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Custom error handling on iOS Michael Ochs, Mobile Developer, HRS

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Agenda • Responsibility • NSError / NSErrorRecoveryAttempting • Presentation • UIResponder • What’s next?

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Error handling before

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How not to do it

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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
 }


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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
 }


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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

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Responsibility

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Responsibility • Globally in the app • Usable by every potential error receiver • Easy to use • Little setup effort • Error recovery

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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:

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Responsibility Trigger action Create error Present error Recover from error Retry action

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Responsibility Trigger action Consumer Creator Create error Present error Recover from error Retry action

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Responsibility Trigger action Create error Present error Recover from error Retry action Controller Data Application

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NSError

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NSError • NSLocalizedDescriptionKey • NSLocalizedFailureReasonErrorKey • NSLocalizedRecoverySuggestionErrorKey • NSLocalizedRecoveryOptionsErrorKey • NSRecoveryAttempterErrorKey

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NSError The corresponding value is a localized string representation of the error that, if present, will be returned by localizedDescription. NSLocalizedDescriptionKey

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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

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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

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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

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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

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NSError • NSLocalizedDescriptionKey • NSLocalizedFailureReasonErrorKey • NSLocalizedRecoverySuggestionErrorKey • NSLocalizedRecoveryOptionsErrorKey • NSRecoveryAttempterErrorKey

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NSError - (BOOL)doSaveOperation:(NSError **)error {
 
 NSError *error = [NSError errorWithDomain:MyErrorDomain
 code:MyErrorCode
 userInfo:userInfo];
 *error = error;
 
 return NO;
 } Error creation

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NSError NSDictionary *userInfo = @{
 NSLocalizedFailureReasonErrorKey:
 @"Something went wrong",
 
 NSLocalizedRecoverySuggestionErrorKey:
 @"Not successful. Try again!",
 
 NSLocalizedRecoveryOptionsErrorKey:
 @[ @"Cancel", @"Retry" ],
 
 NSRecoveryAttempterErrorKey:
 recoveryAttempter
 }; Error creation

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NSError NSDictionary *userInfo = @{
 NSLocalizedFailureReasonErrorKey:
 @"Something went wrong",
 
 NSLocalizedRecoverySuggestionErrorKey:
 @"Not successful. Try again!",
 
 NSLocalizedRecoveryOptionsErrorKey:
 [recoveryAttempter localizedRecoveryOptions],
 
 NSRecoveryAttempterErrorKey:
 recoveryAttempter
 }; Error creation

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NSErrorRecoveryAttempting

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NSErrorRecoveryAttempting • Informal protocol on NSObject • Integrated with NSError • Handles error recovery

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NSErrorRecoveryAttempting - (void)attemptRecoveryFromError:(NSError *)error
 optionIndex:(NSUInteger)recoveryOptionIndex
 delegate:(id)delegate
 didRecoverSelector:(SEL)didRecoverSelector
 contextInfo:(void *)contextInfo;
 
 - (BOOL)attemptRecoveryFromError:(NSError *)error
 optionIndex:(NSUInteger)recoveryOptionIndex;

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NSErrorRecoveryAttempting - (void)attemptRecoveryFromError:(NSError *)error
 optionIndex:(NSUInteger)recoveryOptionIndex
 delegate:(id)delegate
 didRecoverSelector:(SEL)didRecoverSelector
 contextInfo:(void *)contextInfo;
 
 - (BOOL)attemptRecoveryFromError:(NSError *)error
 optionIndex:(NSUInteger)recoveryOptionIndex;

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NSErrorRecoveryAttempting - (void)attemptRecoveryFromError:(NSError *)error
 optionIndex:(NSUInteger)recoveryOptionIndex
 delegate:(id)delegate
 didRecoverSelector:(SEL)didRecoverSelector
 contextInfo:(void *)contextInfo;
 
 - (BOOL)attemptRecoveryFromError:(NSError *)error
 optionIndex:(NSUInteger)recoveryOptionIndex;

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NSErrorRecoveryAttempting HRSErrorRecoveryAttempter

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NSErrorRecoveryAttempting - (void)addRecoveryOptionWithTitle:(NSString *)title
 recoveryAttempt:(BOOL(^)())recoveryBlock;
 
 - (NSArray *)localizedRecoveryOptions; HRSErrorRecoveryAttempter

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NSErrorRecoveryAttempting - (void)addRecoveryOptionWithTitle:(NSString *)title
 recoveryAttempt:(BOOL(^)())recoveryBlock;
 
 - (NSArray *)localizedRecoveryOptions; HRSErrorRecoveryAttempter

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NSErrorRecoveryAttempting - (void)addRecoveryOptionWithTitle:(NSString *)title
 recoveryAttempt:(BOOL(^)())recoveryBlock;
 
 - (NSArray *)localizedRecoveryOptions; HRSErrorRecoveryAttempter

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NSErrorRecoveryAttempting HRSErrorRecoveryAttempter *recoveryAttempter =
 [HRSErrorRecoveryAttempter new];
 
 [recoveryAttempter addCancelRecoveryOption];
 
 [recoveryAttempter addRecoveryOptionWithTitle:@"Retry"
 recoveryAttempt:^{
 return YES;
 }]; HRSErrorRecoveryAttempter

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NSErrorRecoveryAttempting HRSErrorRecoveryAttempter *recoveryAttempter =
 [HRSErrorRecoveryAttempter new];
 
 [recoveryAttempter addCancelRecoveryOption];
 
