Under the Hood of Core Data - Core Data - page 5

To initialize the NSManagedObjectModel , we first need to locate it within the
application bundle. We call upon the NSBundle and retrieve the -mainBundle ,
which represents the application bundle. From there, we call -URLForResource:
withExtension: using the name of our data model—in this case PPRecipes —and the
extension .momd . We use one of the macros discussed in Appendix 2, Macros
in the Precompiled Header , on page 221 , and we verify that we did receive a
NSURL . We then initialize the NSManagedObjectModel with that NSURL . We again
verify that everything worked correctly by checking the new instance against
nil .

And that's all that is involved in constructing the NSManagedObjectModel . Our
next step is to construct the NSPersistentStoreCoordinator , which uses the NSManage-
dObjectModel we just initialized.

1.2 NSPersistentStoreCoordinator

The NSPersistentStoreCoordinator is the true maestro of Core Data. The NSPersis-
tentStoreCoordinator is responsible for persisting, loading, and caching data. Think
of the NSPersistentStoreCoordinator as the heart of Core Data. Having said this, we
do very little work with the NSPersistentStoreCoordinator directly. We work with it
upon initialization, but we almost never touch it again over the life of the
application.

As part of our initialization, we perform two steps with the NSPersistentStoreCoor-
dinator . First, we initialize it, which requires a valid NSManagedObjectModel . Once
it is initialized, we add one or more NSPersistentStore instances. An NSPersistentStore
is a representation of a location in which the data is saved/persisted. Typically,
this persistence is done to disk. However, that step is not required; it could
be in memory or even over the network. For now, let's focus on disk persis-
tence. The NSPersistentStore is responsible for describing the file format used.
This file format can be one of several: SQLite, binary, or atomic. (There's also
an XML store for OS X, but I do not recommend using it because it is not
available on iOS, nor does it perform very well.) To keep our focus, we will
use the SQLite format in this first iteration of our application and explore the
other formats later in the topic.

In previous versions of Core Data and the sample projects, the initialization
of the NSPersistentStoreCoordinator and the addition of the NSPersistentStore were done
in a single method. This example tended to lead to a number of issues for
developers because they did not fully understand the impact of the example.
Therefore, we are going to do this initialization in a more complex way, but
it will be a way that will not paint us into a corner.