Sane in the Domain

How Swift type system can help you model and test your application domain

Transcript

1. Sane in the domain How Swift type system can help you model and test your application domain Michał Ciuba
2. Agenda • What Domain means • Measurements • Phantom types • Domain-specific error types • Functions are types: unit tests example
3. Domain / Entities layer “Business rules or procedures that (…) would make or save the business money, irrespective of whether they were implemented on a computer” - Robert C. Martin, “Clean Architecture”
4. Domain examples • Entities: Driver, Customer, Booking • Business rule: Driver has a list of Bookings (rides booked by Customers)
5. Not Domain examples • class Driver: NSManagedObject • data storage is an implementation detail, not a core business rule
6. Domain types • use “plain” objects (not inheriting from any base class) • powerful Swift value types: structs, enum • compiler-generated Equatable, Codable, init • exhaustiveness checks for enums
7. Measurement using built-in Swift types
8. Mars Climate Orbiter 1999 NASA mission 125 million dollars burned up in the Martian atmosphere
9. Why? • Software developed by two teams • Each team used different units
 (kilograms vs pounds)
10. C (1970s) type system: primitive types // is it in kilograms? Or pounds? func calculateForce(forMass: Double) -> Double
11. Modern type system import Foundation public struct Measurement<UnitType> where UnitType : Unit
12. Measurement let orbiterMass = Measurement<UnitMass>(value: 0.42, unit: .kilograms) let stardustMass = Measurement<UnitMass>(value: 1E-5, unit: .pounds) let totalMass = orbiterMass + stardustMass // 0.42000453592 kg
13. Measurement let orbiterLength = Measurement<UnitLength>(value: 0.84, unit: .meters) let stardustMass = Measurement<UnitMass>(value: 1E-5, unit: .pounds) let totalLength = orbiterLength + stardustMass //won't compile
14. Measurement • Predefined 21 unit types - physical quantities (time, distance, speed…) • You can define your own units • locale-aware MeasurementFormatter
15. Phantom types
16. protocol MediaUploading { func uploadPhoto(at url: URL) func uploadVideo(at url: URL) } protocol MediaLibraryPicking { func selectPhoto() -> URL func selectVideo() -> URL }
17. Mistakes happen let url = mediaLibraryPicker.selectVideo() // a couple of method calls later mediaUploader.uploadPhoto(at: url) //
18. Phantom types struct LocalFile<FileType> { let url: URL } struct Photo {} struct Video {} let photo = LocalFile<Photo>(url: someURL)
19. Explicit interfaces protocol MediaUploading { func uploadPhoto(file: LocalFile<Photo>) func uploadVideo(file: LocalFile<Video>) } protocol MediaLibraryPicking { func selectPhoto() -> LocalFile<Photo> func selectVideo() -> LocalFile<Video> }
20. Compile-time checked let file = mediaLibraryPicker.selectVideo() // a couple of method calls later mediaUploader.uploadPhoto(file: file) // won’t compile!
21. Domain-specific error types Talixo case study
22. Driver: Start job
23. Location timeout error
24. “Too Early” error
25. “Too early” error HTTP 400 Bad Request { "error": { "message": "You are starting the job too early", "code": "JOB_STARTED_TOO_EARLY" } }
26. Default error
27. func handle(error: Error, errorCode: String?) { switch (error) { case AFError.responseValidationFailed where errorCode == "JOB_STARTED_TOO_EARLY": showTooEarlyErrorMessage() case is CLError: showLocationErrorMessage() default: showDefaultErrorMessage() } }
28. Wild error appears! HTTP 400 Bad Request { "error": { “message": "This booking is assigned to a different vehicle from the one currently in use. Use a correct vehicle to continue. “, “code” :"INVALID_VEHICLE" } }
29. Wait, what?
30. Expected message
31. func handle(error: Error, errorCode: String?) { switch (error) { case AFError.responseValidationFailed where errorCode == "JOB_STARTED_TOO_EARLY": showTooEarlyErrorMessage() case is CLError: showLocationErrorMessage() default: showDefaultErrorMessage() } }
