Objective-C Fundamentals

Beyond the basics
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How do you use your new model classes now? We look at the addTask method in
TasksViewController.m together, and you’ll be able to convert the rest of the application
yourself. If you get stuck, check out the code for the completed project online.
First, make sure you import Person.h and Task.h and that you change the person
instance variable type from NSManagedObject to Person. The new addTask method
will look like the following listing.
Listing 12.16
Modifying addTask in TasksViewController.m to use Person class
- (void)addTask {
Task *task = [NSEntityDescription
insertNewObjectForEntityForName:@"Task"
inManagedObjectContext:[person managedObjectContext]];
task.name =
[NSString stringWithFormat:@"Task %i", [person.tasks count] + 1];
task.isDone = [NSNumber numberWithBool:NO];
[person addTasksObject:task];
}
NSError *error = nil;
if (![[person managedObjectContext] save:&error]) {
NSLog(@"Unresolved error %@, %@", error, [error userInfo]);
} else {
}
That looks much nicer, doesn’t it? And notice that the addTasksObject: method of
the Person class is used to add new tasks to the current person. That makes your code
a lot more readable, and it’s much clearer to see what’s going on. It won’t be a problem
for you to change the rest of the code to use the new model classes.
12.5 Beyond the basics
You’ve built your first Core Data application. You know how to create a data model
and how to fetch, save, and even filter and sort data. That’s really all you need to know
to use Core Data in your own applications. As mentioned earlier, however, Core Data
is a complex framework, and there’s still more to learn. It’s beyond the scope of this
chapter to cover Core Data in depth, but in the next three sections, we touch on a few
important topics and provide further resources to point you in the right direction.
12.5.1 Changing the data model
Change is the one thing that’s constant in every software project. The day will likely
come when you’ll have to add a property to an entity, add a new entity, or add a new
relationship to your data model. When you do, you’ll have to take some extra steps to
ensure that your user’s data will still work with the new version of your application.
You won’t change your data model—instead, you’ll add a new one but keep the old
version so it can read data that was saved with that version of your data model.
Data migration can be a complex task, but when the change to the data model
is small (for example, adding a new property or making a non-optional property