgress.

BrightFutures' primary author isThomas Visser. He is lead iOS Engineer atHighstreet. We welcome any feedback and pull requests. Get your name onthis list!

BrightFutures was inspired by Facebook'sBFTasks, the Promises & Futures implementation inScala and Max Howell'sPromiseKit.

BrightFutures is available under the MIT license. See the LICENSE file for more info.