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The Great iOS Concurrency ! " Bake-Off ! Swift Usergroup NL ! | Amsterdam | February 2019 @DesignatedNerd | bakkenbaeck.no | justhum.com

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What is Concurrency?

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Concurrency vs. Parallelism

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Metaphor Time!

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Concurrency: "One thing should do these tasks at as close to the same time as possible."

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Parallelism: "Many things should do all these identical mini-tasks at the same time."

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!

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Taking the result of an asynchronous function

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Taking the result of an asynchronous function Giving it to another asynchronous function

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Taking the result of an asynchronous function Giving it to another asynchronous function (and another and another)

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The Problem: ! Callback Hell

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Teh Codez! https://github.com/designatednerd/ OperationComparison

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As a user, I want to see a profile picture when viewing a profile.

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As a developer, I want a list of profiles with icons to not drop frames when scrolling.

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User Data -> Icon Image -> Resized Icon Image static func fetchUser(failureCompletion: @escaping (Error) -> Void, successCompletion: @escaping (User) -> Void) static func fetchImage(for user: User, failureCompletion: @escaping (Swift.Error) -> Void, successCompletion: @escaping (UIImage) -> Void) static func resizeImage(_ image: UIImage, to size: CGSize, failureCompletion: @escaping (Swift.Error) -> Void, successCompletion: @escaping (UIImage) -> Void)

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Not Horrible... self.activityIndicator.startAnimating() FakeAPI.fetchUser(failureCompletion: { [weak self] error in guard let self = self else { return } self.activityIndicator.stopAnimating() self.operationLabel.text = "Error fetching user: \(error)" }, successCompletion: { [weak self] user in guard let self = self else { return } self.operationLabel.text = "Fetching image..." })

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Not ideal... self.activityIndicator.startAnimating() FakeAPI.fetchUser(failureCompletion: { [weak self] error in guard let self = self else { return } self.activityIndicator.stopAnimating() self.operationLabel.text = "Error fetching user: \(error)" }, successCompletion: { [weak self] user in guard let self = self else { return } self.operationLabel.text = "Fetching image..." RealAPI.fetchImage(for: user, failureCompletion: { [weak self] error in guard let self = self else { return } self.activityIndicator.stopAnimating() self.operationLabel.text = "Error fetching image: \(error)" }, successCompletion: { [weak self] image in guard let self = self else { return } self.operationLabel.text = "Resizing image..." }) })

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...Does anyone have a ? self.activityIndicator.startAnimating() FakeAPI.fetchUser(failureCompletion: { [weak self] error in guard let self = self else { return } self.activityIndicator.stopAnimating() self.operationLabel.text = "Error fetching user: \(error)" }, successCompletion: { [weak self] user in guard let self = self else { return } self.operationLabel.text = "Fetching image..." RealAPI.fetchImage(for: user, failureCompletion: { [weak self] error in guard let self = self else { return } self.activityIndicator.stopAnimating() self.operationLabel.text = "Error fetching image: \(error)" }, successCompletion: { [weak self] image in guard let self = self else { return } self.operationLabel.text = "Resizing image..." ImageResizer.resizeImage(image, to: self.imageView.frame.size, failureCompletion: { [weak self] error in guard let self = self else { return } self.activityIndicator.stopAnimating() self.operationLabel.text = "Error resizing image: \(error)" }, successCompletion: { [weak self] resizedImage in guard let self = self else { return } self.activityIndicator.stopAnimating() self.imageView.image = resizedImage self.operationLabel.text = "Complete in \(self.formattedSecondsSince(start))!" }) }) })

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¯\_(ϑ)_/¯ Avoid UI interaction and don't care about retain cycles!

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¯\_(ϑ)_/¯ Avoid UI interaction and don't care about retain cycles!* * - not recommended

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FakeAPI.fetchUser(failureCompletion: { error in debugPrint("Error fetching user: \(error)") }, successCompletion: { user in RealAPI.fetchImage(for: user, failureCompletion: { error in debugPrint("Error fetching image: \(error)") }, successCompletion: { image in ImageResizer.resizeImage(image, to: self.imageView.frame.size, failureCompletion: { error in debugPrint("Error resizing image: \(error)") }, successCompletion: { resizedImage in self.imageView.image = resizedImage debugPrint("Complete in \(self.formattedSecondsSince(start))!" }) }) })

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Think Indent Different

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FakeAPI.fetchUser( errorCompletion: { error in debugPrint("Error fetching user: \(error)") }, successCompletion: { user in RealAPI.fetchImage( for: user, failureCompletion: { error in debugPrint("Error fetching image: \(error)") }, successCompletion: { image in ImageResizer.resizeImage( image, to: self.imageView.frame.size, failureCompletion: { error in debugPrint("Error resizing image: \(error)") }, successCompletion: { resizedImage in self.imageView.image = resizedImage debugPrint("Complete in \(self.formattedSecondsSince(start))!" }) }) })

