Core Data:: Core Data is an object graph and persistence framework provided by Apple in the Mac OS X and iOS operating systems. It was introduced in Mac OS X 10.4 Tiger and iOS with iPhone SDK 3.0. It allows dataorganised by the relational entity–attribute model to be serialised into XML, binary, or SQLite stores.
Core Data Features
The Core Data framework provides generalized and automated solutions to common tasks associated with object life-cycle and object graphmanagement, including persistence. Its features include:
- Change tracking and undo support.Core Data provides built-in management of undo and redo beyond basic text editing.
- Relationship maintenance.Core Data manages change propagation, including maintaining the consistency of relationships among objects.
- Futures (faulting).Core Data can reduce the memory overhead of your program by lazily loading objects. It also supports partially materialized futures, and copy-on-write data sharing.
- Automatic validation of property values.Core Data’s managed objects extend the standard key-value coding validation methods that ensure that individual values lie within acceptable ranges so that combinations of values make sense.
- Schema migration.Dealing with a change to your application’s schema can be difficult, in terms of both development effort and runtime resources. Core Data’s schema migration tools simplify the task of coping with schema changes, and in some cases allow you to perform extremely efficient in-place schema migration.
- Optional integration with the application’s controller layer to support user interface synchronization.Core Data provides the NSFetchedResultsController object on iOS, and integrates with Cocoa Bindings on OS X.
- Full, automatic, support for key-value coding and key-value observing.In addition to synthesizing key-value coding and key-value observing compliant accessor methods for attributes, Core Data synthesizes the appropriate collection accessors for to-many relationships.
- Grouping, filtering, and organizing data in memory and in the user interface.
- Automatic support for storing objects in external data repositories.
- Sophisticated query compilation.Instead of writing SQL, you can create complex queries by associating an NSPredicate object with a fetch request. NSPredicate provides support for basic functions, correlated subqueries, and other advanced SQL. With Core Data, it also supports proper Unicode, locale-aware searching, sorting, and regular expressions.
- Merge policies.Core Data provides built in version tracking and optimistic locking to support automatic multi-writer conflict resolution.
Why Should You Use Core Data?
There are a number of reasons why it may be appropriate for you to use Core Data. One of the simplest metrics is that, with Core Data, the amount of code you write to support the model layer of your application is typically 50% to 70% smaller as measured by lines of code. This is primarily due to the features listed above—the features Core Data provides are features you don’t have to implement yourself. Moreover they’re features you don’t have to test yourself, and in particular you don’t have to optimize yourself.
Core Data has a mature code base whose quality is maintained through unit tests, and is used daily by millions of customers in a wide variety of applications. The framework has been highly optimized over several releases. It takes advantage of information provided in the model and runtime features not typically employed in application-level code. Moreover, in addition to providing excellent security and error-handling, it offers best memory scalability of any competing solution. Put another way: you could spend a long time carefully crafting your own solution optimized for a particular problem domain, and not gain any performance advantage over what Core Data offers for free for any application.
In addition to the benefits of the framework itself, Core Data integrates well with the OS X tool chain. The model design tools allow you to create your schema graphically, quickly and easily. You can use templates in the Instruments application to measure Core Data’s performance, and to debug various problems. On OS X desktop, Core Data also integrates with Interface Builder to allow you to create user interfaces from your model. These aspects help to further shorten your application design, implementation, and debugging cycles.
What Core Data Is Not
Having given an overview of what Core Data is and does, and why it may be useful, it is also useful to correct some common misperceptions and state what it is not.
- Core Data is not a relational database or a relational database management system (RDBMS).Core Data provides an infrastructure for change management and for saving objects to and retrieving them from storage. It can use SQLite as one of its persistent store types. It is not, though, in and of itself a database. (To emphasize this point: you could for example use just an in-memory store in your application. You could use Core Data for change tracking and management, but never actually save any data in a file.)
- Core Data is not a silver bullet.Core Data does not remove the need to write code. Although it is possible to create a sophisticated application solely using the Xcode data modeling tool and Interface Builder, for more real-world applications you will still have to write code.
- Core Data does not rely on Cocoa bindings.Core Data integrates well with Cocoa bindings and leverages the same technologies—and used together they can significantly reduce the amount of code you have to write—but it is possible to use Core Data without bindings. You can readily create a Core Data application without a user interface