Custom Error Handling on iOS

This is a talk about the approach we take at HRS to handle errors globally within our app by taking advantage of the responder chain and the very powerful but rarely known `NSErrorRecoveryAttempting` protocol. I talk about our implementation of this protocol, how we present errors, the path an error takes through our app, and how error handling can be much less painful than it usually is.

Transcript

2. Custom error handling on iOS Michael Ochs, Mobile Developer, HRS
3. Agenda • Responsibility • NSError / NSErrorRecoveryAttempting • Presentation • UIResponder • What’s next?
4. Error handling before
5. How not to do it
6. How not to do it NSError *error = nil;
 BOOL success = [self.data writeToFile:@"my/path"
 options:NSDataWritingAtomic
 error:&error];
 if (!success) {
 // TODO: Implement error handling
 }

7. How not to do it NSError *error = nil;
 BOOL success = [self.data writeToFile:@"my/path"
 options:NSDataWritingAtomic
 error:&error];
 if (!success) {
 // TODO: Implement error handling
 }

8. How not to do it • Implemented in UIViewController subclasses • Custom in each controller • Various error messages for the same error • The same error message for various errors
9. Responsibility
10. Responsibility • Globally in the app • Usable by every potential error receiver • Easy to use • Little setup effort • Error recovery
11. Responsibility ! • what went wrong • does a retry make sense • how to recover ! • what action led to the error • is a retry possible • how to retry Creator of the error knows: Consumer of the error knows:
12. Responsibility Trigger action Create error Present error Recover from error Retry action
13. Responsibility Trigger action Consumer Creator Create error Present error Recover from error Retry action
14. Responsibility Trigger action Create error Present error Recover from error Retry action Controller Data Application
15. NSError
16. NSError • NSLocalizedDescriptionKey • NSLocalizedFailureReasonErrorKey • NSLocalizedRecoverySuggestionErrorKey • NSLocalizedRecoveryOptionsErrorKey • NSRecoveryAttempterErrorKey
17. NSError The corresponding value is a localized string representation of the error that, if present, will be returned by localizedDescription. NSLocalizedDescriptionKey
18. NSError The corresponding value is a localized string representation containing the reason for the failure that, if present, will be returned by localizedFailureReason. This string provides a more detailed explanation of the error than the description. NSLocalizedFailureReasonErrorKey
19. NSError The corresponding value is a string containing the localized recovery suggestion for the error. This string is suitable for displaying as the secondary message in an alert panel. NSLocalizedRecoverySuggestionErrorKey
20. NSError The corresponding value is an array containing the localized titles of buttons appropriate for displaying in an alert panel. The first string is the title of the right-most and default button, the second the one to the left, and so on. The recovery options should be appropriate for the recovery suggestion returned by localizedRecoverySuggestion. NSLocalizedRecoveryOptionsErrorKey
21. NSError The corresponding value is an object that conforms to the NSErrorRecoveryAttempting informal protocol. The recovery attempter must be an object that can correctly interpret an index into the array returned by localizedRecoveryOptions. NSRecoveryAttempterErrorKey
22. NSError • NSLocalizedDescriptionKey • NSLocalizedFailureReasonErrorKey • NSLocalizedRecoverySuggestionErrorKey • NSLocalizedRecoveryOptionsErrorKey • NSRecoveryAttempterErrorKey
23. NSError - (BOOL)doSaveOperation:(NSError **)error {
 
 NSError *error = [NSError errorWithDomain:MyErrorDomain
 code:MyErrorCode
 userInfo:userInfo];
 *error = error;
 
 return NO;
 } Error creation
24. NSError NSDictionary *userInfo = @{
 NSLocalizedFailureReasonErrorKey:
 @"Something went wrong",
 
 NSLocalizedRecoverySuggestionErrorKey:
 @"Not successful. Try again!",
 
 NSLocalizedRecoveryOptionsErrorKey:
 @[ @"Cancel", @"Retry" ],
 
 NSRecoveryAttempterErrorKey:
 recoveryAttempter
 }; Error creation
25. NSError NSDictionary *userInfo = @{
 NSLocalizedFailureReasonErrorKey:
 @"Something went wrong",
 
 NSLocalizedRecoverySuggestionErrorKey:
 @"Not successful. Try again!",
 
 NSLocalizedRecoveryOptionsErrorKey:
 [recoveryAttempter localizedRecoveryOptions],
 