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Try to break it a p a r t

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private func fetchUser() { FakeAPI.fetchUser( failureCompletion: { error in debugPrint("Error fetching user: \(error)") }, successCompletion: { user in self.fetchImage(for: user) }) } private func fetchImage(for user: User) { RealAPI.fetchImage( for: user, failureCompletion: { error in debugPrint("Error fetching image: \(error)") }, successCompletion: { image in self.resizeImage(image, to: self.imageView.frame.size) }) } private func resizeImage(_ image: UIImage, to size: CGSize) { ImageResizer.resizeImage( image, to: self.imageView.frame.size, failureCompletion: { error in debugPrint("Error resizing image: \(error)") }, successCompletion: { [weak self] resizedImage in guard let self = self else { return } self.imageView.image = resizedImage debugPrint("Complete in \(self.formattedSecondsSince(start))!") }) }

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The problem: Dependent functions

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private func fetchUser() { FakeAPI.fetchUser( failureCompletion: { error in debugPrint("Error fetching user: \(error)") }, successCompletion: { user in self.fetchImage(for: user) }) } private func fetchImage(for user: User) { RealAPI.fetchImage( for: user, failureCompletion: { error in debugPrint("Error fetching image: \(error)") }, successCompletion: { image in self.resizeImage(image, to: self.imageView.frame.size) }) } private func resizeImage(_ image: UIImage, to size: CGSize) { ImageResizer.resizeImage( image, to: self.imageView.frame.size, failureCompletion: { error in debugPrint("Error resizing image: \(error)") }, successCompletion: { [weak self] resizedImage in guard let self = self else { return } self.imageView.image = resizedImage debugPrint("Complete in \(self.formattedSecondsSince(start))!") }) }

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private func fetchUser() { FakeAPI.fetchUser( failureCompletion: { error in debugPrint("Error fetching user: \(error)") }, successCompletion: { user in self.fetchImage(for: user) }) } private func fetchImage(for user: User) { RealAPI.fetchImage( for: user, failureCompletion: { error in debugPrint("Error fetching image: \(error)") }, successCompletion: { image in self.resizeImage(image, to: self.imageView.frame.size) }) } private func resizeImage(_ image: UIImage, to size: CGSize) { ImageResizer.resizeImage( image, to: self.imageView.frame.size, failureCompletion: { error in debugPrint("Error resizing image: \(error)") }, successCompletion: { [weak self] resizedImage in guard let self = self else { return } self.imageView.image = resizedImage debugPrint("Complete in \(self.formattedSecondsSince(start))!") }) }

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FakeAPI.fetchUser( errorCompletion: { error in debugPrint("Error fetching user: \(error)") }, successCompletion: { user in RealAPI.fetchImage( for: user, failureCompletion: { error in debugPrint("Error fetching image: \(error)") }, successCompletion: { image in ImageResizer.resizeImage( image, to: self.imageView.frame.size, failureCompletion: { error in debugPrint("Error resizing image: \(error)") }, successCompletion: { resizedImage in self.imageView.image = resizedImage debugPrint("Complete in \(self.formattedSecondsSince(start))!" }) }) })

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A slight help: The Result Type

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! Swift 5.0 https://github.com/apple/swift/blob/master/ stdlib/public/core/ Result.swift

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Result.Swift public enum Result { /// A success, storing a `Success` value. case success(Success) /// A failure, storing a `Failure` value. case failure(Failure) }

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User Data -> Icon Image -> Resized Icon Image static func fetchUser(failureCompletion: @escaping (Error) -> Void, successCompletion: @escaping (User) -> Void) static func fetchImage(for user: User, failureCompletion: @escaping (Swift.Error) -> Void, successCompletion: @escaping (UIImage) -> Void) static func resizeImage(_ image: UIImage, to size: CGSize, failureCompletion: @escaping (Swift.Error) -> Void, successCompletion: @escaping (UIImage) -> Void)

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User Data -> Icon Image -> Resized Icon Image static func fetchUser(completion: @escaping (Result) -> Void) static func fetchImage(for user: User, completion: @escaping (Result) -> Void) static func resizeImage(_ image: UIImage, to size: CGSize, failureCompletion: @escaping (Result) -> Void)

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FakeAPI.fetchUser { userResult in switch userResult { case .success(let user): RealAPI.fetchImage(for: user) { imageResult in switch imageResult { case .success(let image): ImageResizer.resizeImage(image, to: self.imageView.frame.size) { resizedImageResult in switch resizedImageResult { case .success(let resizedImage): self.imageView.image = resizedImage debugPrint("Complete") case .failure(let error): debugPrint("Error resizing image: \(error)") } } case .failure(let error): debugPrint("Error fetching image: \(error)") } } case .failure(let error): debugPrint("Error fetching user: \(error)") } }

