Class

NSManagedObject

A base class that implements the behavior required of a Core Data model object.

Declaration

class NSManagedObject : NSObject

Overview

A managed object is associated with an entity description (NSEntityDescription) that provides metadata about the object, including the name of the entity that the object represents and the names of its attributes and relationships. A managed object is also associated with a managed object context that tracks changes to the object graph.

You can’t use instances of direct subclasses of NSObject (or any other class that doesn’t inherit from NSManagedObject) with a managed object context. You may create custom subclasses of NSManagedObject, although this isn’t always required. If no custom logic is needed, you can create a complete Object graph with NSManagedObject instances.

If you instantiate a managed object directly, you must call the designated initializer init(entity:insertInto:).

Data Storage

In some respects, an NSManagedObject acts like a dictionary—it’s a generic container object that provides efficient storage for the properties defined by its associated NSEntityDescription instance. NSManagedObject supports a range of common types for attribute values, including string, date, and number (see NSAttributeDescription for full details). Therefore, typically you don’t need to define instance variables in subclasses. Sometimes, however, you want to use types that aren’t supported directly, such as colors and C structures. For example, in a graphics application you might want to define a Rectangle entity that has color and bounds attributes that are an instance of NSColor and an NSRect struct, respectively. For some types you can use a transformable attribute, for others this may require you to create a subclass of NSManagedObject.

Faulting

Managed objects typically represent data held in a persistent store. In some situations a managed object may be a fault—an object whose property values haven’t yet been loaded from the external data store. When you access persistent property values, the fault “fires” and the data is retrieved from the store automatically. This can be a comparatively expensive process (potentially requiring a round trip to the persistent store), and you may wish to avoid unnecessarily firing a fault. See Faulting and Uniquing for more details on faults.

You can safely invoke the following methods and properties on a fault without causing it to fire: isEqual(_:), hash, superclass, class, self(), isProxy(), isKind(of:), isMember(of:), conforms(to:), responds(to:), description, managedObjectContext, entity, objectID, isInserted, isUpdated, isDeleted, faultingState, and isFault. Because isEqual and hash don’t cause a fault to fire, managed objects can typically be placed in collections without firing a fault. Note, however, that invoking key-value coding methods on the collection object might in turn result in an invocation of valueForKey: on a managed object, which would fire the fault.

Although the description property doesn’t cause a fault to fire, if you implement a custom description that accesses the object’s persistent properties, this does cause a fault to fire. You are strongly discouraged from overriding description in this way.

Subclassing Notes

In combination with the entity description in the managed object model, NSManagedObject provides a rich set of default behaviors including support for arbitrary properties and value validation. If you decide to subclass NSManagedObject to implement custom features, make sure you don’t disrupt Core Data’s behavior.

Methods and Properties You Must Not Override

NSManagedObject itself customizes many features of NSObject so that managed objects can be properly integrated into the Core Data infrastructure. Core Data relies on the NSManagedObject implementation of the following methods and properties, which you therefore absolutely must not override: primitiveValue(forKey:), setPrimitiveValue(_:forKey:), isEqual(_:), hash, superclass, class, self(), isProxy(), isKind(of:), isMember(of:), conforms(to:), responds(to:), managedObjectContext, entity, objectID, isInserted, isUpdated, isDeleted, and isFault, alloc, allocWithZone:, new, instancesRespond(to:), instanceMethod(for:), method(for:), methodSignatureForSelector:, instanceMethodSignatureForSelector:, or isSubclass(of:).

Methods and Properties You Shouldn't Override

As with any class, you are strongly discouraged from overriding the key-value observing methods such as willChangeValue(forKey:) and didChangeValue(forKey:withSetMutation:using:). Avoid overriding description—if this method fires a fault during a debugging operation, the results may be unpredictable. Also avoid overriding init(entity:insertInto:), or dealloc. Changing values in the init(entity:insertInto:) method won't be noticed by the context, and if you aren't careful, those changes may not be saved. Perform most initialization customization in one of the awake… methods. If you do override init(entity:insertInto:), make sure you adhere to the requirements set out in the method description. See init(entity:insertInto:).

Don’t override dealloc because didTurnIntoFault() is usually a better time to clear values—a managed object may not be reclaimed for some time after it has been turned into a fault. Core Data doesn’t guarantee that dealloc will be called in all scenarios (such as when the application quits). Therefore, don’t include required side effects (like saving or changes to the file system, user preferences, and so on) in these methods.