 NSRecoveryAttempterErrorKey:
 recoveryAttempter
 }; Error creation
26. NSErrorRecoveryAttempting
27. NSErrorRecoveryAttempting • Informal protocol on NSObject • Integrated with NSError • Handles error recovery
28. NSErrorRecoveryAttempting - (void)attemptRecoveryFromError:(NSError *)error
 optionIndex:(NSUInteger)recoveryOptionIndex
 delegate:(id)delegate
 didRecoverSelector:(SEL)didRecoverSelector
 contextInfo:(void *)contextInfo;
 
 - (BOOL)attemptRecoveryFromError:(NSError *)error
 optionIndex:(NSUInteger)recoveryOptionIndex;
29. NSErrorRecoveryAttempting - (void)attemptRecoveryFromError:(NSError *)error
 optionIndex:(NSUInteger)recoveryOptionIndex
 delegate:(id)delegate
 didRecoverSelector:(SEL)didRecoverSelector
 contextInfo:(void *)contextInfo;
 
 - (BOOL)attemptRecoveryFromError:(NSError *)error
 optionIndex:(NSUInteger)recoveryOptionIndex;
30. NSErrorRecoveryAttempting - (void)attemptRecoveryFromError:(NSError *)error
 optionIndex:(NSUInteger)recoveryOptionIndex
 delegate:(id)delegate
 didRecoverSelector:(SEL)didRecoverSelector
 contextInfo:(void *)contextInfo;
 
 - (BOOL)attemptRecoveryFromError:(NSError *)error
 optionIndex:(NSUInteger)recoveryOptionIndex;
31. NSErrorRecoveryAttempting HRSErrorRecoveryAttempter
32. NSErrorRecoveryAttempting - (void)addRecoveryOptionWithTitle:(NSString *)title
 recoveryAttempt:(BOOL(^)())recoveryBlock;
 
 - (NSArray *)localizedRecoveryOptions; HRSErrorRecoveryAttempter
33. NSErrorRecoveryAttempting - (void)addRecoveryOptionWithTitle:(NSString *)title
 recoveryAttempt:(BOOL(^)())recoveryBlock;
 
 - (NSArray *)localizedRecoveryOptions; HRSErrorRecoveryAttempter
34. NSErrorRecoveryAttempting - (void)addRecoveryOptionWithTitle:(NSString *)title
 recoveryAttempt:(BOOL(^)())recoveryBlock;
 
 - (NSArray *)localizedRecoveryOptions; HRSErrorRecoveryAttempter
35. NSErrorRecoveryAttempting HRSErrorRecoveryAttempter *recoveryAttempter =
 [HRSErrorRecoveryAttempter new];
 
 [recoveryAttempter addCancelRecoveryOption];
 
 [recoveryAttempter addRecoveryOptionWithTitle:@"Retry"
 recoveryAttempt:^{
 return YES;
 }]; HRSErrorRecoveryAttempter
36. NSErrorRecoveryAttempting HRSErrorRecoveryAttempter *recoveryAttempter =
 [HRSErrorRecoveryAttempter new];
 
 [recoveryAttempter addCancelRecoveryOption];
 
 [recoveryAttempter addRecoveryOptionWithTitle:@"Retry"
 recoveryAttempt:^{
 return YES;
 }]; HRSErrorRecoveryAttempter
37. NSErrorRecoveryAttempting HRSErrorRecoveryAttempter *recoveryAttempter =
 [HRSErrorRecoveryAttempter new];
 
 [recoveryAttempter addCancelRecoveryOption];
 
 [recoveryAttempter addRecoveryOptionWithTitle:@"Retry"
 recoveryAttempt:^{
 return YES;
 }]; HRSErrorRecoveryAttempter
38. NSErrorRecoveryAttempting HRSErrorRecoveryAttempter *recoveryAttempter =
 [HRSErrorRecoveryAttempter new];
 
 [recoveryAttempter addCancelRecoveryOption];
 
 [recoveryAttempter addRecoveryOptionWithTitle:@"Retry"
 recoveryAttempt:^{
 return YES;
 }]; HRSErrorRecoveryAttempter
39. Presentation
40. Presentation @interface NSResponder (NSErrorPresentation)
 
 - (BOOL)presentError:(NSError *)anError;
 
 - (void)presentError:(NSError *)error
 modalForWindow:(NSWindow *)aWindow
 delegate:(id)delegate
 didPresentSelector:(SEL)didPresentSelector
 contextInfo:(void *)contextInfo;
 
