Objective-C Fundamentals

206 CHAPTER 10 Error and exception handling
[alert show];
[alert release];
message:message
delegate:nil
cancelButtonTitle:nil
otherButtonTitles:@"OK", nil];
Listing 10.2 first checks whether the error object has a userInfo dictionary and whether
that dictionary has a value for the NSUnderlyingErrorKey. If so, it displays its localized-
Description. If not, it displays the main error object’s localizedDescription.
This technique doesn’t produce the best message for users in all cases. During
development it’s always important to test for different error conditions and look at the
contents of the userInfo dictionary because different frameworks and different subsystems
provide different amounts and kinds of information in the NSError objects
they create. There’s no one-size-fits-all solution to display error messages to your users.
In most cases, you know what could go wrong in a certain piece of code. Use that
knowledge to create and display the most helpful and user-friendly message possible.
What if you want to create your own NSError objects? Let’s look at that next.
10.2 Creating NSError objects
Adopting Cocoa’s error-handling pattern in your own code is simple. You just need to
accept a pointer to an NSError pointer as an argument to your method, check if one
has been provided by the caller, and create an instance of NSError with the information
describing the error if something went wrong.
Let’s look at an example.
10.2.1 Introducing RentalManagerAPI
Pretend you have to build a framework that talks to the
my-awesome-rental-manager.com website. You want to
give this framework to third-party developers to use in
their iOS projects, and naturally you want to build a beautiful
API that handles errors gracefully. One of the framework’s
methods takes a dictionary of values that represents
a classified ad, which is used here for simplicity’s sake; in
real life you’d use a custom class to represent such an ad.
The method publishes the ad to the website after checking
that all required values are present. If not, it creates
and reports an error that indicates which fields are missing,
as shown in figure 10.1.
Since we’re concentrating on how to create and handle
errors in this chapter, we won’t go into every detail of
setting up a demo project or designing the user interface
for this application in Interface Builder. If you want
to use the techniques you’ve learned in the previous
Figure 10.1 The
RentalManagerAPI
application displaying an error