32. Errors as Domain types enum ApplicationError: String { case tooEarly = "JOB_STARTED_TOO_EARLY" case invalidVehicle = "INVALID_VEHICLE" }
33. Errors as Domain types enum ApplicationError: String { case tooEarly = "JOB_STARTED_TOO_EARLY" case invalidVehicle = "INVALID_VEHICLE" } enum StartJobError: Swift.Error { case locationTimeout case applicationError(ApplicationError) case serviceUnavailable }
34. func handle(error: StartJobError) { switch(error) { case .locationTimeout: showLocationErrorMessage() case .applicationError(.tooEarly): showTooEarlyErrorMessage() case .applicationError(.wrongVehicle): showSelectVehicleAlert() } } // checked for exhaustiveness // (all ApplicationError values have to be handled)
35. func handle(error: StartJobError) { switch(error) { case .locationTimeout: showLocationErrorMessage() case .applicationError(.tooEarly): showTooEarlyErrorMessage() case .applicationError(.wrongVehicle): showSelectVehicleAlert() case .serviceUnavailable: showDefaultErrorMessage() } } No “default” case
36. Error conversion func convertToDomain(error: Swift.Error, withCode code: String) -> StartJobError { if case AFError.responseValidationFailed = error, let appError = ApplicationError(rawValue: code) { return .applicationError(appError) } if error is CLError { return .locationTimeout } // and so on... return .serviceUnavailable }
37. Functions are type Simplify async tests
38. Example: async calculation class AsyncCalculator { let serialQueue = DispatchQueue(label: "serial") func calculate(completion: @escaping (Int) ->()) { serialQueue.async { self.doComplexCalculations() completion(42) } } }
39. Async tests - Expectations func testCalculationHasCorrectResult() { // Arrange let expect = expectation(description: name) // Act calculator.calculate { result in XCTAssertEqual(result, 42) expect.fulfill() } // Assert waitForExpectations(timeout: 1) // }
40. Dependency injection? class AsyncCalculator { let serialQueue = DispatchQueue(label: "serial") func calculate(completion: @escaping (Int) ->()) { serialQueue.async { self.doComplexCalculations() completion(42) } } }
41. Dependency injection? class AsyncCalculator { var dispatcher = ? let serialQueue = DispatchQueue(label: "serial") func calculate(completion: @escaping (Int) ->()) { dispatcher { self.doComplexCalculations() completion(42) } } }
42. Inject a function! typealias DispatchedWork = () -> Void typealias Dispatcher = (@escaping DispatchedWork) -> Void class AsyncCalculator { internal lazy var dispatcher: Dispatcher }
43. Inject a function! typealias DispatchedWork = () -> Void typealias Dispatcher = (@escaping DispatchedWork) -> Void class AsyncCalculator { internal lazy var dispatcher: Dispatcher = self.serialQueue.async(execute:) let serialQueue = DispatchQueue(label: "serial") }
44. Inject a function! typealias DispatchedWork = () -> Void typealias Dispatcher = (@escaping DispatchedWork) -> Void class AsyncCalculator { internal lazy var dispatcher: Dispatcher = self.serialQueue.async(execute:) let serialQueue = DispatchQueue(label: "serial") func calculate(completion: @escaping (Int) -> Void) { dispatcher { self.doComplexCalculations() completion(42) } } } Inject a function!
45. Inject a function - test case private func instantDispatcher(_ closure: @escaping () -> Void ) { closure() } func testCalculationHasCorrectResult() { // Arrange calculator.dispatcher = instantDispatcher }
46. Synchronous tests private func instantDispatcher(_ closure: @escaping () -> Void ) { closure() } func testCalculationHasCorrectResult() { // Arrange calculator.dispatcher = instantDispatcher var result: Int? //Act calculator.calculate { result = $0 } //Assert XCTAssertEqual(result, 42) }
47. Wrap up • Use plain Swift objects (value types when possible) to model your Domain • Don’t pass around primitive types (like Double, URL…) • Error types are part of your Domain • Functions are types