 - (NSError *)willPresentError:(NSError *)anError;
 
 @end OS X
41. Presentation @interface NSResponder (NSErrorPresentation)
 
 - (BOOL)presentError:(NSError *)anError;
 
 - (void)presentError:(NSError *)error
 modalForWindow:(NSWindow *)aWindow
 delegate:(id)delegate
 didPresentSelector:(SEL)didPresentSelector
 contextInfo:(void *)contextInfo;
 
 - (NSError *)willPresentError:(NSError *)anError;
 
 @end OS X
42. Presentation @interface NSResponder (NSErrorPresentation)
 
 - (BOOL)presentError:(NSError *)anError;
 
 - (void)presentError:(NSError *)error
 modalForWindow:(NSWindow *)aWindow
 delegate:(id)delegate
 didPresentSelector:(SEL)didPresentSelector
 contextInfo:(void *)contextInfo;
 
 - (NSError *)willPresentError:(NSError *)anError;
 
 @end OS X
43. Presentation @interface NSResponder (NSErrorPresentation)
 
 - (BOOL)presentError:(NSError *)anError;
 
 - (void)presentError:(NSError *)error
 modalForWindow:(NSWindow *)aWindow
 delegate:(id)delegate
 didPresentSelector:(SEL)didPresentSelector
 contextInfo:(void *)contextInfo;
 
 - (NSError *)willPresentError:(NSError *)anError;
 
 @end OS X
44. Presentation @interface UIResponder (HRSCustomErrorPresentation)
 
 - (void)presentError:(NSError *)anError
 completionHandler:(void (^)(BOOL didRecover))completionHandler;
 
 - (NSError *)willPresentError:(NSError *)anError;
 
 @end iOS
45. Presentation @interface UIResponder (HRSCustomErrorPresentation)
 
 - (void)presentError:(NSError *)anError
 completionHandler:(void (^)(BOOL didRecover))completionHandler;
 
 - (NSError *)willPresentError:(NSError *)anError;
 
 @end iOS
46. Presentation @interface UIResponder (HRSCustomErrorPresentation)
 
 - (void)presentError:(NSError *)anError
 completionHandler:(void (^)(BOOL didRecover))completionHandler;
 
 - (NSError *)willPresentError:(NSError *)anError;
 
 @end iOS
47. Presentation - (IBAction)save:(id)sender {
 NSError *error = nil;
 BOOL success = [self.data writeToFile:@"my/path"
 options:NSDataWritingAtomic
 error:&error];
 if (!success) {
 [self presentError:error
 completionHandler:^(BOOL didRecover) {
 if (didRecover) {
 [self save:sender];
 }
 }];
 }
 } iOS
48. Presentation - (IBAction)save:(id)sender {
 NSError *error = nil;
 BOOL success = [self.data writeToFile:@"my/path"
 options:NSDataWritingAtomic
 error:&error];
 if (!success) {
 [self presentError:error
 completionHandler:^(BOOL didRecover) {
 if (didRecover) {
 [self save:sender];
 }
 }];
 }
 } iOS
49. Presentation - (IBAction)save:(id)sender {
 NSError *error = nil;
 BOOL success = [self.data writeToFile:@"my/path"
 options:NSDataWritingAtomic
 error:&error];
 if (!success) {
 [self presentError:error
 completionHandler:^(BOOL didRecover) {
 if (didRecover) {
 [self save:sender];
 }
 }];
 }
 } iOS
50. Architecture
51. Architecture - presentError:completionHandler: NSError <NSErrorRecoveryAttempter> - doSomething: UIAlertView <UIAlertViewDelegate>
52. Architecture - presentError:completionHandler: NSError <NSErrorRecoveryAttempter> - doSomething: UIAlertView <UIAlertViewDelegate> completionHandler:
 void(^)(BOOL didRecover) HRSErrorRecoveryAttempter HRSErrorPresentationDelegate
53. The path of an error
54. NSError UIAlertView -attemptRecoveryFromError:
 optionIndex: didRecover void (^completionHandler)(BOOL didRecover) The path of an error UIViewController Operation AppDelegate -save: - doSomething: -presentError:completionHandler:
55. UIResponder
56. UIResponder - presentError:completionHandler:
57. UIResponder Subclasses • UIView • UIViewController • UIApplication • <UIApplicationDelegate> • SKNode
58. UIApplication UIWindow UIResponder UIView <UIApplicationDelegate> - (UIResponder *)nextResponder; UIView UIView UIViewController UIView UIViewController
59. UIResponder @interface DataSource : UIResponder <UITableViewDataSource>
 @property (nonatomic, weak) id<DataSourceDelegate> delegate;
 @end
 
