Straying From the Happy Path: Taking Control of Errors in Swift

Have you ever seen a code comment saying “this should never happen”? Have you ever wondered what would happen if it did? Swift has a diverse set of error-handling capabilities, from using throw to send errors up the stack to using a Result to handle errors in asynchronous methods. In this talk, we’ll look at the landscape of handling errors in Swift, create new ways of expressing and handling them, and show how even the most impossible code-level situations can have 100% test coverage. By the end, you’ll be taking control of the errors in your code, instead of letting them take control of you.

Transcript

1. Straying From the Happy Path Taking Control of Errors in Swift Jeff Kelley (@SlaunchaMan) #CodeMash 2020, January 10th, 2020
2. Agenda The History of Error-Handling: Objective-C and Swift The Swift Error Type Testing Errors Error Handling Through Types: Combine and SwiftUI Jeff Kelley |
3. Agenda The History of Error-Handling: Objective-C and Swift The Swift Error Type Testing Errors Error Handling Through Types: Combine and SwiftUI Jeff Kelley |
4. Error Handling in Objective-C Error Pointers Objective-C used a pointer to an NSError object to vend errors back to the calling code. When the removeItemAtPath:error: method finishes, if an error occurred, success would be NO and error might have a pointer to an NSError in it. NSError *error; BOOL success = [[NSFileManager defaultManager] removeItemAtPath:@"/tmp/dontneedthisanymore" error:&error]; So, how do we handle this kind of error? Jeff Kelley |
5. Error Handling in Objective-C Error Pointers You might think to do it this way: NSError *error; BOOL success = [[NSFileManager defaultManager] removeItemAtPath:@"/tmp/dontneedthisanymore" error:&error]; if (error != nil) { // Handle the error } Jeff Kelley |
6. Error Handling in Objective-C Error Pointers But in reality, you need to do it this way: NSError *error; BOOL success = [[NSFileManager defaultManager] removeItemAtPath:@"/tmp/dontneedthisanymore" error:&error]; if (!success) { if (error != nil) { // Handle the error } } The value in error is not sufficient for determining the result of the operation. Jeff Kelley |
7. Error Handling in Objective-C Completion Handlers When writing asynchronous code, we often use completion handlers to convey the results of the operation. HKHealthStore *store = [[HKHealthStore alloc] init]; [store startWatchAppWithWorkoutConfiguration:config completion:^(BOOL success, NSError * _Nullable error) { if (!success) { NSLog(@"Error starting workout: %@", [error localizedDescription]); // Alert the user that their workout didn’t start } }]; This pattern is so common, we have a type for it in Swift. Jeff Kelley |
8. Errors in Swift Result Result is perfect for asynchronous code—it clearly defines the type you’ll have in both the success and failure cases. /// A value that represents either a success or a failure, including an /// associated value in each case. public enum Result<Success, Failure> where Failure : Error { /// A success, storing a `Success` value. case success(Success) /// A failure, storing a `Failure` value. case failure(Failure) … } Jeff Kelley |
9. Result in Practice Network Requests A typical place you’ll see a Result is in fetching something from the network: let url = URL(string: "https://catfactsapi.cool/api/catfactoftheday")! performRequest(url) { result in switch result { case .success(let data): // Parse data into model object case .failure(let error): // Handle error } } Jeff Kelley |
10. Result in Practice map() We can use map(_:) to transform the success case of a Result into a new value. func map<NewSuccess>( _ transform: (Success) -> NewSuccess ) -> Result<NewSuccess, Failure> 1 If you want to transform the failure type, there’s a corresponding mapError(_:) method. Jeff Kelley |
