updatePersonInfo(_ person: Person) { let group = DispatchGroup() var image: UIImage? var invitesCount: Int? var friends = [Friend]() group.enter() person.loadImage { fetchedImage in defer { group.leave() } image = fetchedImage } group.enter() person.loadInvitesCount { count in defer { group.leave() } invitesCount = count } group.enter() person.loadFriends { allFriends in friends = allFriends } group.notify(queue: DispatchQueue.main) { [weak self] in /$ Have all 3 values only here, /$ what about errors though ?! self?'imgAvatar.image = image self?'btnInvite.setTitle( "You have \(invitesCount) invites", for: .normal ) self?'friends = friends } } Other options: Semaphore DispatchWorkItems Combine Publishers How do we deal with accumulated errors here? How do we short-circuit? How do we cancel work? Which queue are we running on?
the hardest topic for me to grasp. Not obvious, too many methodologies, too many dark corners, and I’m not the only one: Fan moment: Matt Thomposon (NSHipster) created Alamofire and AFNetworking * tl;dr: Async is HARD
SwiftUI, Swift and other modern standards - working with asynchronous work is an obvious standard But it didn’t get any easier. Yet … Our code is modern now, sort of …
error handling Bad Serial Blocking Single thread Poor resource utilization Good Concurrent Non-blocking Multi-threaded Superior resource optimization Bad Difficult to follow (sometimes) unpredictable Handling errors is involved / error-prone Synchronous Asynchronous
is non-optimal in Swift For example, this very smart person: Main author of LLVM, Clang & Swift Chris Lattner https://gist.github.com/lattner/31ed37682ef1576b16bca1432ea9f782 Swift Concurrency Manifesto: First commit: August 11, 2017
many other programming languages async/await is a new syntax and part of a set of language features titled “Swift Concurrency”. (each with its own implementation) and more …
function It also marks a suspension point person.loadImage { [weak self] fetchedImage in switch fetchedImage { case .success(let image): self?'image = image case .failure(let error): handleError(error) } } do { self.image = try await person.loadImage() } catch { handleError(error) } ❓What is async/await?
can only be called from within an asynchronous context. This means either from a function which itself is async, or a task. func updatePersonInfo() async throws { let image = try await person.loadImage() } func updatePersonInfo() { Task { do { let image = try await person.loadImage() } catch { /$ handle error } } } Called from an async function: Called from a Task: ❓What is async/await?
[weak self] friends in /$ access image, count and friends } } } let image = await person.loadImage() let count = await person.loadInvitesCount() let friends = await person.loadFriends() /$ Access image, count and friends This might be confusing, but from a functionality perspective, these two pieces of code are equal: person.loadImage { image in person.loadInvitesCount { count in person.loadFriends { [weak self] friends in /$ access image, count and friends } } } ❓What is async/await?
two pieces of code are equal: These only seem synchronous, but each of them suspends and waits for its result before resuming to the next steps ❓What is async/await? let image = await person.loadImage() let count = await person.loadInvitesCount() let friends = await person.loadFriends() /$ Access image, count and friends
two pieces of code are equal: These only seem synchronous, but each of them suspends and waits for its result before resuming to the next steps ❓What is async/await? let image = await person.loadImage() let count = await person.loadInvitesCount() let friends = await person.loadFriends() /$ Access image, count and friends
Bad Serial Blocking Single thread Poor resource utilization Good Concurrent Non-blocking Multi-threaded Superior resource optimization Bad Difficult to follow (sometimes) unpredictable Handling errors is involved / error-prone Synchronous Asynchronous + = async/await ❓What is async/await?