 @implementation DataSource
 
 - (UIResponder *)nextResponder {
 if ([self.delegate isKindOfClass:[UIResponder class]]) {
 return self.delegate;
 }
 return nil;
 }
 
 @end
60. UIResponder @interface DataSource : UIResponder <UITableViewDataSource>
 @property (nonatomic, weak) id<DataSourceDelegate> delegate;
 @end
 
 @implementation DataSource
 
 - (UIResponder *)nextResponder {
 if ([self.delegate isKindOfClass:[UIResponder class]]) {
 return self.delegate;
 }
 return nil;
 }
 
 @end
61. UIResponder @interface DataSource : UIResponder <UITableViewDataSource>
 @property (nonatomic, weak) id<DataSourceDelegate> delegate;
 @end
 
 @implementation DataSource
 
 - (UIResponder *)nextResponder {
 if ([self.delegate isKindOfClass:[UIResponder class]]) {
 return self.delegate;
 }
 return nil;
 }
 
 @end
62. UIResponder UIResponder (HRSCustomErrorPresentation)
63. UIResponder - (NSError *)willPresentError:(NSError *)anError {
 return anError;
 } UIResponder (HRSCustomErrorPresentation)
64. UIResponder - (void)presentError:(NSError *)anError
 completionHandler:(void (^)(BOOL didRecover))completionHandler {
 
 anError = [self willPresentError:anError];
 if (anError == nil) {
 return;
 }
 
 UIResponder *nextResponder = ([self nextResponder] ?:
 [UIApplication sharedApplication]);
 [nextResponder presentError:anError
 completionHandler:completionHandler];
 } UIResponder (HRSCustomErrorPresentation)
65. UIResponder - (void)presentError:(NSError *)anError
 completionHandler:(void (^)(BOOL didRecover))completionHandler {
 
 anError = [self willPresentError:anError];
 if (anError == nil) {
 return;
 }
 
 UIResponder *nextResponder = ([self nextResponder] ?:
 [UIApplication sharedApplication]);
 [nextResponder presentError:anError
 completionHandler:completionHandler];
 } UIResponder (HRSCustomErrorPresentation)
66. UIResponder - (void)presentError:(NSError *)anError
 completionHandler:(void (^)(BOOL didRecover))completionHandler {
 
 anError = [self willPresentError:anError];
 if (anError == nil) {
 return;
 }
 
 UIResponder *nextResponder = ([self nextResponder] ?:
 [UIApplication sharedApplication]);
 [nextResponder presentError:anError
 completionHandler:completionHandler];
 } UIResponder (HRSCustomErrorPresentation)
67. UIResponder - (void)presentError:(NSError *)anError
 completionHandler:(void (^)(BOOL didRecover))completionHandler {
 
 anError = [self willPresentError:anError];
 if (anError == nil) {
 return;
 }
 
 UIResponder *nextResponder = ([self nextResponder] ?:
 [UIApplication sharedApplication]);
 [nextResponder presentError:anError
 completionHandler:completionHandler];
 } UIResponder (HRSCustomErrorPresentation)
68. UIResponder AppDelegate (HRSCustomErrorPresentation)
69. UIResponder - (NSError *)willPresentError:(NSError *)anError {
 if (anError.recoveryAttempter == nil) {
 NSDictionary *userInfo = @{ … };
 anError = [NSError errorWithDomain:anError.domain
 code:anError.code
 userInfo:userInfo];
 }
 return anError;
 } AppDelegate (HRSCustomErrorPresentation)
70. UIResponder - (NSError *)willPresentError:(NSError *)anError {
 if (anError.recoveryAttempter == nil) {
 NSDictionary *userInfo = @{ … };
 anError = [NSError errorWithDomain:anError.domain
 code:anError.code
 userInfo:userInfo];
 }
 return anError;
 } AppDelegate (HRSCustomErrorPresentation)
71. UIResponder - (NSError *)willPresentError:(NSError *)anError {
 if (anError.recoveryAttempter == nil) {
 NSDictionary *userInfo = @{ … };
 anError = [NSError errorWithDomain:anError.domain
 code:anError.code
 userInfo:userInfo];
 }
 return anError;
 } AppDelegate (HRSCustomErrorPresentation)
72. UIResponder - (void)presentError:(NSError *)anError
 completionHandler:(void (^)(BOOL))completionHandler {
 
