Error handling made easy

UX doesn't only come down to looks and speed. Error handling is quite as important, and in order to get it right it has to be easy and straightforward. However, for most, it is still a mundane task with painfully too many cases to consider. In this talk we propose a recipe for reducing this friction and for adding complex error handling with just a few lines of code.

Presented in try!Swift New York in 2017
https://academy.realm.io/posts/try-swift-nyc-2017-eleni-papanikolopoulou-kostas-kremizas-error-handling/?fbclid=IwAR2ZrdxgN6gcPA3vNya7wOOYOgiM7Z4XtbtXJEgfuL4LE60hJAqn7LltQUg

Presented in #Pragma Conference in Verona in 2017
https://www.youtube.com/watch?v=yiE1R0qd1Gg

Transcript

1. Eleni Papanikolopoulou iOS Developer @Workable @elenipapanikolo ERROR HANDLING MADE EASY Kostas Kremizas iOS Developer @Workable @kremizask
2. Definitions
3. Error = “a value used to report that an error condition occurred and normal functionality was skipped” Definitions – Matt Gallagher, Cocoa With Love
4. Error Handling = “code that looks for errors and performs different actions based on the presence of those errors” Error = “a value used to report that an error condition occurred and normal functionality was skipped” Definitions – Matt Gallagher, Cocoa With Love
5. – Swift Apprentice, Chapter 22 (Error Handling) “Error handling is the art of failing gracefully”
6. Why bother?
7. Why bother? More and more critical actions in apps
8. Why bother? More and more critical actions in apps Apps often rely on unreliable sources
9. Why bother? More and more critical actions in apps Apps often rely on unreliable sources Bad error handling => users don't TRUST our app
11. Why so cryptic? It’s just copywriting and easy to fix
12. Why so cryptic? It’s just copywriting and easy to fix Right?
14. Steps
15. Steps For each method that can error out:
16. Steps 1. List possible errors For each method that can error out:
17. Steps 1. List possible errors 2. Handle each error For each method that can error out:
18. 1. List possible errors
19. Technical • Network • Disk • Server unavailability 1. List possible errors
20. Technical • Network • Disk • Server unavailability 1. List possible errors • Authorization • Validation • Stale or bad data Business
21. 2. Handle each error
22. 2. Handle each error Present a relevant error message
23. 2. Handle each error Present a relevant error message Additional actions per case
24. 2. Handle each error Present a relevant error message Additional actions per case Log & report the error (always)
25. – Jan L. A. van de Snepscheut “In theory there is no difference between theory and practice. 
 In practice there is.”
26. Email client app Unlucky Bob
27. Adding error handling to a simple login screen
28. Adding error handling to a simple login screen unlucky.bob@gmail.com **********
29. Adding error handling to a simple login screen unlucky.bob@gmail.com **********
30. @IBAction func loginTapped(_ sender: Any) { api.login(email, password) { (response, error) in } }
31. @IBAction func loginTapped(_ sender: Any) { api.login(email, password) { (response, error) in } } if let error = error { } switch error { } case let error as HttpError where error.status == 401: showAlert(message: "Incorrect email or password")
32. @IBAction func loginTapped(_ sender: Any) { api.login(email, password) { (response, error) in } } if let error = error { } switch error { } case let error as HttpError where error.status == 401: showAlert(message: "Incorrect email or password")
33. @IBAction func loginTapped(_ sender: Any) { api.login(email, password) { (response, error) in } } if let error = error { } switch error { } case let error as HttpError where error.status == 401: showAlert(message: "Incorrect email or password") case let error as NSError where error.domain == NSURLErrorDomain && error.code == NSURLErrorNotConnectedToInternet: showWarning(“You appear to be offline. Please check your connection.")
34. @IBAction func loginTapped(_ sender: Any) { api.login(email, password) { (response, error) in } } if let error = error { } switch error { } case let error as HttpError where error.status == 401: showAlert(message: "Incorrect email or password") case let error as NSError where error.domain == NSURLErrorDomain && error.code == NSURLErrorNotConnectedToInternet: showWarning(“You appear to be offline. Please check your connection.")