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FakeAPI.fetchUser { userResult in switch userResult { case .success(let user): RealAPI.fetchImage(for: user) { imageResult in switch imageResult { case .success(let image): ImageResizer.resizeImage(image, to: self.imageView.frame.size) { resizedImageResult in switch resizedImageResult { case .success(let resizedImage): self.imageView.image = resizedImage debugPrint("Complete") case .failure(let error): debugPrint("Error resizing image: \(error)") } } case .failure(let error): debugPrint("Error fetching image: \(error)") } } case .failure(let error): debugPrint("Error fetching user: \(error)") } }

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FakeAPI.fetchUser { userResult in switch userResult { case .success(let user): RealAPI.fetchImage(for: user) { imageResult in switch imageResult { case .success(let image): ImageResizer.resizeImage( image, to: self.imageView.frame.size) { resizedImageResult in switch resizedImageResult { case .success(let resizedImage): self.imageView.image = resizedImage debugPrint("Complete") case .failure(let error): debugPrint("Error resizing image: \(error)") } } case .failure(let error): debugPrint("Error fetching image: \(error)") } } case .failure(let error): debugPrint("Error fetching user: \(error)") } }

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FakeAPI.fetchUser { userResult in switch userResult { case .success(let user): RealAPI.fetchImage(for: user) { imageResult in switch imageResult { case .success(let image): ImageResizer.resizeImage( image, to: self.imageView.frame.size) { resizedImageResult in switch resizedImageResult { case .success(let resizedImage): self.imageView.image = resizedImage debugPrint("Complete") case .failure(let error): debugPrint("Error resizing image: \(error)") } } case .failure(let error): debugPrint("Error fetching image: \(error)") } } case .failure(let error): debugPrint("Error fetching user: \(error)") } }

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How do we chain operations which need each others' results clearly?

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How do we chain operations which need each others' results in an easy-to-follow way?

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How do we chain operations which need each others' results so we can figure out where something's going wrong?

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! " Bake-Off!

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Three competitors

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Three competitors (out of way more than three)

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The Problem: ! Callback Hell

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Three competitors

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Three competitors 1. Operator Overloading

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Three competitors 1. Operator Overloading 2. PromiseKit

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Three competitors 1. Operator Overloading 2. PromiseKit 3. RxSwift

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! Signature Challenge:

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! Signature Challenge: Making Code More Readable

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Competitor #1: Operator Overloading

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Competitor #1: Operator Overloading "We don't need no stinkin' libraries!"

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typealias ResultCompletion = (Result) -> Void infix operator -->: MultiplicationPrecedence func -->(_ firstFunction: @escaping (@escaping ResultCompletion) -> Void, _ secondFunction: @escaping (T, @escaping ResultCompletion) -> Void) -> (@escaping ResultCompletion) -> Void { return { completion in firstFunction { result in switch result { case .success(let item): secondFunction(item) { result2 in completion(result2) } case .error(let error): completion(.error(error)) } } } }

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typealias ResultCompletion = (Result) -> Void infix operator -->: MultiplicationPrecedence func -->(_ firstFunction: @escaping (@escaping ResultCompletion) -> Void, _ secondFunction: @escaping (T, @escaping ResultCompletion) -> Void) -> (@escaping ResultCompletion) -> Void { return { completion in firstFunction { result in switch result { case .success(let item): secondFunction(item) { result2 in completion(result2) } case .error(let error): completion(.error(error)) } } } }

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typealias ResultCompletion = (Result) -> Void infix operator -->: MultiplicationPrecedence func -->(_ firstFunction: @escaping (@escaping ResultCompletion) -> Void, _ secondFunction: @escaping (T, @escaping ResultCompletion) -> Void) -> (@escaping ResultCompletion) -> Void { return { completion in firstFunction { result in switch result { case .success(let item): secondFunction(item) { result2 in completion(result2) } case .error(let error): completion(.error(error)) } } } }

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PEMDAS

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PEMDAS (parentheses, exponents, multiplication, division, addition, subtraction)

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typealias ResultCompletion = (Result) -> Void infix operator -->: MultiplicationPrecedence func -->(_ firstFunction: @escaping (@escaping ResultCompletion) -> Void, _ secondFunction: @escaping (T, @escaping ResultCompletion) -> Void) -> (@escaping ResultCompletion) -> Void { return { completion in firstFunction { result in switch result { case .success(let item): secondFunction(item) { result2 in completion(result2) } case .error(let error): completion(.error(error)) } } } }

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typealias ResultCompletion = (Result) -> Void infix operator -->: MultiplicationPrecedence func -->(_ firstFunction: @escaping (@escaping ResultCompletion) -> Void, _ secondFunction: @escaping (T, @escaping ResultCompletion) -> Void) -> (@escaping ResultCompletion) -> Void { return { completion in firstFunction { result in switch result { case .success(let item): secondFunction(item) { result2 in completion(result2) } case .error(let error): completion(.error(error)) } } } }