of asynchronous work which could be a child of a different Task It has several initializers that let us create new asynchronous contexts: Task { … } Task(priority:) { ." } Creating tasks that inherit context, task values, etc: class ViewController: UIViewController { override func viewDidLoad() { super.viewDidLoad() Task { /$ Executes asynchronously, /$ but on the main thread print(Thread.isMainThread) /$ true } } }
{ ." } Creating tasks that are detached / independent: class ViewController: UIViewController { override func viewDidLoad() { super.viewDidLoad() Task.detached { /$ Executes asynchronously, /$ not on the main thread print(Thread.isMainThread) /$ false } } } A Task represents a unit of asynchronous work which could be a child of a different Task It has several initializers that let us create new asynchronous contexts:
a Success and Failure, But the default for these initializers (and most common scenario here) is Void and Never, accordingly Task { ..+ } = Task<Void, Never> { ..+ } let task = Task<String, Never> { return someExpensiveComputation() } let result = await task.value It also exposes an await-able value getter:
Pool model A pool of threads available to run asynchronous functions ! Tasks must always make forward progress or suspend No more than one thread per CPU core (as opposed to GCD) Avoid thread explosion Avoid the overhead and performance penalty of thread switching Can suspend tasks and resume them later to prioritize other, more important, pieces of work, thanks to a lightweight abstraction called a Continuation
Swift know it can “give up the thread” This means an async function can suspend, wait for results, and then resume as needed, while letting other async functions utilize threads in the pool func getFoo() async { let item = await getAmazingItem() doSomething(with: item) } suspends getFoo() and gives up its current thread (possibly) suspend getAmazingItem() or immediately run depending on thread availability and priorities
Swift know it can “give up the thread” This means an async function can suspend, wait for results, and then resume as needed, while letting other async functions utilize threads in the pool This results in optimum CPU utilization, prevents thread hops and avoids possible over-committing of threads func getFoo() async { let item = await getAmazingItem() doSomething(with: item) } suspends getFoo() and gives up its current thread (possibly) suspend getAmazingItem() or immediately run depending on thread availability and priorities When getAmazingItem() finishes, getFoo() resumes (It may resume on any thread)
count, error2 in person.loadFriends { [weak self] friends, error3 in /$ Somehow deal with the accumulation of all errors /$ What about cancellation and early short circuit?! } } } Error handling pre-async/await is … painful, to say the least. It’s very easy to miss handling errors, and handle all the edge cases Error handling
try await person.loadFriends() } catch { /$ handle error } async/await makes error handling feel natural to Swift, and identical to synchronous functions syntactically If any of the tasks fail, consecutive tasks will simply not run (or cancel) . More async/await
prevents us from writing incorrect code: func loadInsanelyLargeResource(completion: (Result<Resource, Error>) -) Void) { guard let resource = loadOptionalResource() else { return } completion(.success(resource)) } Oops, forgot to complete with an error! . More async/await
Resource { guard let resource = loadOptionalResource() else { return } return resource } The compiler and runtime also prevents us from writing incorrect code: Impossible to forget error handling . More async/await
Resource { guard let resource = loadOptionalResource() else { throw Error.invalidResource } return resource } The compiler and runtime also prevents us from writing incorrect code: . More async/await
make properties async, and even throw from them! struct Person { let avatarURL: URL } let shai = Person(..+) userImage.image = try await shai.avatar . More async/await var avatar: UIImage? { get async throws { let (data, _) = try await URLSession.shared.data(from: avatarURL) return UIImage(data: data) } }
reason about . More async/await Refresh token about to expire? Renew refresh token # Refresh access token / Fetch user and return 0 Error handling for each phase Yes No, refresh token is valid
if token.isAboutToExpire(within: 15) { service.renew { error in if let error = error { completion(.failure(error)) return } refreshTokenIfNeeded(token, completion: completion) } return } service.getAuthenticatedUser(for: token) { user in switch user { case .success(let user): completion(.success(user)) case .failure: service.notifyExpiration { error in if let error = error { completion(.failure(error)) } } } } } JWT Refresh with Closures We have to recurse back into the function since we can’t “continue” the control flow after a closure Messy error handling and hard to follow . More async/await