35. @IBAction func loginTapped(_ sender: Any) { api.login(email, password) { (response, error) in } } if let error = error { } switch error { } case let error as HttpError where error.status == 401: showAlert(message: "Incorrect email or password") case let error as NSError where error.domain == NSURLErrorDomain && error.code == NSURLErrorNotConnectedToInternet: showWarning(“You appear to be offline. Please check your connection.") case let error as SignInError where error == .invalidEmail: showInvalidEmail()
36. @IBAction func loginTapped(_ sender: Any) { api.login(email, password) { (response, error) in } } if let error = error { } switch error { } case let error as HttpError where error.status == 401: showAlert(message: "Incorrect email or password") case let error as NSError where error.domain == NSURLErrorDomain && error.code == NSURLErrorNotConnectedToInternet: showWarning(“You appear to be offline. Please check your connection.") case let error as SignInError where error == .invalidEmail: showInvalidEmail()
37. @IBAction func loginTapped(_ sender: Any) { api.login(email, password) { (response, error) in } } if let error = error { } switch error { } case let error as HttpError where error.status == 401: showAlert(message: "Incorrect email or password") case let error as NSError where error.domain == NSURLErrorDomain && error.code == NSURLErrorNotConnectedToInternet: showWarning(“You appear to be offline. Please check your connection.") case let error as SignInError where error == .invalidEmail: showInvalidEmail() default: showError(message: "Sorry, there was a problem. Please try again")
38. @IBAction func loginTapped(_ sender: Any) { api.login(email, password) { (response, error) in } } if let error = error { } switch error { } case let error as HttpError where error.status == 401: showAlert(message: "Incorrect email or password") case let error as NSError where error.domain == NSURLErrorDomain && error.code == NSURLErrorNotConnectedToInternet: showWarning(“You appear to be offline. Please check your connection.") case let error as SignInError where error == .invalidEmail: showInvalidEmail() default: showError(message: "Sorry, there was a problem. Please try again") Logger.log(error)
39. Add error handling to a Compose email screen
40. Add error handling to a Compose email screen natalie.sung@gmail.com Issue Dear Natalie, I would like to ask you about a recent purchase of your product. I've having issues with... Thanks, Bob
41. Add error handling to a Compose email screen natalie.sung@gmail.com Issue Dear Natalie, I would like to ask you about a recent purchase of your product. I've having issues with... Thanks, Bob
42. @IBAction func sendTapped(_ sender: Any) { api.send(emailData) { (response, error) in } } if let error = error { } switch error { } case let error as HttpError where error.status == 401: showSignInScreen()
43. @IBAction func sendTapped(_ sender: Any) { api.send(emailData) { (response, error) in } } if let error = error { } switch error { } case let error as HttpError where error.status == 401: showSignInScreen()
44. case let error as NSError where error.domain == NSURLErrorDomain && error.code == NSURLErrorNotConnectedToInternet: showWarning(“You appear to be offline. Please check your connection.") @IBAction func sendTapped(_ sender: Any) { api.send(emailData) { (response, error) in } } if let error = error { } switch error { } case let error as HttpError where error.status == 401: showSignInScreen()
45. case let error as NSError where error.domain == NSURLErrorDomain && error.code == NSURLErrorNotConnectedToInternet: showWarning(“You appear to be offline. Please check your connection.") @IBAction func sendTapped(_ sender: Any) { api.send(emailData) { (response, error) in } } if let error = error { } switch error { } case let error as HttpError where error.status == 401: showSignInScreen()
46. case let error as NSError where error.domain == NSURLErrorDomain && error.code == NSURLErrorNotConnectedToInternet: showWarning(“You appear to be offline. Please check your connection.") @IBAction func sendTapped(_ sender: Any) { api.send(emailData) { (response, error) in } } if let error = error { } switch error { } case let error as HttpError where error.status == 401: showSignInScreen() default: showError(message: "Sorry, there was a problem. Please try again")
47. case let error as NSError where error.domain == NSURLErrorDomain && error.code == NSURLErrorNotConnectedToInternet: showWarning(“You appear to be offline. Please check your connection.") @IBAction func sendTapped(_ sender: Any) { api.send(emailData) { (response, error) in } } if let error = error { } switch error { } case let error as HttpError where error.status == 401: showSignInScreen() default: showError(message: "Sorry, there was a problem. Please try again")
48. case let error as NSError where error.domain == NSURLErrorDomain && error.code == NSURLErrorNotConnectedToInternet: showWarning(“You appear to be offline. Please check your connection.") @IBAction func sendTapped(_ sender: Any) { api.send(emailData) { (response, error) in } } if let error = error { } switch error { } case let error as HttpError where error.status == 401: showSignInScreen() default: showError(message: "Sorry, there was a problem. Please try again") showMessage("Don't worry, a draft has been saved.")