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typealias ResultCompletion = (Result) -> Void infix operator -->: MultiplicationPrecedence func -->(_ firstFunction: @escaping (@escaping ResultCompletion) -> Void, _ secondFunction: @escaping (T, @escaping ResultCompletion) -> Void) -> (@escaping ResultCompletion) -> Void { return { completion in firstFunction { result in switch result { case .success(let item): secondFunction(item) { result2 in completion(result2) } case .error(let error): completion(.error(error)) } } } }

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typealias ResultCompletion = (Result) -> Void infix operator -->: MultiplicationPrecedence func -->(_ firstFunction: @escaping (@escaping ResultCompletion) -> Void, _ secondFunction: @escaping (T, @escaping ResultCompletion) -> Void) -> (@escaping ResultCompletion) -> Void { return { completion in firstFunction { result in switch result { case .success(let item): secondFunction(item) { result2 in completion(result2) } case .error(let error): completion(.error(error)) } } } }

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typealias ResultCompletion = (Result) -> Void infix operator -->: MultiplicationPrecedence func -->(_ firstFunction: @escaping (@escaping ResultCompletion) -> Void, _ secondFunction: @escaping (T, @escaping ResultCompletion) -> Void) -> (@escaping ResultCompletion) -> Void { return { completion in firstFunction { result in switch result { case .success(let item): secondFunction(item) { result2 in completion(result2) } case .error(let error): completion(.error(error)) } } } }

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User Data -> Icon Image -> Resized Icon Image static func fetchUser(completion: @escaping (Result) -> Void) static func fetchImage(for user: User, completion: @escaping (Result) -> Void) static func resizeImage(_ image: UIImage, to size: CGSize, failureCompletion: @escaping (Result) -> Void)

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User Data -> Icon Image -> Resized Icon Image static func fetchUser(completion: @escaping (Result) -> Void) static func fetchImage(for user: User, completion: @escaping (Result) -> Void) static func resizeImage(_ image: UIImage, to size: CGSize, failureCompletion: @escaping (Result) -> Void)

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User Data -> Icon Image -> Resized Icon Image static func fetchUser(completion: @escaping (Result) -> Void) static func fetchImage(for user: User, completion: @escaping (Result) -> Void) static func resizeImage(_ image: UIImage, to size: CGSize, failureCompletion: @escaping (Result) -> Void)

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! Wrapper Methods private func resizeImageForImageView(_ image: UIImage, completion: @escaping (Swift.Result) -> Void) { ImageResizer.resizeImage(image, to: self.imageView.frame.size, completion: completion) }

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! Wrapper Methods private func resizeImageForImageView(_ image: UIImage, completion: @escaping (Swift.Result) -> Void) { self.operationLabel.text = "Resizing image (Custom Operator)..." ImageResizer.resizeImage(image, to: self.imageView.frame.size, completion: completion) } private func fetchUserImage(for user: User, completion: @escaping (Swift.Result) -> Void) { self.operationLabel.text = "Fetching image (Custom Operator)..." RealAPI.fetchImage(for: user, completion: completion) }

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self.activityIndicator.startAnimating() let chain = API.fetchUser --> fetchUserImage --> resizeImageForImageView chain { result in self.activityIndicator.stopAnimating() switch result { case .success(let image): self.imageView.image = image self.operationLabel.text = "Complete (Custom Operator)!" case .error(let error): self.operationLabel.text = "Error occurred (Custom Operator): \(error)" } }

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self.activityIndicator.startAnimating() let chain = API.fetchUser --> fetchUserImage --> resizeImageForImageView chain { result in self.activityIndicator.stopAnimating() switch result { case .success(let image): self.imageView.image = image self.operationLabel.text = "Complete (Custom Operator)!" case .error(let error): self.operationLabel.text = "Error occurred (Custom Operator): \(error)" } }

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self.activityIndicator.startAnimating() let chain = API.fetchUser --> fetchUserImage --> resizeImageForImageView chain { result in self.activityIndicator.stopAnimating() switch result { case .success(let image): self.imageView.image = image self.operationLabel.text = "Complete (Custom Operator)!" case .error(let error): self.operationLabel.text = "Error occurred (Custom Operator): \(error)" } }

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typealias ResultCompletion = (Result) -> Void infix operator -->: MultiplicationPrecedence func -->(_ firstFunction: @escaping (@escaping ResultCompletion) -> Void, _ secondFunction: @escaping (T, @escaping ResultCompletion) -> Void) -> (@escaping ResultCompletion) -> Void { return { completion in firstFunction { result in switch result { case .success(let item): secondFunction(item) { result2 in completion(result2) } case .error(let error): completion(.error(error)) } } } }

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typealias ResultCompletion = (Result) -> Void infix operator -->: MultiplicationPrecedence func -->(_ firstFunction: @escaping (@escaping ResultCompletion) -> Void, _ secondFunction: @escaping (T, @escaping ResultCompletion) -> Void) -> (@escaping ResultCompletion) -> Void { return { completion in firstFunction { result in switch result { case .success(let item): secondFunction(item) { result2 in completion(result2) } case .error(let error): completion(.error(error)) } } } }

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Pro: You Do It Yourself

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Con: You Do It Yourself

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Competitor #2: PromiseKit

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Competitor #2: PromiseKit "OK, fine. Maybe we do need a library."