{ if token.isAboutToExpire(within: 15) { try await service.renew() } return try await service.getAuthenticatedUser(for: token) } catch { try await service.notifyExpiration() } } No need for recursion, the function simply suspends, waits for the renewal and proceeds Trivial error handling and control flow, like synchronous Swift code . More async/await JWT Refresh with async/await
[weak self] friends in /$ access image, count and friends } } } let image = await person.loadImage() let count = await person.loadInvitesCount() let friends = await person.loadFriends() /$ Access image, count and friends Asynchronous execution with straight-line “synchronous” code is awesome. But, it still waits for each task to complete. We can do better! person.loadImage { image in person.loadInvitesCount { count in person.loadFriends { [weak self] friends in /$ access image, count and friends } } } let image = await person.loadImage() let count = await person.loadInvitesCount() let friends = await person.loadFriends() /$ Access image, count and friends Back to our person info example… . More async/await
[weak self] friends in /$ access image, count and friends } } } func updatePersonInfo(_ person: Person) async { async let image = person.loadImage() async let count = person.loadInvitesCount() async let friends = person.loadFriends() await handleInfo(image, count, friends) } func updatePersonInfo(_ person: Person) { let group = DispatchGroup() var image: UIImage? var invitesCount: Int? var friends = [Friend]() group.enter() person.loadImage { fetchedImage in defer { group.leave() } image = fetchedImage } group.enter() person.loadInvitesCount { count in defer { group.leave() } invitesCount = count } group.enter() person.loadFriends { allFriends in friends = allFriends } group.notify(queue: DispatchQueue.main) { [weak self] in /$ Have all 3 values only here, /$ what about errors though ?! handleInfo(image, invitesCount, friends) } } async let bindings let you run multiple independent tasks in parallel, while awaiting all of their results together . More async/await
uses a “Cooperative Cancellation” mechanism. struct Person { let avatarURL: URL var avatar: UIImage? { get async throws { let (data, _) = try await URLSession.shared.data(from: avatarURL) return UIImage(data: data) } } } This basically means each task is responsible to short-circuit its own execution if it’s canceled. . More async/await
uses a “Cooperative Cancellation” mechanism. struct Person { let avatarURL: URL var avatar: UIImage? { get async throws { let (data, _) = try await URLSession.shared.data(from: avatarURL) return UIImage(data: data) } } } Task might have been canceled by the time fetching the data finished This basically means each task is responsible to short-circuit its own execution if it’s canceled. . More async/await
uses a “Cooperative Cancellation” mechanism. This basically means each task is responsible to short-circuit its own execution if it’s canceled. struct Person { let avatarURL: URL var avatar: UIImage? { get async throws { let (data, _) = try await URLSession.shared.data(from: avatarURL) try Task.checkCancellation() return UIImage(data: data) } } } Throws a special CancellationError if the task was canceled and stop the control flow . More async/await
different flow based on whether or not the task was cancelled: struct Person { let avatarURL: URL var avatar: UIImage? { get async throws { let (data, _) = try await URLSession.shared.data(from: avatarURL) if Task.isCancelled { return UIImage(named: "placeholder") } return UIImage(data: data) } } } . More async/await
people, and we want to fetch all of their images, and arrange them into an array of assets (many) async requests You might be tempted to do it this way: func getAllUserPhotos() async -) [UIImage] { var result = [UIImage]() for person in people { await result.append(person.loadAvatar()) } return result }
quite some time since it’s serial: (many) async requests func getAllUserPhotos() async -) [UIImage] { var result = [UIImage]() for person in people { await result.append(person.loadAvatar()) } return result } Suspends and waits for each image in its turn
perform many concurrent tasks might be to reach for async let bindings But this isn’t feasible / useful when we’re talking about varying / dynamic amount of async functions /$ ??? func getAllUserPhotos() async -) [UIImage] { var result = [UIImage]() for person in people { async let thing = person.loadAvatar() } return result } /" ???