11. Result in Practice map(_:) A common use of map(_:) is to transform data from the network into a model object: let url = URL(string: "https://catfactsapi.cool/api/catfactoftheday")! performRequest(url) { result in // result is a Result<Data, Error> let mappedResult = result.map(CatFact.init) // mappedResult is a Result<CatFact, Error> switch result.map { { case .success(let catFact): // Handle parsed cat fact case .failure(let error): // Handle error } } Jeff Kelley |
12. Error-Handling is Unenforceable. You can lead a developer to error-handling APIs, but you can’t make them use them. Photo by Brett Jordan on Unsplash
13. Result in Actual Practice How many times have you written code like this? let url = URL(string: "https://catfactsapi.cool/api/catfactoftheday")! performRequest(url) { result in switch result { case .success(let data): // Parse data into model object case .failure(let error): // TODO: Handle Error break } } How many times do you forget about that TODO? Jeff Kelley |
14. Result in Actual Practice Or even have the TODO? let url = URL(string: "https://catfactsapi.cool/api/catfactoftheday")! performRequest(url) { result in if case .success(let data) = result { // Handle data } } Jeff Kelley |
15. Objective-C Error Handling in Practice NSError *error; BOOL success = [[NSFileManager defaultManager] removeItemAtPath:@"/tmp/dontneedthisanymore" error:&error]; if (success) { // Proceed with what you were doing } else { // TODO: Handle error. NSLog(@"Error: %@", error.localizedDescription); } I mean, it could be worse. At least we’re logging the error. Jeff Kelley |
16. Objective-C Error Handling in Practice NSError *error; BOOL success = [[NSFileManager defaultManager] removeItemAtPath:@"/tmp/dontneedthisanymore" error:&error]; if (success) { // Proceed with what you were doing } Jeff Kelley |
17. Objective-C Error Handling in Practice BOOL success = [[NSFileManager defaultManager] removeItemAtPath:@"/tmp/dontneedthisanymore" error:NULL]; if (success) { // Proceed with what you were doing } Jeff Kelley |
18. Agenda The History of Error-Handling: Objective-C and Swift The Swift Error Type Testing Errors Error Handling Through Types: Combine and SwiftUI Jeff Kelley |
19. Swift Errors Objective-C Remember the removeItemAtPath:error: method we used in Objective-C? - (BOOL)removeItemAtPath:(NSString *)path error:(NSError * _Nullable *)error; Jeff Kelley |
20. Swift Errors Swift Here’s how that method appears in Swift. Now it’s removeItem(atPath:). func removeItem(atPath path: String) throws Notice that the error parameter is gone—where did it go? Jeff Kelley |
21. Swift Errors If we just call removeItem(atPath:), this won’t compile. FileManager.default .removeItem(atPath: "/tmp/dontneedthisanymore") Jeff Kelley |
22. Swift Errors If we just call removeItem(atPath:), this won’t compile. FileManager.default .removeItem(atPath: "/tmp/dontneedthisanymore") // ! Call can throw but is not marked with 'try' Jeff Kelley |
23. Swift Errors Any method marked throws needs us to try calling it first: do { try FileManager.default .removeItem(atPath: "/tmp/dontneedthisanymore") } catch { print(error.localizedDescription) } There’s our error parameter. Jeff Kelley |
24. The Swift Error Type public protocol Error { } Jeff Kelley |
25. The Swift Error Type Custom Error Types If you know specific error cases you can run into, you can create an enum to represent these states. enum CatFactParsingError: Error { case invalidData case unknown case actuallyADogFact } Jeff Kelley |
26. The Swift Error Type Catching Specific Errors And once you have these cases, you can catch them: do { try parseCatFact() } catch CatFactParsingError { print("Bad data!") } catch { // Gotta catch ‘em all! print("Some other error: \(error.localizedDescription)") } Jeff Kelley |
27. Swift Errors Just like Result, you can simply ignore the errors. try? FileManager.default .removeItem(atPath: "/tmp/dontneedthisanymore") // or try! FileManager.default .removeItem(atPath: "/tmp/dontneedthisanymore") Jeff Kelley |
