Objective-C Fundamentals

Summary
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10.3.2 Catching exceptions
Code that you expect to potentially throw an exception should be wrapped in a @try
block, followed by one or more @catch() blocks that will catch exceptions that might
get thrown or raised by the code in the @try block. Optionally, you can add a
@finally block. The code in a @finally block gets run regardless of whether or not
an exception occurred. Why can there be multiple @catch blocks? Because you can
catch different kinds of exceptions, and the Objective-C runtime will pick the bestmatching
@catch block for the exception that’s been thrown. Note that each @try,
@catch, and @finally block has its own scope, so you can’t access an object in the
@finally block that’s been declared in the @try block, for example. If you want to
accomplish that, you must declare the variable outside of the @try block, in its enclosing
scope. The following listing shows a simple example.
Listing 10.7
Catching an exception
@try {
NSLog(@"Trying to add nil to an array...");
NSMutableArray *array = [NSMutableArray array];
[array addObject:nil];
}
@catch (NSException *e) {
NSLog(@"Caught exception: %@, Reason: %@", [e name], [e reason]);
}
@finally {
NSLog(@"Executed whether an exception was thrown or not.");
}
Inside the @try block, you attempt to insert nil into an array. The exception is caught
by the @catch block, and afterwards the @finally block is run. The console output of
this code looks like this:
Trying to add nil to an array...
Caught exception: NSInvalidArgumentException, Reason: *** -
[NSMutableArray insertObject:atIndex:]: attempt to insert nil object at 0
Executed whether or not an exception was thrown.
These exception-handling fundamentals should get you far, especially because exceptions
are rarely used in Cocoa development—but they’re useful for catching programming
errors.
10.4 Summary
Error handling in Cocoa and Cocoa Touch is both simple and flexible. Don’t ignore
errors in your applications: handle them gracefully and tell your users about errors
they need to know about in an understandable way. The concept of exceptions is present
in Cocoa as well but plays a minor role compared to other frameworks and languages.
Use them as intended: to catch errors during development.
In chapter 11, we dive into an important Cocoa concept that lets you access and
manipulate values in powerful ways: Key-Value Coding and NSPredicate.