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User Data -> Icon Image -> Resized Icon Image static func fetchUser(completion: @escaping (Result) -> Void) static func fetchImage(for user: User, completion: @escaping (Result) -> Void) static func resizeImage(_ image: UIImage, to size: CGSize, failureCompletion: @escaping (Result) -> Void)

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User Data -> Icon Image -> Resized Icon Image static func fetchUser() -> Promise static func fetchImage(for user: User) -> Promise static func resizeImage(_ image: UIImage, to size: CGSize) -> Promise

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self.activityIndicator.startAnimating() FakeAPI.fetchUser() .then { user in RealAPI.fetchImage(for: user) } .then { image in ImageResizer.resizeImage(image, to: self.imageView.frame.size) } .ensure { self.activityIndicator.stopAnimating() } .done { resizedImage in self.imageView.image = resizedImage self.operationLabel.text = "Complete (PromiseKit)!" } .catch { [weak self] error in self?.operationLabel.text = "Error occurred (PromiseKit): \(error)" }

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self.activityIndicator.startAnimating() FakeAPI.fetchUser() .then { user in RealAPI.fetchImage(for: user) } .then { image in ImageResizer.resizeImage(image, to: self.imageView.frame.size) } .ensure { self.activityIndicator.stopAnimating() } .done { resizedImage in self.imageView.image = resizedImage self.operationLabel.text = "Complete (PromiseKit)!" } .catch { [weak self] error in self?.operationLabel.text = "Error occurred (PromiseKit): \(error)" }

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self.activityIndicator.startAnimating() FakeAPI.fetchUser() .then { user in RealAPI.fetchImage(for: user) } .then { image in ImageResizer.resizeImage(image, to: self.imageView.frame.size) } .ensure { self.activityIndicator.stopAnimating() } .done { resizedImage in self.imageView.image = resizedImage self.operationLabel.text = "Complete (PromiseKit)!" } .catch { [weak self] error in self?.operationLabel.text = "Error occurred (PromiseKit): \(error)" }

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self.activityIndicator.startAnimating() FakeAPI.fetchUser() .then { user in RealAPI.fetchImage(for: user) } .then { image in ImageResizer.resizeImage(image, to: self.imageView.frame.size) } .ensure { self.activityIndicator.stopAnimating() } .done { resizedImage in self.imageView.image = resizedImage self.operationLabel.text = "Complete (PromiseKit)!" } .catch { [weak self] error in self?.operationLabel.text = "Error occurred (PromiseKit): \(error)" }

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self.activityIndicator.startAnimating() FakeAPI.fetchUser() .then { user in RealAPI.fetchImage(for: user) } .then { image in ImageResizer.resizeImage(image, to: self.imageView.frame.size) } .ensure { self.activityIndicator.stopAnimating() } .done { resizedImage in self.imageView.image = resizedImage self.operationLabel.text = "Complete (PromiseKit)!" } .catch { [weak self] error in self?.operationLabel.text = "Error occurred (PromiseKit): \(error)" }

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self.activityIndicator.startAnimating() FakeAPI.fetchUser() .then { user in RealAPI.fetchImage(for: user) } .then { image in ImageResizer.resizeImage(image, to: self.imageView.frame.size) } .ensure { self.activityIndicator.stopAnimating() } .done { resizedImage in self.imageView.image = resizedImage self.operationLabel.text = "Complete (PromiseKit)!" } .catch { [weak self] error in self?.operationLabel.text = "Error occurred (PromiseKit): \(error)" }

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self.activityIndicator.startAnimating() FakeAPI.fetchUser() .then { user in RealAPI.fetchImage(for: user) } .then { image in ImageResizer.resizeImage(image, to: self.imageView.frame.size) } .ensure { self.activityIndicator.stopAnimating() } .done { resizedImage in self.imageView.image = resizedImage self.operationLabel.text = "Complete (PromiseKit)!" } .catch { [weak self] error in self?.operationLabel.text = "Error occurred (PromiseKit): \(error)" }

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Solution #3: RxSwift

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Solution #3: RxSwift "Use ALL the libraries!"

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User Data -> Icon Image -> Resized Icon Image static func fetchUser() -> Promise static func fetchImage(for user: User) -> Promise static func resizeImage(_ image: UIImage, to size: CGSize) -> Promise

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User Data -> Icon Image -> Resized Icon Image static func fetchUser() -> Single static func fetchImage(for user: User) -> Single static func resizeImage(_ image: UIImage, to size: CGSize) -> Single

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Observable

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Observable "I could get many values from this, until it tells me things are complete or an error has occurred."

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Single

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Single "This will either succeed or fail once."