28. throws You can of course use throws in your own code: func deleteTempoararyData() throws { try FileManager.default .removeItem(atPath: "/tmp/dontneedthisanymore") } Jeff Kelley |
29. throws You can also use this with methods that return values: func retrieveTemporaryData() throws -> String? { let path = "/tmp/dontneedthisanymore" let temporaryData = try String(contentsOfFile: path) try FileManager.default.removeItem(atPath: path) return temporaryData } Jeff Kelley |
30. Advanced throws We use map(_:) all the time without try, thanks to the rethrows keyword: func map<T>( _ transform: (Self.Element) throws -> T ) rethrows -> [T] Jeff Kelley |
31. Advanced throws You can write this yourself, just use try inside of your rethrows method: extension Collection { public func map<T>( _ transform: (Element) throws -> T ) rethrows -> [T] { var result: [T] = [] for item in self { let transformed = try transform(item) result.append(transformed) } return result } } Jeff Kelley |
32. The Error Type The Error protocol has a localizedDescription property: protocol Error { var localizedDescription: String { get } } You can use this property when you need to display the error: CatLog("Error parsing cat fact: \(error.localizedDescription)") Jeff Kelley |
33. Effective Swift Errors Logging You may find yourself writing code like this a lot: healthStore.add(samples, to: workout) { success, error in if let error = error { Log("Error adding samples to workout: " + error.localizedDescription) } } Jeff Kelley |
34. Effective Swift Errors Logging The pattern involves logging the error if there is one with some additional context. if let error = error { Log("Error <some context here>: " + error.localizedDescription) } Jeff Kelley |
35. Effective Swift Errors Logging We can use an extension on Error to wrap this code: extension Error { func log(context: String, filename: String = #file, lineNumber: Int = #line) { Log("Error \(context): \(localizedDescription)", type: .error, filename: filename, lineNumber: lineNumber) } } Jeff Kelley |
36. Effective Swift Errors Logging Here it is in use: healthStore.add(samples, to: workout) { success, error in error?.log(context: "adding samples to workout") } Jeff Kelley |
37. Agenda The History of Error-Handling: Objective-C and Swift The Swift Error Type Testing Errors Error Handling Through Types: Combine and SwiftUI Jeff Kelley |
38. Testing Result The Happy Path Since Result is Equatable when its Success and Failure types are Equatable, we can use XCTAssertEqual(_:_:_:file:line:) class ResultTests: XCTestCase { func testTheHappyPath() { let url = URL(string: "https://catfactsapi.cool/api/catfactoftheday")! let expectation = self.expectation("The request finishes") performRequest(url) { result in XCTAssertEqual(result, .success) expectation.fulfill() } waitForExpectations(timeout: 2) } } Jeff Kelley |
39. Testing Result The Failure Case We can do the same thing for the failure case: class ResultTests: XCTestCase { func testTheFailurePath() { let url = URL(string: "https://catfactsapi.cool/api/dogfactoftheday")! let expectation = self.expectation("The request finishes") performRequest(url) { result in XCTAssertEqual(result, .failure(NetworkError.notFound)) expectation.fulfill() } waitForExpectations(timeout: 2) } Jeff Kelley |
40. fatalError() If you can’t handle an error, sometimes the right thing to do is crash: do { try somethingThatCouldThrow() } catch { fatalError(error.localizedDescription) } Jeff Kelley |
41. Testing fatalError() Jeff Kelley |
42. Testing throws Consider the following code: enum ThrowingError: Error { case passedFalse } func throwIfPassedFalse( _ trueOrFalse: Bool ) throws { guard trueOrFalse else { throw ThrowingError.passedFalse } } Jeff Kelley |
43. Testing throws All we need to do is assert that the error is thrown. func testThrowingError() { XCTAssertThrowsError( try throwIfPassedFalse(false), "Expected method to throw") { error in XCTAssertEqual( error as? ThrowingError, ThrowingError.passedFalse) } } Jeff Kelley |