might be to reach for async let bindings (many) concurrent async requests func getAllUserPhotos() async -) [UIImage] { var result = [UIImage]() for person in people { async let thing = person.loadAvatar() result.append(thing) } return result } But this isn’t feasible / useful when we’re talking about varying / dynamic amount of async functions
might be to reach for async let bindings (many) concurrent async requests func getAllUserPhotos() async -) [UIImage] { var result = [UIImage]() for person in people { async let thing = person.loadAvatar() result.append(thing) } return result } But this isn’t feasible / useful when we’re talking about varying / dynamic amount of async functions
tasks, in a hierarchical way Task groups await withTaskGroup(of: ..+) { group in await withThrowingTaskGroup(of: ..+) { group in Type of each child task’s result, or - each individual resource we want to await in the group await withTaskGroup(of: UIImage.self) { group in
you need to the group Task groups func getAllUserPhotos() async -) [UIImage] { await withTaskGroup(of: UIImage.self) { group in for person in people { group.addTask { await person.loadAvatar() } } var result = [UIImage]() for await image in group { result.append(image) } return result } }
async results, as if they were a simple Swift Collection Task groups func getAllUserPhotos() async -) [UIImage] { await withTaskGroup(of: UIImage.self) { group in for person in people { group.addTask { await person.loadAvatar() } } var result = [UIImage]() for await image in group { result.append(image) } return result } }
others: Task groups func getAllUserPhotos() async -) [UIImage] { await withTaskGroup(of: UIImage.self) { group in for person in people { group.addTask(priority: person.isImportant ? .high : nil) { await person.loadAvatar() } } var result = [UIImage]() for await image in group { result.append(image) } return result } }
about Structured Concurrency, we talk about hierarchy. Having this dependency hierarchy lets Swift do smart things such as propagate cancellation to child tasks. Child Task 1 Child Task 2 Child Task 3 Child Task 4 Task<UIImage, Never> Task<UIImage, Never> Task<UIImage, Never> Task<UIImage, Never> TaskGroup<UIImage> Parent task Task groups are a prime example of this, as well as async let bindings
- we can create Tasks that aren’t part of any hierarchy, and don’t have a parent task. We can store them, and cancel them or interact with them later on. A good example we’ve already talked about: Task { … } Task.detached { … }
with async functions that return a single result. But often, you will have asynchronous work that might return multiple values - Sockets, Publishers, File handles, and more… Time for Async Sequences
Swift Sequence, except that every value is awaited asynchronously You’ve already seen an example of this with Task Groups: await withTaskGroup(of: UIImage.self) { group in for person in people { group.addTask { await person.loadAvatar() } } var result = [UIImage]() for await image in group { result.append(image) } return result }
CSV file with hundreds of thousands of lines, and we want to fetch and parse it into a struct: 1,Shai 2,Elia 3,Ethan 4,Ariel 5,Natan 6,Yuval 7,Igor ..+ 300000,Steve
the file in its entirety and then iterating over all of its available lines: let (data, _) = try await URLSession.shared.data(for: URLRequest(url: largeCSV)) var people = [Person]() let lines = String(data: data, encoding: .utf8)?'components(separatedBy: "\n") ?, [] for line in lines { people.append(Person(csvLine: line)) } Greedily fetch the entire file, parse it as a single large String and split into lines
the new bytes(for:) API: let (bytes, _) = try await URLSession.shared.bytes(for: URLRequest(url: largeCSV)) var people = [Person]() for try await line in bytes.lines { people.append(Person(csvLine: line)) } Processes the lines lazily, while the data transfer is ongoing
reading local files: Reads the entire file synchronously, and then parses the lines let data = FileManager.default.contents(atPath: localFile.absoluteString) ?, Data() var people = [Person]() let lines = String(data: data, encoding: .utf8)?'components(separatedBy: "\n") ?, [] for line in lines { people.append(Person(csvLine: line)) }