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FakeAPI.rx.fetchUser() .flatMap { user in RealAPI.rx.fetchImage(for: user) } .flatMap { image in ImageResizer.rx.resizeImage(image, to: self.imageView.frame.size) } .subscribe( onSuccess: { resizedImage in self.imageView.image = resizedImage self.operationLabel.text = "Complete (RxSwift) in \(self.formattedSecondsSince(start))!" }, onError: { error in self.operationLabel.text = "Error occurred (RxSwift): \(error)" } ) .disposed(by: bag)

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FakeAPI.rx.fetchUser() .flatMap { user in RealAPI.rx.fetchImage(for: user) } .flatMap { image in ImageResizer.rx.resizeImage(image, to: self.imageView.frame.size) } .subscribe( onSuccess: { resizedImage in self.imageView.image = resizedImage self.operationLabel.text = "Complete (RxSwift) in \(self.formattedSecondsSince(start))!" }, onError: { error in self.operationLabel.text = "Error occurred (RxSwift): \(error)" } ) .disposed(by: bag)

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FakeAPI.rx.fetchUser() .flatMap { user in RealAPI.rx.fetchImage(for: user) } .flatMap { image in ImageResizer.rx.resizeImage(image, to: self.imageView.frame.size) } .subscribe( onSuccess: { resizedImage in self.imageView.image = resizedImage self.operationLabel.text = "Complete (RxSwift) in \(self.formattedSecondsSince(start))!" }, onError: { error in self.operationLabel.text = "Error occurred (RxSwift): \(error)" } ) .disposed(by: bag)

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FakeAPI.rx.fetchUser() .flatMap { user in RealAPI.rx.fetchImage(for: user) } .flatMap { image in ImageResizer.rx.resizeImage(image, to: self.imageView.frame.size) } .subscribe( onSuccess: { resizedImage in self.imageView.image = resizedImage self.operationLabel.text = "Complete (RxSwift) in \(self.formattedSecondsSince(start))!" }, onError: { error in self.operationLabel.text = "Error occurred (RxSwift): \(error)" } ) .disposed(by: bag)

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FakeAPI.rx.fetchUser() .flatMap { user in RealAPI.rx.fetchImage(for: user) } .flatMap { image in ImageResizer.rx.resizeImage(image, to: self.imageView.frame.size) } .subscribe( onSuccess: { resizedImage in self.imageView.image = resizedImage self.operationLabel.text = "Complete (RxSwift) in \(self.formattedSecondsSince(start))!" }, onError: { error in self.operationLabel.text = "Error occurred (RxSwift): \(error)" } ) .disposed(by: bag)

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FakeAPI.rx.fetchUser() .flatMap { user in RealAPI.rx.fetchImage(for: user) } .flatMap { image in ImageResizer.rx.resizeImage(image, to: self.imageView.frame.size) } .subscribe( onSuccess: { resizedImage in self.imageView.image = resizedImage self.operationLabel.text = "Complete (RxSwift) in \(self.formattedSecondsSince(start))!" }, onError: { error in self.operationLabel.text = "Error occurred (RxSwift): \(error)" } ) .disposed(by: bag)

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! Technical Challenge:

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! Technical Challenge: Handling Threading

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Operator Overloading private func fetchUserImage(for user: User, completion: @escaping (Result) -> Void) { self.operationLabel.text = "Fetching image (Custom Operator)..." API.fetchImage(for: user, completion: completion) }

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Operator Overloading private func fetchUserImage(for user: User, completion: @escaping (Result) -> Void) { self.operationLabel.text = "Fetching image (Custom Operator)..." DispatchQueue.global(qos: .background).async { API.fetchImage(for: user, completion: completion) } }

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Operator Overloading private func fetchUserImage(for user: User, completion: @escaping (Result) -> Void) { self.operationLabel.text = "Fetching image (Custom Operator)..." DispatchQueue.global(qos: .background).async { API.fetchImage(for: user, completion: completion) } }

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Operator Overloading private func fetchUserImage(for user: User, completion: @escaping (Result) -> Void) { self.operationLabel.text = "Fetching image (Custom Operator)..." DispatchQueue.global(qos: .background).async { API.fetchImage(for: user, completion: completion) } }

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PromiseKit FakeAPI.fetchUser() .then { user in RealAPI.fetchImage(for: user) } .then { image in ImageResizer.resizeImage(image, to: self.imageView.frame.size) } .ensure { self.activityIndicator.stopAnimating() } .done { resizedImage in self.imageView.image = resizedImage self.operationLabel.text = "Complete (PromiseKit)!" } .catch { [weak self] error in self?.operationLabel.text = "Error occurred (PromiseKit): \(error)" }

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PromiseKit FakeAPI.fetchUser() .then(on: .global(qos: .background)) { user in RealAPI.fetchImage(for: user) } .then(on: .global(qos: .background)) { image in ImageResizer.resizeImage(image, to: self.imageView.frame.size) } .ensure { self.activityIndicator.stopAnimating() } .done { resizedImage in self.imageView.image = resizedImage self.operationLabel.text = "Complete (PromiseKit)!" } .catch { [weak self] error in self?.operationLabel.text = "Error occurred (PromiseKit): \(error)" }

