BrightFutures is a simple Futures & Promises library for iOS and OS X written in Swift.
BrightFutures uses Control Flow-like syntax to wrap complicated calculations and provide an asynchronous interface to their result when it becomes available.
The goal of this project is to port Scala's Futures & Promises (guide, api) to Swift. Second to this readme, the Scala documentation should therefore also be of help.
BrightFutures is compatible with Swift 1.0. The project is currently moving towards a 1.0 release. Issue #12 has been created to track the progress towards that goal. Please feel free to chime in with your feedback or requests! Until 1.0, the API could change significantly.
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, error in
if !error {
Posts.fetchPosts(user, success: { posts in
// 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 { user, _ in
Posts.fetchPosts(user)
}.onSuccess { posts in
// do something with the user's posts
}.onFailure { error in
// either logging in or fetching posts failed
}Both User.logIn and Posts.fetchPosts now immediately return a Future. 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 from User.logIn fails, e.g. the username and password did not match, flatMap and onSuccess are skipped and onFailure is called with the error that occurred while logging in. If the login attempt succeeded, the resulting user object is passed to flatMap, which 'turns' the user into an array of his or her posts. If the posts could not be fetched, onSuccess is skipped and onFailure 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 or by looking at the tests.
If you already have a function (or really any expression) defined that you just want to execute asynchronously, you can just wrap it in a future() call and turn it into a Future:
future(fibonacci(10)).onSuccess { value in
// value is 55
}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. That is where the control-flow syntax comes in.
Because Swift allows to omit parenthesis for a function call if the only parameter is closure, we can pretend to add a future construct to the language:
let f = future { error in
fibonacci(10)
}
f.onSuccess { value in
// value will be 55
}error is an inout parameter that can be set in the closure if the calculation failed. If error is non-nil after the execution of the closure, the future is considered to have failed.
Now let's assume the role of an API author who wants to use BrightFutures. The 'producer' of a future is called a Promise. A promise contains a future that you can immediately hand to the client. The promise is kept around while performing the asynchronous operation, until calling Promise.success(result) or Promise.error(error) when the operation ended. Futures can only be completed through a Promise.
func complicatedQuestion() -> Future<String> {
let promise = Promise<String>()
Queue.async {
// do a complicated task
promise.success("forty-two")
}
return promise.future
}Queue is a simple wrapper around a dispatch queue.
Using the andThen function on a Future, the order of callbacks can be explicitly defined. The closure passed to andThen is meant to perform side-effects and does not influence the result. andThen returns a new Future with the same result as this future.
var answer = 10
future(4).andThen { result in
switch result {
case .Succeeded(let val):
answer *= val.value
case .Failure(_):
break
}
}.andThen { result in
// short-hand for the switch statement. Closure is executed immediately iff result is .Succeeded
result.succeeded { val in
answer += 2
}
}
// answer will be 42 (not 48)result is an instance of TaskResult, which mimics a typical Try construct as much as the Swift compiler currently allows. Due to limitations of generic enum types, the actual value needs to be wrapped in the .value property. (See #8.)
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. There's also a flatMap function that can be used to map the result of a future to the value of a new Future.
future { _ in
fibonacci(10)
}.map { number, error in
if number > 5 {
return "large"
}
return "small"
}.map { sizeString, _ in
return sizeString == "large"
}.onSuccess { numberIsLarge in
// numberIsLarge is true
}let f = future(1)
let f1 = future(2)
f.zip(f1).onSuccess { (let a, let b) in
// a is 1, b is 2
}future(3).filter { $0 > 5 }.onComplete { result in
// failed with error NoSuchElementError
}
future("Swift").filter { $0.hasPrefix("Sw") }.onComplete { result in
// succeeded with value "Swift"
}If a Future fails, use recover to offer a default or alternative value and continue the callback chain.
future { _ in
// fetch something from the web
if (request.error) { // it could fail
error = request.error
return nil
}
}.recover { _ in // provide an offline default
return 5
}.onSuccess { value in // either the request or the recovery succeeded
//value is 5
}In addition to recover, recoverWith can be used to provide a Future that will provide the value to recover with.
By default, all tasks and callbacks are performed on the global GCD queue. All future-wrapped tasks are performed concurrently, but all callbacks of a single future will be executed serially. You can however change this behavior by providing an execution context when creating a future or adding a callback:
let f = future(context: ImmediateExecutionContext()) { _ in
fibonacci(10)
}
f.onComplete(context: Queue.main) { value in
// update the UI, we're on the main thread
}The calculation of the 10nth Fibonacci number is now performed on the same thread as where the future is created.
BrightFutures also comes with a number of utility functions that simplify working with multiple futures. These functions are part of the FutureUtils class, which is the counterpart of Scala's Future object.
The built-in fold function allows you to turn a list of values into a single value by performing an operation on every element in the list that consumes 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 possible with the built-in fold function, which is why FutureUtils provides one that does work. Our version of fold turns a list of Futures into a single Future that contains the resulting value. This allows us, for example, to calculate the sum of the first 10 Future-wrapped elements of the fibonacci sequence:
// 1+1+2+3+5+8+13+21+34+55
let fibonacciSequence = [future(fibonacci(1)), future(fibonacci(2)), ... future(fibonacci(10))]
FutureUtils.fold(fibonacciSequence, zero: 0, op: { $0 + $1 }).onSuccess { val in
// value is 143
}With FutureUtils.sequence, you can turn a list of Futures into a single Future that contains a list of the results from those futures.
// 1+1+2+3+5+8+13+21+34+55
let fibonacciSequence = [future(fibonacci(1)), future(fibonacci(2)), ... future(fibonacci(10))]
FutureUtils.sequence(fibonacciSequence).onSuccess { fibNumbers in
// fibNumbers is an array of Ints: [1, 1, 2, 3, etc.]
}FutureUtils.traverse combines map and fold 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 of traverse is a single Future containing an array of the values from the Futures returned by the given closure.
FutureUtils.traverse(Array(1...10)) {
future(fibonacci($0))
}.onSuccess { fibNumbers in
// fibNumbers is an array of Ints: [1, 1, 2, 3, etc.]
}You can find more examples in the tests.
BrightFutures is created by me, Thomas Visser. I am an iOS Engineer at Touchwonders. I aspire for this project to have a growing list of contributors.
I really like Facebook's BFTasks, had a good look at the Promises & Futures implementation in Scala and also like what Max Howell is doing with PromiseKit.
I am looking forward to your feedback. I am very much still learning Swift. We all are. Let me know how I could improve BrightFutures by creating an issue, a pull request or by reaching out on twitter. I'm @thomvis88.
BrightFutures is available under the MIT license. See the LICENSE file for more info.
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