49. case let error as NSError where error.domain == NSURLErrorDomain && error.code == NSURLErrorNotConnectedToInternet: showWarning(“You appear to be offline. Please check your connection.") @IBAction func sendTapped(_ sender: Any) { api.send(emailData) { (response, error) in } } if let error = error { } switch error { } case let error as HttpError where error.status == 401: showSignInScreen() default: showError(message: "Sorry, there was a problem. Please try again") showMessage("Don't worry, a draft has been saved.") Logger.log(error)
50. case let error as NSError where error.domain == NSURLErrorDomain && error.code == NSURLErrorNotConnectedToInternet: showWarning(“You appear to be offline. Please check your connection.") @IBAction func sendTapped(_ sender: Any) { api.send(emailData) { (response, error) in } } if let error = error { } switch error { } case let error as HttpError where error.status == 401: showSignInScreen() default: showError(message: "Sorry, there was a problem. Please try again") showMessage("Don't worry, a draft has been saved.") Logger.log(error) common
51. case let error as NSError where error.domain == NSURLErrorDomain && error.code == NSURLErrorNotConnectedToInternet: showWarning(“You appear to be offline. Please check your connection.") @IBAction func sendTapped(_ sender: Any) { api.send(emailData) { (response, error) in } } if let error = error { } switch error { } case let error as HttpError where error.status == 401: showSignInScreen() default: showError(message: "Sorry, there was a problem. Please try again") showMessage("Don't worry, a draft has been saved.") Logger.log(error) common with a twist
52. case let error as NSError where error.domain == NSURLErrorDomain && error.code == NSURLErrorNotConnectedToInternet: showWarning(“You appear to be offline. Please check your connection.") @IBAction func sendTapped(_ sender: Any) { api.send(emailData) { (response, error) in } } if let error = error { } switch error { } case let error as HttpError where error.status == 401: showSignInScreen() default: showError(message: "Sorry, there was a problem. Please try again") showMessage("Don't worry, a draft has been saved.") Logger.log(error) common with a twist NEW
58. We end up with…
59. We end up with… Boilerplate
60. We end up with… Boilerplate Repetition
61. We end up with… Boilerplate Repetition No standard way of handling
62. We end up with… Boilerplate Repetition No standard way of handling Cognitive overhead
63. Bad practices
64. Bad practices Skip cases and use generic error messages
65. Bad practices Skip cases and use generic error messages Don’t abstract and leave duplicate code everywhere
66. Bad practices Skip cases and use generic error messages Don’t abstract and leave duplicate code everywhere Handle errors at the network layer !
67. What would 
 we want ideally?