 anError = [self willPresentError:anError];
 if (anError == nil) {
 return;
 } AppDelegate (HRSCustomErrorPresentation)
73. UIResponder - (void)presentError:(NSError *)anError
 completionHandler:(void (^)(BOOL))completionHandler {
 
 anError = [self willPresentError:anError];
 if (anError == nil) {
 return;
 } AppDelegate (HRSCustomErrorPresentation)
74. UIResponder - (void)presentError:(NSError *)anError
 completionHandler:(void (^)(BOOL))completionHandler {
 
 anError = [self willPresentError:anError];
 if (anError == nil) {
 return;
 } AppDelegate (HRSCustomErrorPresentation)
75. UIResponder HRSErrorPresentationDelegate *delegate = …;
 
 UIAlertView *alertView = [[UIAlertView alloc]
 initWithTitle:[anError localizedFailureReason]
 message:[anError localizedRecoverySuggestion]
 delegate:delegate
 cancelButtonTitle:nil
 otherButtonTitles:nil];
 
 for (NSString *title in [anError localizedRecoveryOptions]) {
 [alertView addButtonWithTitle:title];
 }
 
 [alertView show]; AppDelegate (HRSCustomErrorPresentation)
76. UIResponder HRSErrorPresentationDelegate *delegate = …;
 
 UIAlertView *alertView = [[UIAlertView alloc]
 initWithTitle:[anError localizedFailureReason]
 message:[anError localizedRecoverySuggestion]
 delegate:delegate
 cancelButtonTitle:nil
 otherButtonTitles:nil];
 
 for (NSString *title in [anError localizedRecoveryOptions]) {
 [alertView addButtonWithTitle:title];
 }
 
 [alertView show]; AppDelegate (HRSCustomErrorPresentation)
77. UIResponder HRSErrorPresentationDelegate *delegate = …;
 
 UIAlertView *alertView = [[UIAlertView alloc]
 initWithTitle:[anError localizedFailureReason]
 message:[anError localizedRecoverySuggestion]
 delegate:delegate
 cancelButtonTitle:nil
 otherButtonTitles:nil];
 
 for (NSString *title in [anError localizedRecoveryOptions]) {
 [alertView addButtonWithTitle:title];
 }
 
 [alertView show]; AppDelegate (HRSCustomErrorPresentation)
78. UIResponder HRSErrorPresentationDelegate *delegate = …;
 
 UIAlertView *alertView = [[UIAlertView alloc]
 initWithTitle:[anError localizedFailureReason]
 message:[anError localizedRecoverySuggestion]
 delegate:delegate
 cancelButtonTitle:nil
 otherButtonTitles:nil];
 
 for (NSString *title in [anError localizedRecoveryOptions]) {
 [alertView addButtonWithTitle:title];
 }
 
 [alertView show]; AppDelegate (HRSCustomErrorPresentation)
79. UIResponder HRSErrorPresentationDelegate *delegate = …;
 
 UIAlertView *alertView = [[UIAlertView alloc]
 initWithTitle:[anError localizedFailureReason]
 message:[anError localizedRecoverySuggestion]
 delegate:delegate
 cancelButtonTitle:nil
 otherButtonTitles:nil];
 
 for (NSString *title in [anError localizedRecoveryOptions]) {
 [alertView addButtonWithTitle:title];
 }
 
 [alertView show]; AppDelegate (HRSCustomErrorPresentation)
80. UIResponder objc_setAssociatedObject(alertView,
 DelegateAssociation,
 delegate,
 OBJC_ASSOCIATION_RETAIN);
 
 } AppDelegate (HRSCustomErrorPresentation)
81. What is next?
82. What’s next? • Handle common errors • Provide custom error presentation UI • Add specific error recovery attempter • This is all loosely coupled
83. What’s next? • Available open source • Currently considered beta • Internal interaction might still change • Feedback welcome
84. Related topics • Realmac’s “Cocoa error handling and recovery”
 http://realmacsoftware.com/blog/cocoa-error-handling-and-recovery
85. Feedback / Questions @_mochs michael.ochs@hrs.com ios-coding.com
86. Thank you