44. Testing fatalError() Replacing the call to fatalError() If we put this method in our app code, it’ll take precedence over the original fatalError(): func fatalError( _ message: @autoclosure () -> String = "", file: StaticString = #file, line: UInt = #line ) -> Never { Swift.fatalError(message, file: file, line: line) } Jeff Kelley |
45. Testing fatalError() Replacing fatalError() With a Closure func fatalError( _ message: @autoclosure () -> String = "", file: StaticString = #file, line: UInt = #line ) -> Never { FatalErrorUtilities.fatalErrorClosure(message(), file, line) } struct FatalErrorUtilities { typealias FatalErrorClosure = (String, StaticString, UInt) -> Never fileprivate static var fatalErrorClosure = defaultFatalErrorClosure private static let defaultFatalErrorClosure = { (message: String, file: StaticString, line: UInt) -> Never in Swift.fatalError(message, file: file, line: line) } } Jeff Kelley |
46. Testing fatalError() Swapping Out the Implementation extension FatalErrorUtilities { internal static func replaceFatalError( closure: @escaping FatalErrorClosure ) { fatalErrorClosure = closure } internal static func restoreFatalError() { fatalErrorClosure = defaultFatalErrorClosure } } Jeff Kelley |
47. Testing fatalError() Expect the Error extension XCTestCase { func expectFatalError(file: StaticString = #file, line: UInt = #line, testcase: @escaping () -> Void) { let expectation = self.expectation(description: "expecting fatal error") FatalErrorUtilities.replaceFatalError { (_, _, _) -> Never in expectation.fulfill() self.unreachable() } DispatchQueue.global().async(execute: testcase) waitForExpectations(timeout: 2) { _ in FatalErrorUtilities.restoreFatalError() } } func unreachable() -> Never { while true { RunLoop.current.run() } } } Jeff Kelley |
48. Testing fatalError() Putting it All Together Now we can write this test, and it’ll pass without crashing! ✅ func testCrashing() { expectFatalError { fatalErrorIfPassedFalse(false) } } Jeff Kelley |
49. Testing fatalError() Jeff Kelley |
50. When to Use fatalError() One common use of fatalError() is to stop execution if you get into a situation you shouldn’t be in: func tableView( _ tableView: UITableView, cellForRowAt indexPath: IndexPath ) -> UITableViewCell { guard let cell = tableView.dequeueReusableCell( withIdentifier: "cell") as? MyTableViewCell else { fatalError() } return cell } Jeff Kelley |
51. Swift Assertion Methods Assert.swift in the Swift Standard Library Assert Type Debug Release Unchecked fatalError ✅ ✅ precondition ✅ ✅ ❌ assert ✅ ❌ ❌ Jeff Kelley |
52. Swift Assertion Methods assert() and precondition() These are functionally equivalent: if (!someCondition) { assertionFailure() } assert(someCondition) Jeff Kelley |
53. Effective Swift Errors ProgrammerError We can use throws and a custom Error type to wrap the code for dequeueing table view cells: extension UITableView { enum ProgrammerError: Error { case noCellReturned } func dequeue<T: UITableViewCell>( _ type: T.Type, identifier reuseIdentifier: String ) throws -> T { guard let cell = dequeueReusableCell( withIdentifier: reuseIdentifier) as? T else { throw ProgrammerError.noCellReturned } return cell } } Jeff Kelley |
54. Effective Swift Errors ProgrammerError Now, we can use this in our code: func tableView( _ tableView: UITableView, cellForRowAt indexPath: IndexPath ) -> UITableViewCell { do { return try tableView.dequeue( MyTableViewCell.self, identifier: "cell") } catch { fatalError(error.localizedDescription) } } Jeff Kelley |
55. Effective Swift Errors Passing Errors to fatalError() and Friends Let’s write another replacement for fatalError() that takes an Error instead: func fatalError( _ error: Error, file: StaticString = #file, line: UInt = #line ) -> Never { fatalError(error.localizedDescription, file: file, line: line) } Jeff Kelley |
