Me and Swift and error

Presented at “Swift 2 (& LLDB) Symposium”

1. ૪ ࠴
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3. cockscomb
5. ࡻ ࡨ ࡥ ࢗ ࢊ
6. Swift 2
7. ࡪ ࢫ ߭
8. Error Handling
9. • Error Handling Basics • guard, defer • Closure • Asynchronous callback • Exhaustive
10. Error Handling Basics
11. enum MyError: Int { static let domain = "MyErrorDomain" case DangerousError = -999 } func awesome(error: NSErrorPointer) -> String? { error.memory = NSError(domain: MyError.domain, code: MyError.DangerousError.rawValue, userInfo: nil) return nil } func test() { var error: NSError? let result = awesome(&error) if let r = result { print(r) } if let e = error { print(e) } } test() Swift 1.x
12. enum MyError: ErrorType { case DangerousError } func awesome() throws -> String { if fail { throw MyError.DangerousError } return "awesome" } func test() { do { let result = try awesome() print(result) } catch { print("Error") } } test() Using ErrorType
13. enum MyError: ErrorType { case DangerousError(String) } func awesome() throws -> String { if fail { throw MyError.DangerousError("Too dangerous") } return "awesome" } func test() { do { let result = try awesome() print(result) } catch MyError.DangerousError(let message) { print(message) } catch { print("Error") } } test() Error information
14. enum MyError: ErrorType { case DangerousError } func awesome() throws -> String { if fail { throw MyError.DangerousError } return "awesome" } func test() throws { let result = try awesome() print(result) } do { try test() } catch { print("Error") } Propagate error
15. enum MyError: ErrorType { case DangerousError } func awesome() throws -> String { if fail { throw MyError.DangerousError } return "awesome" } func test() { let result = try! awesome() print(result) } test() Forced-try
16. Error Handling Basics • Before Swift 2, NSErrorPointer • ErrorType • throw, try, catch • Propagation
17. guard, defer
18. struct Dog { enum Error: ErrorType { case Sleeping } enum State { case Sitting case Sleeping case Walking } var state: State = .Sitting mutating func walk() throws { guard state != .Sleeping else { throw Error.Sleeping } state = .Walking } } guard
19. struct Human { enum State { case Standing case Walking } var state: State = .Standing mutating func walk(inout dog: Dog) throws { defer { dog.praise() } try dog.walk() state = .Walking } } defer
20. Closure
21. enum MyError: ErrorType { case DangerousError } func execute(closure: () throws -> Void) throws { try closure() } do { try execute({ throw MyError.DangerousError }) } catch { print("Error") } Closures can throw
22. enum MyError: ErrorType { case DangerousError } func execute(closure: () throws -> Void) throws { try closure() } do { try execute({ print("No error") }) } catch { print("Error") } Assumed no error but catch
23. enum MyError: ErrorType { case DangerousError } func execute(closure: () throws -> Void) rethrows { try closure() } execute({ print("No error") }) Assumed no error
24. Closure • try closure • rethrows
25. Asynchronous callback
26. enum MyError: ErrorType { case DangerousError } func doAsync(callback: (result: String?, error: ErrorType?) -> Void) { dispatch_async(dispatch_get_main_queue()) { callback(result: nil, error: MyError.DangerousError) } } doAsync { (result, error) in if let result = result { print(result) } else if let error = error { print("Error") } } Asynchronous callback
27. enum MyError: ErrorType { case DangerousError } func doAsync(callback: (() throws -> String) -> Void) { dispatch_async(dispatch_get_main_queue()) { callback({ throw MyError.DangerousError }) } } doAsync { (f) in do { let result = try f() } catch { print("Error") } } Asynchronous callback
28. enum ValueOrError<T> { case Value(T) case Error(ErrorType) init(_ value: T) { self = Value(value) } init(_ error: ErrorType) { self = Error(error) } func get() throws -> T { switch self { case .Value(let value): return value case .Error(let error): throw error } } } Value or Error
29. enum MyError: ErrorType { case DangerousError } func doAsync(callback: (ValueOrError<String>) -> Void) { dispatch_async(dispatch_get_main_queue()) { callback(ValueOrError(MyError.DangerousError)) } } doAsync { (v) in do { let result = try v.get() } catch { print("Error") } } Asynchronous callback
30. Asynchronous callback • Optional, Optional • f: () throws -> Result • ValueOrError value
31. func doAsync(callback : String catches -> Void) { if (success) { callback("hooray!") } else { callback(throw MyError.Failed) } } doAsync { result in // handle result } catch { // Catch async thrown error } Suggested in developer forums https://forums.developer.apple.com/thread/4354 Unreal
32. Exhaustive
33. –The Swift Programming Language “Like a switch statement, the compiler attempts to infer whether catch clauses are exhaustive. If such a determination can be made, the error is considered handled.”
34. enum MyError: ErrorType { case DangerousError } func doSomething() throws { throw MyError.DangerousError } func test() { do { try doSomething() } catch MyError.DangerousError { print("Error") } }
35. Errors thrown from here are not handled b ! enum MyError: ErrorType { case DangerousError } func doSomething() throws { throw MyError.DangerousError } func test() { do { try doSomething() } catch MyError.DangerousError { print("Error") } } Not exhaustive
36. Errors thrown from here are not handled because the enclosing catch is not exhaustive. !
37. Errors thrown from here are not handled b ! enum MyError: ErrorType { case DangerousError } func doSomething() throws { throw MyError.DangerousError } func test() { do { try doSomething() } catch MyError.DangerousError { print("Error") } } Not exhaustive
38. • Error Handling Basics • guard, defer • Closure • Asynchronous callback • Exhaustive
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