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PromiseKit FakeAPI.fetchUser() .then(on: .global(qos: .background)) { user in RealAPI.fetchImage(for: user) } .then(on: .global(qos: .background)) { image in ImageResizer.resizeImage(image, to: self.imageView.frame.size) } .ensure { self.activityIndicator.stopAnimating() } .done { resizedImage in self.imageView.image = resizedImage self.operationLabel.text = "Complete (PromiseKit)!" } .catch { [weak self] error in self?.operationLabel.text = "Error occurred (PromiseKit): \(error)" }

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PromiseKit FakeAPI.fetchUser() .then(on: .global(qos: .background)) { user in RealAPI.fetchImage(for: user) } .then(on: .global(qos: .background)) { image in ImageResizer.resizeImage(image, to: self.imageView.frame.size) } .ensure { self.activityIndicator.stopAnimating() } .done { resizedImage in self.imageView.image = resizedImage self.operationLabel.text = "Complete (PromiseKit)!" } .catch { [weak self] error in self?.operationLabel.text = "Error occurred (PromiseKit): \(error)" }

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PromiseKit 6

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PromiseKit 6

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PromiseKit 6

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PromiseKit 6

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PromiseKit 6

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PromiseKit 6

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PromiseKit 6

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PromiseKit FakeAPI.fetchUser() .then(on: .global(qos: .background)) { user in RealAPI.fetchImage(for: user) } .map { image in (image, self.imageView.frame.size) } .then(on: .global(qos: .background)) { tuple in ImageResizer.resizeImage(tuple.0, to: tuple.1) } .then(on: .global(qos: .background)) { image in ImageResizer.resizeImage(image, to: self.imageView.frame.size) } .ensure { self.activityIndicator.stopAnimating() } .done { resizedImage in self.imageView.image = resizedImage self.operationLabel.text = "Complete (PromiseKit)!" } .catch { [weak self] error in self?.operationLabel.text = "Error occurred (PromiseKit): \(error)" }

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PromiseKit FakeAPI.fetchUser() .then(on: .global(qos: .background)) { user in RealAPI.fetchImage(for: user) } .map { image in (image, self.imageView.frame.size) } .then(on: .global(qos: .background)) { tuple in ImageResizer.resizeImage(tuple.0, to: tuple.1) } .then(on: .global(qos: .background)) { image in ImageResizer.resizeImage(image, to: self.imageView.frame.size) } .ensure { self.activityIndicator.stopAnimating() } .done { resizedImage in self.imageView.image = resizedImage self.operationLabel.text = "Complete (PromiseKit)!" } .catch { [weak self] error in self?.operationLabel.text = "Error occurred (PromiseKit): \(error)" }

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RxSwift:

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RxSwift: ! Scheduler

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It's not a queue!

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It's not a queue! (it's just way easier to think of it as one)

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observeOn: tell me what happened here

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RxSwift FakeAPI.rx.fetchUser() .flatMap { user in RealAPI.rx.fetchImage(for: user) } .flatMap { image in ImageResizer.rx.resizeImage(image, to: self.imageView.frame.size) } .subscribe( onSuccess: { resizedImage in self.imageView.image = resizedImage self.operationLabel.text = "Complete (RxSwift) in \(self.formattedSecondsSince(start))!" }, onError: { error in self.operationLabel.text = "Error occurred (RxSwift): \(error)" } ) .disposed(by: bag)

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RxSwift FakeAPI.rx.fetchUser() .flatMap { user in RealAPI.rx.fetchImage(for: user) } .flatMap { image in ImageResizer.rx.resizeImage(image, to: self.imageView.frame.size) } .observeOn(MainScheduler.instance) .subscribe( onSuccess: { resizedImage in self.imageView.image = resizedImage self.operationLabel.text = "Complete (RxSwift) in \(self.formattedSecondsSince(start))!" }, onError: { error in self.operationLabel.text = "Error occurred (RxSwift): \(error)" } ) .disposed(by: bag)

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RxSwift FakeAPI.rx.fetchUser() .flatMap { user in RealAPI.rx.fetchImage(for: user) } .flatMap { image in ImageResizer.rx.resizeImage(image, to: self.imageView.frame.size) } .observeOn(MainScheduler.instance) .subscribe( onSuccess: { resizedImage in self.imageView.image = resizedImage self.operationLabel.text = "Complete (RxSwift) in \(self.formattedSecondsSince(start))!" }, onError: { error in self.operationLabel.text = "Error occurred (RxSwift): \(error)" } ) .disposed(by: bag)

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subscribeOn: actually do stuff here