reading local files: let handle = try FileHandle(forReadingFrom: localFile) var people = [Person]() for try await line in handle.bytes.lines { people.append(Person(csvLine: line)) } Read the lines one-by-ones using the new bytes API on FileHandle
get right in asynchronous programming is Data Races A data race usually involves shared mutable state, with two or more threads trying to access that state at the same time - and at least one of these accesses is a write Shared State One of these threads might get the wrong piece of data, or flat-out crash and burn 5 Thread 1 Thread 2
data races, usually we solve them with more involved mechanisms: NSLock NSRecursiveLock os_unfair_lock Mutex Serial Dispatch Queue These work and provide mutually exclusive access, but very hard to get just right
provide automatic synchronization and isolation for its state Actor Read/Write Synchronous Different Actor Its origins are rooted in high-scale languages such as Akka and Erlang Read/Write Asynchronous
entire types with @MainActor, to make sure they always run on the main thread: @MainActor class AwesomeViewModel { func doesUIThing1() { /$ Runs on Main Thread } func doesUIThing2() { /$ Runs on Main Thread } }
don’t run on the main actor by marking them as nonisolated: @MainActor class AwesomeViewModel { func doesUIThing() { /$ Runs on Main Thread } nonisolated func doesNonUIThing() { /$ Not isolated to Main Actor } }
a portion of the Actors story, there are many topics we haven’t looked into at all, such as: ⚪ @Sendable ⚪ @globalActor ⚪ @preconcurrency ⚪ nonisolated ⚪ Using the Thread Sanitizer with Actors ⚪ And much more …
async functions, but you’ll often want to create your own to wrap other closure- based functions, for example: You can do this using these built-in functions: withCheckedContinuation withCheckedThrowingContinuation withUnsafeContinuation withUnsafeThrowingContinuation 7 Using in your codebase
wrap it as an async function like so: func loadImage(completion: (Result<UIImage, Error>) -) Void) func loadImage() async throws -) UIImage { try await withCheckedThrowingContinuation { continuation in loadImage { result in switch result { case .success(let image): continuation.resume(returning: image) case .failure(let error): continuation.resume(throwing: error) } } } } 7 Using in your codebase
with a Result: func loadImage() async throws -) UIImage { try await withCheckedThrowingContinuation { continuation in loadImage { result in continuation.resume(with: result) } } } Note: You mustn’t resume the continuation more than once! 7 Using in your codebase
case finished } func monitorStocks(stockChange: (StockChange) -) Void) { ..+ } 7 Using in your codebase As mentioned before, AsyncSequence is simply a protocol. But luckily, you don’t have to implement it yourself. You can simply use AsyncStream, instead
AsyncStream<Stock> { continuation in monitorStocks { change in switch change { case .change(let stock): continuation.yield(stock) case .finished: continuation.finish() } } } } Task { for await stock in monitorStocks() { print("Current stock: \(stock)") } } 7 Using in your codebase As mentioned before, AsyncSequence is simply a protocol. But luckily, you don’t have to implement it yourself. You can simply use AsyncStream, instead
using NS_SWIFT_ASYNC_NAME and NS_REFINED_FOR_SWIFT_ASYNC - (void)fetchNumbers:(void(^)(NSArray<NSNumber */*))completion NS_SWIFT_ASYNC_NAME(numbers()); 7 Using in your codebase
this AsyncSequence thing is pretty similar to a Combine Publisher! 7 Using in your codebase Well, you’re absolutely right - and you can even use a Publisher this way using the values property: Combine & Sink: Combine & async/await: numbers .map { $0 * 2 } .prefix(3) .dropFirst() .sink( receiveCompletion: { _ in print("Done!") }, receiveValue: { print("Value \($0)") } ) .store(in: &subscriptions) let publisher = numbers .map { $0 * 2 } .prefix(3) .dropFirst() for await number in publisher.values { print("Value \(number)") } print("Done!")
operators?! How can I use them in async/ await, without Combine? 7 Using in your codebase Apple seems to be shifting its focus heavily towards async/await, as evident by their latest release of swift-async-algorithms: A set of Combine-like operators for AsyncSequences: https://github.com/apple/swift-async-algorithms
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