Objective-C Fundamentals

210 CHAPTER 10 Error and exception handling
}
}
UIAlertView *av = [[UIAlertView alloc]
initWithTitle:@"Error"
message:message
delegate:nil
cancelButtonTitle:@"OK"
otherButtonTitles:nil];
[av show];
[av release];
Display error
message
You should now have a good fundamental understanding of how to deal with errors in
Cocoa and how to create your own.
10.3 Exceptions
Exceptions play a different role in Objective-C and Cocoa than they do in languages
like Java. In Cocoa, exceptions should be used only for programmer errors, and the
program should quit quickly after such an exception is caught. Exceptions should
never be used to report or handle expected errors (like an unreachable host or a file
that can’t be found). Instead, use them to catch programming errors like an argument
being nil that should never be nil or an array having only one item when it’s
expected to have at least two. You get the idea.
So how do you create and throw exceptions?
10.3.1 Throwing exceptions
Exceptions are instances of NSException (or of a subclass thereof). They consist of a
name, a reason, and an optional userInfo dictionary, much like NSError. You can use
any string as an exception name or use one of Cocoa’s predefined names that can be
found at http://developer.apple.com/library/ios/#documentation/Cocoa/Conceptual/
Exceptions/Concepts/PredefinedExceptions.html.
Exceptions can be thrown either by using the @throw directive or by calling the
raise method on an instance of NSException. The following listing shows an example
of creating and throwing an exception with a predefined exception name.
Listing 10.6
Throwing an exception
if (aRequiredValue == nil) {
@throw [NSException
exceptionWithName:NSInvalidArgumentException
reason:@"aRequiredValue is nil"
userInfo:nil];
}
Uncaught exceptions cause an application to terminate. During development, that
might be just what you want to be alerted that you did something wrong in your program
code or logic. How would you go about catching exceptions, though?