 [recoveryAttempter addRecoveryOptionWithTitle:@"Retry"
 recoveryAttempt:^{
 return YES;
 }]; HRSErrorRecoveryAttempter

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NSErrorRecoveryAttempting HRSErrorRecoveryAttempter *recoveryAttempter =
 [HRSErrorRecoveryAttempter new];
 
 [recoveryAttempter addCancelRecoveryOption];
 
 [recoveryAttempter addRecoveryOptionWithTitle:@"Retry"
 recoveryAttempt:^{
 return YES;
 }]; HRSErrorRecoveryAttempter

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NSErrorRecoveryAttempting HRSErrorRecoveryAttempter *recoveryAttempter =
 [HRSErrorRecoveryAttempter new];
 
 [recoveryAttempter addCancelRecoveryOption];
 
 [recoveryAttempter addRecoveryOptionWithTitle:@"Retry"
 recoveryAttempt:^{
 return YES;
 }]; HRSErrorRecoveryAttempter

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Presentation

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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

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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

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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

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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

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Presentation @interface UIResponder (HRSCustomErrorPresentation)
 
 - (void)presentError:(NSError *)anError
 completionHandler:(void (^)(BOOL didRecover))completionHandler;
 
 - (NSError *)willPresentError:(NSError *)anError;
 
 @end iOS

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Presentation @interface UIResponder (HRSCustomErrorPresentation)
 
 - (void)presentError:(NSError *)anError
 completionHandler:(void (^)(BOOL didRecover))completionHandler;
 
 - (NSError *)willPresentError:(NSError *)anError;
 
 @end iOS

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Presentation @interface UIResponder (HRSCustomErrorPresentation)
 
 - (void)presentError:(NSError *)anError
 completionHandler:(void (^)(BOOL didRecover))completionHandler;
 
 - (NSError *)willPresentError:(NSError *)anError;
 
 @end iOS

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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

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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

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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

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Architecture

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Architecture - presentError:completionHandler: NSError - doSomething: UIAlertView

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Architecture - presentError:completionHandler: NSError - doSomething: UIAlertView completionHandler:
 void(^)(BOOL didRecover) HRSErrorRecoveryAttempter HRSErrorPresentationDelegate

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The path of an error

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NSError UIAlertView -attemptRecoveryFromError:
 optionIndex: didRecover void (^completionHandler)(BOOL didRecover) The path of an error UIViewController Operation AppDelegate -save: - doSomething: -presentError:completionHandler:

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UIResponder

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UIResponder - presentError:completionHandler:

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UIResponder Subclasses • UIView • UIViewController • UIApplication • • SKNode

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UIApplication UIWindow UIResponder UIView - (UIResponder *)nextResponder; UIView UIView UIViewController UIView UIViewController

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UIResponder @interface DataSource : UIResponder 
 @property (nonatomic, weak) id delegate;
 @end
 
 @implementation DataSource
 
 - (UIResponder *)nextResponder {
 if ([self.delegate isKindOfClass:[UIResponder class]]) {
 return self.delegate;
 }
 return nil;
 }
 
 @end

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UIResponder @interface DataSource : UIResponder 
 @property (nonatomic, weak) id delegate;
 @end
 
 @implementation DataSource
 
 - (UIResponder *)nextResponder {
 if ([self.delegate isKindOfClass:[UIResponder class]]) {
 return self.delegate;
 }
 return nil;
 }
 
 @end

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UIResponder @interface DataSource : UIResponder 
 @property (nonatomic, weak) id delegate;
 @end
 
 @implementation DataSource
 
 - (UIResponder *)nextResponder {
 if ([self.delegate isKindOfClass:[UIResponder class]]) {
 return self.delegate;
 }
 return nil;
 }
 
 @end

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UIResponder UIResponder (HRSCustomErrorPresentation)

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UIResponder - (NSError *)willPresentError:(NSError *)anError {
 return anError;
 } UIResponder (HRSCustomErrorPresentation)

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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)

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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)

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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)

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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)

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UIResponder AppDelegate (HRSCustomErrorPresentation)

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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)

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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)

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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)

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UIResponder - (void)presentError:(NSError *)anError
 completionHandler:(void (^)(BOOL))completionHandler {
 
 anError = [self willPresentError:anError];
 if (anError == nil) {
 return;
 } AppDelegate (HRSCustomErrorPresentation)

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UIResponder - (void)presentError:(NSError *)anError
 completionHandler:(void (^)(BOOL))completionHandler {
 
 anError = [self willPresentError:anError];
 if (anError == nil) {
 return;
 } AppDelegate (HRSCustomErrorPresentation)

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UIResponder - (void)presentError:(NSError *)anError
 completionHandler:(void (^)(BOOL))completionHandler {
 
 anError = [self willPresentError:anError];
 if (anError == nil) {
 return;
 } AppDelegate (HRSCustomErrorPresentation)

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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)

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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)

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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)

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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)

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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)

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UIResponder objc_setAssociatedObject(alertView,
 DelegateAssociation,
 delegate,
 OBJC_ASSOCIATION_RETAIN);
 
 } AppDelegate (HRSCustomErrorPresentation)

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What is next?

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What’s next? • Handle common errors • Provide custom error presentation UI • Add specific error recovery attempter • This is all loosely coupled

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What’s next? • Available open source • Currently considered beta • Internal interaction might still change • Feedback welcome

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Related topics • Realmac’s “Cocoa error handling and recovery”
 http://realmacsoftware.com/blog/cocoa-error-handling-and-recovery

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Feedback / Questions @_mochs [email protected] ios-coding.com

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Thank you