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Core Data Programming Guide

from https://developer.apple.com/library/ios/documentation/Cocoa/Conceptual/CoreData/cdProgrammingGuide.html

Technology Overview

  • Beyond simple persistence and querying, core data offers change tracking and undo support, futures, value validation, UI synchronization, and automatic multi-writer conflict resolution.
  • Use it because it's mature, proven, robust, efficient, and integrates well with the development toolchain.

Core Data Basics

Basic Core Data Architecture

  • The managed object context, or context, provides access to a collection of framework objects, or the persistence stack, that mediate between the objects in your application and external data stores.
  • Model objects that tie into in the Core Data framework are known as managed objects, and are registered with the context.
  • The context tracks changes to these objects, thereby providing undo and redo support. If you save the changes you've made, the context ensures that your objects are in a valid state.
  • A fetch request specifies an entity name to retrieve; optionally it can specify a predicate and an array of sort orderings.
  • If a context already contains a managed object for an object returned from a fetch, then the fetch results contain that existing managed object.
  • A persistent store coordinator presents a façade to the managed object contexts so that a group of persistent stores appears as a single aggregate store. This allows partitioning data between these persistent stores.
  • In order to use a SQLite database, Core Data must create and manage the database itself.

Managed Objects and the Managed Object Model

  • A managed object model is a schema that provides a description of the managed objects.
  • A model consists of a collection of entity description objects that serve as metadata, such as its name, as well as its attributes and relationships.

Managed Object Models

  • A managed object mdoel, or NSManagedObjectModel instance, describes the schema.
  • A model contains NSEntityDescription objects that represent its entities. Each may have NSAttributeDescription and NSRelationshipDescription instances, and the entity may also have fetched properties, represented by NSFetchedPropertyDescription instances.
  • Transient properties are properties that you define as part of the model, but which are not saved with the entity to the persistent store.
  • Optional attributes are discouraged, becuase SQL has special comparison behavior for NULL that is unlike Objective-C's nil. NULL in a database is not the same as 0 or an empty string.
  • You can predefine fetch requests and store them in a managed object model as named templates. This is like compiling SQLite statements.

Using a Managed Object Model

Creating and Loading a Managed Object Model

  • An xcdatamodeld "source" directory is compiled into a momd deployment directory, and an xcdatamodel "source" file is compiled into a mom deployment file.
  • If you use Xcode to create a non-document application that uses Core Data, the application delegate includes code to retrieve a data model.

Accessing and Using a Managed Object Model at Runtime

  • To get the model at runtime, call persistentStoreCoordinator on the context and managedObjectModel on that, or call managedObjectModel on any entity.
  • Through the model you can create and access fetch request templates.

Managed Objects

Basics

  • You can subclass NSManagedObject to provide custom accessor or validation methods, to use non-standard attributes, to specify dependent keys, to calculate derived values, or to implement any other custom logic.
  • A managed object is associated with an entity description that provides metadata, and with a managed object context.

Properties and Data Storage

  • A NSManagedObject is a generic container object that efficiently provides storage for the properties defined by its associated NSEntityDescription object.
  • NSManagedObject represents date attributes using NSDate objects, and stores times internally as an NSTimeInterval value since the reference date.

Custom Managed Object Classes

  • Core Data dynamically generates efficient attribute accessor methods and relationship accessor methods for properties that are defined in the entity of a managed object's corresponding managed object model.
  • For performance reasons, Core Data typically does not copy object values, even if the value class adopts the NSCopying protocol.

Object Life-Cycle—Initialization and Deallocation

  • Core Data "owns" the life-cycle of managed objects. The framework can instantiate, destroy, and resurrect these objects as required.
  • To perform additional initialization of a managed object, don't override init. Instead, override awakeFromInsert or awakeFromFetch.
  • awakeFromInsert is invoked only once, when an object is first created. awakeFromFetch is invoked when an object is re-initialized from a persistent store, during a fetch.

Validation

  • To validate property values, implement methods of the form validate<Key>:error:, as defined by the NSKeyValueCoding protocol.
  • To validate inter-property values, override validateForUpdate: and/or related validation methods.

Faulting

  • A managed object may be a "fault" if its property values have not yet been loaded from the external data stores. Accessing these values "fires" the fault, and thereby loads them.