In summary, for init(entity:insertInto:) and dealloc, Core Data reserves exclusive control over the life cycle of the managed object (that is, raw memory management). This is so that the framework can provide features such as uniquing and by consequence, relationship maintenance, as well as much better performance than would be possible otherwise.

Additional Override Considerations

The following methods are intended to be fine grained and aren’t suitable for large-scale operations. Don’t fetch or save in these methods. In particular, they shouldn’t have side effects on the managed object context.

  • init(entity:insertInto:)

  • didTurnIntoFault()

  • willTurnIntoFault()

  • dealloc

In addition, if you plan to override awakeFromInsert, awakeFromFetch, and validation methods, first invoke super.method(), the superclass’s implementation. Don’t modify relationships in awakeFromFetch()—see the method description for details.

Custom Accessor Methods

Typically, you don’t need to write custom accessor methods for properties that are defined in the entity of a managed object’s corresponding managed object model. If you need to do so, follow the implementation patterns described in Managed Object Accessor Methods in Core Data Programming Guide.

Core Data automatically generates accessor methods (and primitive accessor methods) for you. For attributes and to-one relationships, Core Data generates the standard get and set accessor methods; for to-many relationships, Core Data generates the indexed accessor methods as described in Achieving Basic Key-Value Coding Compliance in Key-Value Coding Programming Guide. You do however need to declare the accessor methods or use Objective-C properties to suppress compiler warnings. For a full discussion, see Managed Object Accessor Methods in Core Data Programming Guide.

Custom Instance Variables

By default, NSManagedObject stores its properties in an internal structure as objects, and in general Core Data is more efficient working with storage under its own control rather than by using custom instance variables.

NSManagedObject provides support for a range of common types for attribute values, including string, date, and number (see NSAttributeDescription for full details). If you want to use types that aren’t supported directly, like colors and C structures, you can either use transformable attributes or create a subclass of NSManagedObject.

Sometimes it’s convenient to represent variables as scalars—in drawing applications, for example, where variables represent dimensions and x and y coordinates and are frequently used in calculations. To represent attributes as scalars, you declare instance variables as you do in any other class. You also need to implement suitable accessor methods as described in Managed Object Accessor Methods.

If you define custom instance variables for example to store derived attributes or other transient properties, clean up these variables in didTurnIntoFault() rather than dealloc.

Validation Methods

NSManagedObject provides consistent hooks for validating property and inter-property values. You typically shouldn’t override validateValue(_:forKey:). Instead implement methods of the form validate<Key>:error:, as defined by the NSKeyValueCoding protocol. If you want to validate inter-property values, you can override validateForUpdate() and/or related validation methods.

Don’t call validateValue:forKey:error: within custom property validation methods—if you do, you create an infinite loop when validateValue:forKey:error: is invoked at runtime. If you do implement custom validation methods, don’t call them directly. Instead, call validateValue:forKey:error: with the appropriate key. This ensures that any constraints defined in the managed object model are applied.

If you implement custom inter-property validation methods like validateForUpdate(), call the superclass’s implementation first. This ensures that individual property validation methods are also invoked. If there are multiple validation failures in one operation, collect them in an array and add the array—using the key NSDetailedErrorsKey—to the userInfo dictionary in the NSError object you return. For an example, see Managed Object Validation.

Topics

Initializing a Managed Object

init(entity: NSEntityDescription, insertInto: NSManagedObjectContext?)

Initializes a managed object from an entity description and inserts it into the specified managed object context.

init(context: NSManagedObjectContext)

Initializes a managed object subclass and inserts it into the specified managed object context.

Getting a Managed Object’s Identity

var entity: NSEntityDescription

The entity description of the managed object.

var objectID: NSManagedObjectID

The object ID of the managed object.

class func entity() -> NSEntityDescription

Returns the entity description that is associated with this subclass.

Getting State Information

var managedObjectContext: NSManagedObjectContext?

The managed object context with which the managed object is registered.

var hasChanges: Bool

A Boolean value that indicates whether the managed object has been inserted, has been deleted, or has unsaved changes.

var isInserted: Bool

A Boolean value that indicates whether the managed object has been inserted in a managed object context.

var isUpdated: Bool

A Boolean value that indicates whether the managed object has unsaved changes.

var isDeleted: Bool

A Boolean value that indicates whether the managed object will be deleted during the next save.

var isFault: Bool

A Boolean value that indicates whether the managed object is a fault.

var faultingState: Int

The faulting state of the managed object.

func hasFault(forRelationshipNamed: String) -> Bool

Returns a Boolean value that indicates whether the relationship for a given key is a fault.

var hasPersistentChangedValues: Bool

A Boolean value that indicates whether the managed object has persistent changes.