69. Make error handling as easy as ordering a pizza
70. tomato sauce tomato basil dough ham buffalo mozzarella olives
71. “Can I have a pizza with dough, buffalo mozzarella, tomato, tomato sauce, basil, olives and ham please?” - noone ever
72. Instead we say… “The usual…” or “Capricciosa pizza with extra olives without the mushrooms”
73. What we actually want
74. A set of default actions for common errors What we actually want
75. A set of default actions for common errors An easy way to customise these defaults Add new cases Override existing ones Add actions for unknown errors Add actions for all errors (logging) What we actually want
76. ErrorHandler A library that provides a declarative fluent API for flexible error handling
77. ErrorHandler Basic API
78. ErrorHandler public func on(matches: Error -> Bool, do action: @escaping ErrorAction) -> ErrorHandler Basic API
79. ErrorHandler public func on(matches: Error -> Bool, do action: @escaping ErrorAction) -> ErrorHandler public func always(do action: @escaping ErrorAction) -> ErrorHandler Basic API
80. ErrorHandler public func on(matches: Error -> Bool, do action: @escaping ErrorAction) -> ErrorHandler public func always(do action: @escaping ErrorAction) -> ErrorHandler public func onNoMatch(do action: @escaping ErrorAction) -> ErrorHandler Basic API
81. ErrorHandler public func on(matches: Error -> Bool, do action: @escaping ErrorAction) -> ErrorHandler public func handle(_ error: Error) public func always(do action: @escaping ErrorAction) -> ErrorHandler public func onNoMatch(do action: @escaping ErrorAction) -> ErrorHandler Basic API
82. public func on(matches: Error -> Bool, do action: @escaping ErrorAction) -> ErrorHandler public func handle(_ error: Error) public func always(do action: @escaping ErrorAction) -> ErrorHandler public func on(error: Error & Equatable, do action: @escaping ErrorAction) -> ErrorHandler Basic API public func onNoMatch(do action: @escaping ErrorAction) -> ErrorHandler
83. ErrorHandler() .on(error1, do: actionA) .on(error2, do: actionB) .onNoMatch(do: actionC) .always(do: log) .handle(error)
84. Our strategy
85. Our strategy Setup a default ErrorHandler once
86. Our strategy Setup a default ErrorHandler once Customize the default ErrorHandler when needed based on the context
87. In many cases all we will need is… if let error = error { return }
88. In many cases all we will need is… ErrorHandler.defaultHandler.handle(error) if let error = error { return }
90. Back to our example
91. extension ErrorHandler { class var defaultHandler: ErrorHandler { } }
92. extension ErrorHandler { class var defaultHandler: ErrorHandler { } }
93. extension ErrorHandler { class var defaultHandler: ErrorHandler { } } return ErrorHandler()
94. extension ErrorHandler { class var defaultHandler: ErrorHandler { } } return ErrorHandler() .on(NSError(domain:NSURLErrorDomain, code: NSURLErrorNotConnectedToInternet), do: { (error) -> MatchingPolicy in showWarning(“You appear to be offline. Please check your connection.") return .continueMatching })
95. .on(httpStatus: 401, do: { (error) -> MatchingPolicy in showSignInScreen() return .continueMatching }) extension ErrorHandler { class var defaultHandler: ErrorHandler { } } return ErrorHandler() .on(NSError(domain:NSURLErrorDomain, code: NSURLErrorNotConnectedToInternet), do: { (error) -> MatchingPolicy in showWarning(“You appear to be offline. Please check your connection.") return .continueMatching })
96. .always(do: { (error) -> MatchingPolicy in Logger.log(error) return .continueMatching }) .on(httpStatus: 401, do: { (error) -> MatchingPolicy in showSignInScreen() return .continueMatching }) extension ErrorHandler { class var defaultHandler: ErrorHandler { } } return ErrorHandler() .on(NSError(domain:NSURLErrorDomain, code: NSURLErrorNotConnectedToInternet), do: { (error) -> MatchingPolicy in showWarning(“You appear to be offline. Please check your connection.") return .continueMatching })
