Objective-C Fundamentals

254 CHAPTER 12 Reading and writing application data
UIAlertView and then use the abort() function to exit the application. Apple discourages
the use of the abort() function and asks developers to display a message that
tells the user to quit the application by pressing the Home button. While this works too,
the abort() function has one advantage: it generates a crash log. The log can be
extremely useful for tracking down the problem. It’s up to you to decide how you’ll handle
fatal errors, but abort() isn’t as bad as it’s made out to be if you use it responsibly.
Validation errors must also be dealt with: a required value is missing, a string is too
short, or a number is too large. Such errors can be recovered from, and you should
tell the user about them and the user a chance to fix them.
Listing 12.18 shows one way to handle and display validation errors (for example,
in PersonDetailViewController). Please note that you should adapt this method to
your application and your user’s needs. As it stands, the messages aren’t quite user
friendly but are definitely better than just saying “Saving failed.”
Listing 12.18
One way to handle and display validation errors
- (void)displayValidationError:(NSError *)anError { Make sure it’s a built-in error
if (anError &&
[[anError domain] isEqualToString:@"NSCocoaErrorDomain"]) {
NSArray *errors = nil;
if ([anError code] == NSValidationMultipleErrorsError) {
errors = [[anError userInfo] objectForKey:NSDetailedErrorsKey];
} else {
errors = [NSArray arrayWithObject:anError];
}
if (errors && [errors count] > 0) {
NSString *messages = @"Reason(s):\n";
for (NSError *error in errors) {
NSString *entityName =
[[[[error userInfo]
objectForKey:@"NSValidationErrorObject"]
entity]
name];
NSString *attributeName =
Iterate over
the errors
[[error userInfo] objectForKey:@"NSValidationErrorKey"];
NSString *msg;
Extract multiple errors;
wrap single errors
Get attribute
name
Get entity
name
switch ([error code]) {
Build appropriate
case NSManagedObjectValidationError:
error messages
msg = @"Generic validation error.";
break;
case NSValidationMissingMandatoryPropertyError:
msg = [NSString stringWithFormat:
@"The attribute '%@' mustn't be empty.",
attributeName];
break;
case NSValidationRelationshipLacksMinimumCountError:
msg = [NSString stringWithFormat:
@"The relationship '%@' doesn't have enough entries.",