56. Effective Swift Errors Passing Errors to fatalError() and Friends Now we can just pass the error through directly: func tableView( _ tableView: UITableView, cellForRowAt indexPath: IndexPath ) -> UITableViewCell { do { return try tableView.dequeue( MyTableViewCell.self, identifier: "cell") } catch { fatalError(error) } } Jeff Kelley |
57. Effective Swift Errors Testing Specific Error Conditions We can use this in our tests, too: func expectFatalError<T: Error>(expectedError: T, file: StaticString = #file, line: UInt = #line, testcase: @escaping () -> Void) where T: Equatable { let expectation = self.expectation(description: "expecting fatal error") var assertionError: T? = nil FatalErrorUtilities.replaceFatalError { error, _, _ in assertionError = error as? T expectation.fulfill() self.unreachable() } DispatchQueue.global().async(execute: testcase) waitForExpectations(timeout: 2) { _ in XCTAssertEqual(assertionError, expectedError, file: file, line: line) FatalErrorUtilities.restoreFatalError() } } Jeff Kelley |
58. “But This Should Never Happen” __block void (^recursiveBlock)(void) = ^{ NSLog(@"foo"); recursiveBlock(); }; recursiveBlock(); This code crashed on Release builds, not Debug builds. Jeff Kelley |
59. “But This Should Never Happen” if (someObject != NULL && someObject->someValue != 0) { // Do something very impressive } On a compiler without short-circuit evaluation, this crashed when someObject was NULL. Jeff Kelley |
60. Agenda The History of Error-Handling: Objective-C and Swift The Swift Error Type Testing Errors Error Handling Through Types: Combine and SwiftUI Jeff Kelley |
61. Combine Values That Change Over Time public protocol Publisher { /// The kind of values published by this publisher. associatedtype Output /// The kind of errors this publisher might publish. /// /// Use `Never` if this `Publisher` does not publish errors. associatedtype Failure : Error /// This function is called to attach the specified `Subscriber` to this /// `Publisher` by `subscribe(_:)` /// /// - SeeAlso: `subscribe(_:)` /// - Parameters: /// - subscriber: The subscriber to attach to this `Publisher`. /// once attached it can begin to receive values. func receive<S>(subscriber: S) where S : Subscriber, Self.Failure == S.Failure, Self.Output == S.Input } Jeff Kelley |
62. Combine URLSession’s DataTaskPublisher extension URLSession { public struct DataTaskPublisher : Publisher { /// The kind of values published by this publisher. public typealias Output = (data: Data, response: URLResponse) /// The kind of errors this publisher might publish. /// /// Use `Never` if this `Publisher` does not publish errors. public typealias Failure = URLError /// This function is called to attach the specified `Subscriber` to this /// `Publisher` by `subscribe(_:)` /// /// - SeeAlso: `subscribe(_:)` /// - Parameters: /// - subscriber: The subscriber to attach to this `Publisher`. /// once attached it can begin to receive values. public func receive<S>(subscriber: S) where S : Subscriber, S.Failure == URLSession.DataTaskPublisher.Failure, S.Input == URLSession.DataTaskPublisher.Output } } Jeff Kelley |
63. Combine Operators let url = URL(string: "https://batcave.info/enemies/joker")! let publisher = URLSession.shared.dataTaskPublisher(for: url) let enemyPublisher: AnyPublisher<Enemy, Error> = publisher.tryMap { let decoder = JSONDecoder() let enemy = try decoder.decode(Enemy.self, from: $0.data) return enemy } .eraseToAnyPublisher() Jeff Kelley |
64. Combine Publishers The Never Type NotificationCenter .default .publisher(for: UIResponder.keyboardWillShowNotification) .sink { notification in // Process notification } Jeff Kelley |
65. Combine Publishers The Never Type extension NotificationCenter { /// A publisher that emits elements when broadcasting notifications. public struct Publisher : Publisher { /// The kind of values published by this publisher. public typealias Output = Notification /// The kind of errors this publisher might publish. /// /// Use `Never` if this `Publisher` does not publish errors. public typealias Failure = Never } } Jeff Kelley |