97. .always(do: { (error) -> MatchingPolicy in Logger.log(error) return .continueMatching }) .on(httpStatus: 401, do: { (error) -> MatchingPolicy in showSignInScreen() return .continueMatching }) extension ErrorHandler { class var defaultHandler: ErrorHandler { } } return ErrorHandler() .on(NSError(domain:NSURLErrorDomain, code: NSURLErrorNotConnectedToInternet), do: { (error) -> MatchingPolicy in showWarning(“You appear to be offline. Please check your connection.") return .continueMatching }) .onNoMatch(do: { (error) -> MatchingPolicy in showError(message: "Sorry, there was a problem. Please try again") return .continueMatching })
99. @IBAction func loginTapped(_ sender: Any) { api.login(email, password) { (response, error) in if let error = error { } } }
100. @IBAction func loginTapped(_ sender: Any) { api.login(email, password) { (response, error) in if let error = error { } } } ErrorHandler.defaultHandler .on(httpStatus: 401, do: { (_) -> MatchingPolicy in showAlert(message: "Incorrect email or password") return .stopMatching }) .on(SignInError.invalidEmail, do: { [weak self] (_) -> MatchingPolicy in self?.showInvalidEmail() return .stopMatching }) .handle(error)
102. @IBAction func sendTapped(_ sender: Any) { api.send(emailData) { (response, error) in if let error = error { } } }
103. ErrorHandler.defaultHandler .always(do: { (error) -> MatchingPolicy in self.showMessage("Don't worry, a draft has been saved.") return .continueMatching }) .handle(error) @IBAction func sendTapped(_ sender: Any) { api.send(emailData) { (response, error) in if let error = error { } } }
104. Boilerplate Repetition No standard way of handling Cognitive overhead We get to..
105. Avoid the boilerplate Repetition No standard way of handling Cognitive overhead We get to..
106. Avoid the repetition Avoid the boilerplate No standard way of handling Cognitive overhead We get to..
107. Avoid the repetition Have a standard way of handling Avoid the boilerplate Cognitive overhead We get to..
108. Eliminate cognitive overhead Avoid the repetition Have a standard way of handling Avoid the boilerplate We get to..
110. Additional features
111. Additional features Error Matchers
112. public protocol ErrorMatcher { func matches(_ error: Error) -> Bool } ErrorMatcher handler .on(matcher) { (error) -> MatchingPolicy in showAlert(message: "It's a match!") return .stopMatching }.handle(error)
113. ErrorMatcher ErrorMatcher && ||
114. ErrorMatcher ErrorMatcher && || let notConnectedMatcher = NSErrorMatcher(domain: NSURLErrorDomain, code: NSURLErrorNotConnectedToInternet) let connectionLostMatcher = NSErrorMatcher(domain: NSURLErrorDomain, code: NSURLErrorNetworkConnectionLost)
115. ErrorMatcher ErrorMatcher && || let notConnectedMatcher = NSErrorMatcher(domain: NSURLErrorDomain, code: NSURLErrorNotConnectedToInternet) let connectionLostMatcher = NSErrorMatcher(domain: NSURLErrorDomain, code: NSURLErrorNetworkConnectionLost) let offlineMatcher = notConnectedMatcher || connectionLostMatcher
116. Additional features Error Matchers Tags
117. // At the setup point of our default handler
 return ErrorHandler() // other setup code .tag(notConnectedMatcher, with: "offline") .tag(connectionLostMatcher, with: "offline") // At the point where we handle an error e.g. any controller ErrorHandler() .on(tag: "offline", do: { (error) -> MatchingPolicy in showError("You appear to be offline. Please check you connection.) return .continueMatching }).handle(error) Tag
118. Additional features Error Matchers Tags Extension for http status error handling 
 (Alamofire out of the box)
119. Conclusions Error handling is an integral part of a good UX It can get cumbersome if you want to do it right You can minimize the friction with the right abstractions
120. DESIGN FOR ERRORS
121. Questions? Even if there are many error cases, it’s nothing you can’t .handle() https://github.com/Workable/swift-error-handler.git https://github.com/Workable/java-error-handler.git