Managing Change Events

class var contextShouldIgnoreUnmodeledPropertyChanges: Bool

A Boolean value that indicates whether instances of the class should be marked as having changes if an unmodeled property is changed.

func awakeFromFetch()

Provides an opportunity to add code into the life cycle of the managed object when it is being fulfilled from a fault.

func awakeFromInsert()

Provides an opportunity to add code into the life cycle of the managed object when it is initially created.

func awake(fromSnapshotEvents: NSSnapshotEventType)

Provides an opportunity to add code into the life cycle of the managed object when it is being fulfilled from a snapshot.

func changedValues() -> [String : Any]

Returns a dictionary containing the keys and (old) values of persistent properties that have been changed since the last fetching or saving of the managed object.

func changedValuesForCurrentEvent() -> [String : Any]

Returns a dictionary containing the keys and old values of persistent properties that have changed since the last posting of NSManagedObjectContextObjectsDidChange.

func committedValues(forKeys: [String]?) -> [String : Any]

Returns a dictionary of the last fetched or saved values of the managed object for the properties specified by the given keys.

func prepareForDeletion()

Provides an opportunity to add code into the life cycle of the managed object when it is about to be deleted.

func willSave()

Provides an opportunity to add code into the life cycle of the managed object when it is about to be saved.

func didSave()

Provides an opportunity to add code into the life cycle of the managed object after the managed object’s context completes a save operation.

func willTurnIntoFault()

Provides an opportunity to add code into the life cycle of the managed object before it is converted to a fault.

func didTurnIntoFault()

Provides an opportunity to add code into the life cycle of the managed object when it is turned into a fault.

class func fetchRequest() -> NSFetchRequest<NSFetchRequestResult>

Returns a new fetch request initialized with the entity represented by this subclass.

Supporting Key-Value Coding

func value(forKey: String) -> Any?

Returns the value for the property specified by key.

func setValue(Any?, forKey: String)

Sets the specified property of the managed object to the specified value.

func primitiveValue(forKey: String) -> Any?

Returns the value for the specified property from the managed object’s private internal storage .

func setPrimitiveValue(Any?, forKey: String)

Sets the value of a given property in the managed object's private internal storage.

func objectIDs(forRelationshipNamed: String) -> [NSManagedObjectID]

Returns the object IDs for all of the managed objects that are in the named relationship.

Managing Data Validation

func validateForDelete()

Determines whether the managed object can be deleted in its current state.

func validateForInsert()

Determines whether the managed object can be inserted in its current state.

func validateForUpdate()

Determines whether the managed object's current state is valid.

Validation Error Codes

Error codes related to validation of managed object properties.

let NSValidationKeyErrorKey: String

The error key for the attribute that failed to validate.

let NSValidationObjectErrorKey: String

The error key for the object that failed to validate.

let NSValidationPredicateErrorKey: String

The error key for the predicate that failed to validate.

let NSValidationValueErrorKey: String

The error key for the value that failed to validate.

Supporting Key-Value Observing

func didAccessValue(forKey: String?)

Provides support for key-value observing access notification.

func observationInfo() -> UnsafeMutableRawPointer?

Returns the observation info of the managed object.

func setObservationInfo(UnsafeMutableRawPointer?)

Sets the observation info of the managed object.

func willAccessValue(forKey: String?)

Provides support for key-value observing access notification.

func didChangeValue(forKey: String)

Provides an opportunity to respond when a value of a given property has changed.

func didChangeValue(forKey: String, withSetMutation: NSKeyValueSetMutationKind, using: Set<AnyHashable>)

Provides an opportunity to respond when a change was made to a specified to-many relationship.

func willChangeValue(forKey: String)

Provides an opportunity to respond when a value of a given property is about to change.

func willChangeValue(forKey: String, withSetMutation: NSKeyValueSetMutationKind, using: Set<AnyHashable>)

Provides an opportunity to respond when a change is about to be made to a specified to-many relationship.

Reinitializing Values

struct NSSnapshotEventType

Constants that specify the reason the managed object may need to reinitialize its values.

See Also

Managed Objects and Entities

class NSEntityDescription

A description of an entity in Core Data.

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