66. SwiftUI and Errors public protocol ObservableObject : AnyObject { /// The type of publisher that emits before the object has changed. associatedtype ObjectWillChangePublisher : Publisher = ObservableObjectPublisher where Self.ObjectWillChangePublisher.Failure == Never /// A publisher that emits before the object has changed. var objectWillChange: Self.ObjectWillChangePublisher { get } } Jeff Kelley |
67. SwiftUI and Errors Enforcing Error-Handling Through Types struct CatView: View { @ObservedObject var cat: Cat static let lengthFormatter: LengthFormatter = { let formatter = LengthFormatter() formatter.isForPersonHeightUse = false // it’s a cat return formatter }() var body: some View { VStack { Text(cat.name) Text(cat.breed) Text("\(cat.height, formatter: CatView.lengthFormatter)") } } } Jeff Kelley |
68. SwiftUI and Errors Placeholder Values extension Cat { static var placeholder: Cat { return Cat(name: "Unknown") } } let publisher = catPublisher .catch { _ in Just(.placeholder) } Jeff Kelley |
69. SwiftUI and Errors Showing Alerts struct CatFactView: View { let url = URL(string: "https://catfactsapi.cool/api/catfactoftheday")! @State var fact: String? var body: some View { VStack { Text("Cat Fact of the Day") .font(.title) fact.map { Text($0) } } .onReceive(URLSession.shared.dataTaskPublisher(for: url)) { result in } } } Jeff Kelley |
70. SwiftUI and Errors Showing Alerts struct CatFactView: View { let url = URL(string: "https://catfactsapi.cool/api/catfactoftheday")! @State var fact: String? var body: some View { VStack { Text("Cat Fact of the Day") .font(.title) fact.map { Text($0) } } .onReceive(URLSession.shared.dataTaskPublisher(for: url)) { result in // ! error: instance method 'onReceive(_:perform:)' requires the // types 'URLSession.DataTaskPublisher.Failure' (aka // 'URLError') and 'Never' be equivalent } } } Jeff Kelley |
71. SwiftUI and Errors Showing Alerts struct CatFactView: View { let url = URL(string: "https://catfactsapi.cool/api/catfactoftheday")! @State var fact: String? @State var networkError: URLError? @State var isPresentingError = false var factPublisher: AnyPublisher<String?, Never> { return AnyPublisher( URLSession.shared.dataTaskPublisher(for: url) .map { String(data: $0.0, encoding: .utf8) } .catch { (error: URLError) -> Just<String?> in self.networkError = error self.isPresentingError = true return Just(nil) } ) } var body: some View { VStack { Text("Cat Fact of the Day") .font(.title) fact.map { Text($0) } } .onReceive(factPublisher) { fact in self.fact = fact } .alert(isPresented: $isPresentingError) { Alert(title: Text("An error occurred loading this content."), message: self.networkError.map { Text($0.localizedDescription) }) } } } Jeff Kelley |
72. SwiftUI and Errors Showing Alerts let url = URL(string: "https://catfactsapi.cool/api/catfactoftheday")! var factPublisher: AnyPublisher<String?, Never> { return AnyPublisher( URLSession.shared.dataTaskPublisher(for: url) .map { String(data: $0.0, encoding: .utf8) } .catch { (error: URLError) -> Just<String?> in self.networkError = error self.isPresentingError = true return Just(nil) } ) } Jeff Kelley |
73. SwiftUI and Errors Showing Alerts @State var fact: String? @State var networkError: URLError? @State var isPresentingError = false var body: some View { VStack { Text("Cat Fact of the Day") .font(.title) fact.map { Text($0) } } .onReceive(factPublisher) { fact in self.fact = fact } .alert(isPresented: $isPresentingError) { Alert(title: Text("An error occurred loading this content."), message: self.networkError.map { Text($0.localizedDescription) }) } } Jeff Kelley |
74. Agenda The History of Error-Handling: Objective-C and Swift The Swift Error Type Testing Errors Error Handling Through Types: Combine and SwiftUI Jeff Kelley |
75. Questions? Contact info: Jeff Kelley @SlaunchaMan SlaunchaMan@gmail.com