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RxSwift FakeAPI.rx.fetchUser() .flatMap { user in RealAPI.rx.fetchImage(for: user) } .flatMap { image in ImageResizer.rx.resizeImage(image, to: self.imageView.frame.size) } .subscribeOn(ConcurrentDispatchQueueScheduler(qos: .background)) .observeOn(MainScheduler.instance) .subscribe( onSuccess: { resizedImage in self.imageView.image = resizedImage self.operationLabel.text = "Complete (RxSwift) in \(self.formattedSecondsSince(start))!" }, onError: { error in self.operationLabel.text = "Error occurred (RxSwift): \(error)" } ) .disposed(by: bag)

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Operator overloading: Hard mode

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PromiseKit: Perform on a queue you specify Call back on a pre-defined queue

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RxSwift: Always specify where to do stuff Always specify where to call back

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! Show-Stopper Challenge:

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! Show-Stopper Challenge: Multi-Function Concurrency

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Two functions at the same time Do something with both results

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Operator Overloading

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Operator Overloading

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PromiseKit: Same Type let userPromise1 = FakeAPI.fetchUser() let userPromise2 = FakeAPI.fetchUser() when(fulfilled: [userPromise1, userPromise2]) .done { users in debugPrint("Users: \(users)") } .catch { error in debugPrint("Error: \(error)") }

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PromiseKit: Same Type let userPromise1 = FakeAPI.fetchUser() let userPromise2 = FakeAPI.fetchUser() when(fulfilled: [userPromise1, userPromise2]) .done { users in debugPrint("Users: \(users)") } .catch { error in debugPrint("Error: \(error)") }

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PromiseKit: Same Type let userPromise1 = FakeAPI.fetchUser() let userPromise2 = FakeAPI.fetchUser() when(fulfilled: [userPromise1, userPromise2]) .done { users in debugPrint("Users: \(users)") } .catch { error in debugPrint("Error: \(error)") }

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PromiseKit: Same Type let userPromise1 = FakeAPI.fetchUser() let userPromise2 = FakeAPI.fetchUser() when(fulfilled: userPromise1, userPromise2) .done { user1, user2 in debugPrint("User1: \(user1), User2: \(user2)") } .catch { error in debugPrint("Error: \(error)") }

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PromiseKit: Same Type let userPromise1 = FakeAPI.fetchUser() let userPromise2 = FakeAPI.fetchUser() when(fulfilled: userPromise1, userPromise2) .done { user1, user2 in debugPrint("User1: \(user1), User2: \(user2)") } .catch { error in debugPrint("Error: \(error)") }

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PromiseKit: Different Types let userPromise1 = FakeAPI.fetchUser() let imagePromise = ImageResizer.resizeImage(UIImage(), to: self.imageView.frame.size) when(fulfilled: userPromise1, imagePromise) .done { user, image in debugPrint("User: \(user), image: \(image)") } .catch { error in debugPrint("Error: \(error)") }

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PromiseKit: Different Types let userPromise1 = FakeAPI.fetchUser() let imagePromise = ImageResizer.resizeImage(UIImage(), to: self.imageView.frame.size) when(fulfilled: userPromise1, imagePromise) .done { user, image in debugPrint("User: \(user), image: \(image)") } .catch { error in debugPrint("Error: \(error)") }

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PromiseKit: Different Types let userPromise1 = FakeAPI.fetchUser() let imagePromise = ImageResizer.resizeImage(UIImage(), to: self.imageView.frame.size) when(fulfilled: userPromise1, imagePromise) .done { user, image in debugPrint("User: \(user), image: \(image)") } .catch { error in debugPrint("Error: \(error)") }

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RxSwift

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Rx Marble Diagrams

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RxSwift

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RxSwift

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RxSwift

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RxSwift

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RxSwift

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JUST PICK ONE!

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! PromiseKit

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! PromiseKit FOR ME

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! Killing flies with a sledgehammer

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Sometimes you need a sledgehammer

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Most of the time you only need a fly swatter

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Obligatory Summary Slide

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Obligatory Summary Slide → Harder concurrency is it looks than.

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Obligatory Summary Slide → Concurrency is harder than it looks.

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Obligatory Summary Slide → Concurrency is harder than it looks. → You can try to handle it yourself, but it's going to be a difficult time sink.

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Obligatory Summary Slide → Concurrency is harder than it looks. → You can try to handle it yourself, but it's going to be a difficult time sink. → PromiseKit is great for most concurrency handling and readability

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Obligatory Summary Slide → Concurrency is harder than it looks. → You can try to handle it yourself, but it's going to be a difficult time sink. → PromiseKit is great for most concurrency handling and readability → RxSwift is a giant hammer...but sometimes you need a giant hammer.

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Choose Wisely

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Links! → Vincent Pradeilles' Function Composition Talk https://vimeo.com/292702159 → John Sundell's Tasks post https://www.swiftbysundell.com/posts/task-based- concurrency-in-swift → RxMarbles Visualization https://rxmarbles.com

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! The Future! → Chris Lattner's Concurrency Manifesto https://gist.github.com/lattner/ 31ed37682ef1576b16bca1432ea9f782 → Kotlin Coroutines: Async/Await + More https://kotlinlang.org/docs/reference/coroutines- overview.html