Objective-C Fundamentals

208 CHAPTER 10 Error and exception handling
}
if (anError != NULL) {
NSString *description = @"The ad could not be \
published because some required \
values are missing.";
NSString *recoverySuggestion = @"Please provide \
the missing values and try again.";
NSArray *keys = [NSArray arrayWithObjects:
NSLocalizedDescriptionKey,
NSLocalizedRecoverySuggestionErrorKey,
RMAMissingValuesKey, nil];
NSArray *values = [NSArray arrayWithObjects:
description,
recoverySuggestion,
missingValues, nil];
NSDictionary *userDict = [NSDictionary
dictionaryWithObjects:values
forKeys:keys];
*anError = [[[NSError alloc] initWithDomain:
RMAErrorDomain
code:RMAValidationError
userInfo:userDict] autorelease];
}
return NO;
} else {
return YES;
}
Caller
interested
in errors?
Create an
instance of
NSError
@end
Let’s examine the code. In the header file, some constants are set up: a custom error
domain, some special keys for the userInfo dictionary, and some error codes. Using
constants and enumerators for these things makes your code much more readable
and maintainable. In the implementation of the publishAd:error: method, you’ll
notice one interesting thing right away: the use of the @throw directive. We briefly
look at exceptions in the next section of this chapter, but here you see one common
use case for them: catching programming errors. Your method should never be called
without an instance of NSDictionary, and with this exception you make sure that such
an error gets caught during development.
The rest of the method isn’t rocket science: it checks if any required values are
missing and collects them in an array. If some values are missing, you create a description
and some recovery suggestions and put them in a dictionary together with the array
of missing values. With that, you create an instance of NSError and have the pointer to
a pointer (anError) point to it by using the asterisk in front of the variable name
(*anError). That tells the compiler that you want to change the value of the pointer
that anError is pointing to, not the value of anError itself. This is a little confusing, no
doubt, but you’ll wrap your head around it in no time.
Finally you return NO when an error occurred and YES otherwise. Now how do you
call this method and handle the error it might return?