Class

NSManagedObjectContext

An instance of NSManagedObjectContext represents a single “object space” or scratch pad in an application. Its primary responsibility is to manage a collection of managed objects. These objects form a group of related model objects that represent an internally consistent view of one or more persistent stores. A single managed object instance exists in one and only one context, but multiple copies of an object can exist in different contexts. Thus object uniquing is scoped to a particular context.

Overview

Life-cycle Management

The context is a powerful object with a central role in the life-cycle of managed objects, with responsibilities from life-cycle management (including faulting) to validation, inverse relationship handling, and undo/redo. Through a context you can retrieve or “fetch” objects from a persistent store, make changes to those objects, and then either discard the changes or—again through the context—commit them back to the persistent store. The context is responsible for watching for changes in its objects and maintains an undo manager so you can have finer-grained control over undo and redo. You can insert new objects and delete ones you have fetched, and commit these modifications to the persistent store.

All objects fetched from an external store are registered in a context together with a global identifier (an instance of NSManagedObjectID) that’s used to uniquely identify each object to the external store.

Parent Store

Managed object contexts have a parent store from which they retrieve data representing managed objects and through which they commit changes to managed objects.

Prior to OS X v10.7 and iOS v5.0, the parent store is always a persistent store coordinator. In macOS 10.7 and later and iOS v5.0 and later, the parent store may be another managed object context. Ultimately the root of a context’s ancestry must be a persistent store coordinator. The coordinator provides the managed object model and dispatches requests to the various persistent stores containing the data.

If a context’s parent store is another managed object context, fetch and save operations are mediated by the parent context instead of a coordinator. This pattern has a number of usage scenarios, including:

  • Performing background operations on a second thread or queue.

  • Managing discardable edits, such as in an inspector window or view.

As the first scenario implies, a parent context can service requests from children on different threads. You cannot, therefore, use parent contexts created with the thread confinement type (see Concurrency).

When you save changes in a context, the changes are only committed “one store up.” If you save a child context, changes are pushed to its parent. Changes are not saved to the persistent store until the root context is saved. (A root managed object context is one whose parent context is nil.) In addition, a parent does not pull changes from children before it saves. You must save a child context if you want ultimately to commit the changes.

Notifications

A context posts notifications at various points—see NSManagedObjectContextObjectsDidChangeNotification for example. Typically, you should register to receive these notifications only from known contexts:

[[NSNotificationCenter defaultCenter] addObserver:self
                                      selector:@selector(<#Selector name#>)
                                      name:NSManagedObjectContextDidSaveNotification
                                      object:<#A managed object context#>];

Several system frameworks use Core Data internally. If you register to receive these notifications from all contexts (by passing nil as the object parameter to a method such as addObserver:selector:name:object:), then you may receive unexpected notifications that are difficult to handle.

Concurrency

Core Data uses thread (or serialized queue) confinement to protect managed objects and managed object contexts (see Core Data Programming Guide). A consequence of this is that a context assumes the default owner is the thread or queue that allocated it—this is determined by the thread that calls its init method. You should not, therefore, initialize a context on one thread then pass it to a different thread. Instead, you should pass a reference to a persistent store coordinator and have the receiving thread/queue create a new context derived from that. If you use NSOperation, you must create the context in main (for a serial queue) or start (for a concurrent queue).

In macOS 10.7 and later and iOS v5.0 and later, when you create a context you can specify the concurrency pattern with which you will use it using initWithConcurrencyType:. When you create a managed object context using initWithConcurrencyType:, you have two options for its thread (queue) association

  • Confinement (NSConfinementConcurrencyType)

    For backwards compatibility, this is the default. You promise that context will not be used by any thread other than the one on which you created it. In general, to make the behavior explicit you’re encouraged to use one of the other types instead.

    You can only use this concurrency type if the managed object context’s parent store is a persistent store coordinator.

  • Private queue (NSPrivateQueueConcurrencyType)

    The context creates and manages a private queue.

  • Main queue (NSMainQueueConcurrencyType)

    The context is associated with the main queue, and as such is tied into the application’s event loop, but it is otherwise similar to a private queue-based context. You use this queue type for contexts linked to controllers and UI objects that are required to be used only on the main thread.

If you use contexts using the confinement pattern, you send the contexts messages directly; it’s up to you to ensure that you send the messages from the right queue.

You use contexts using the queue-based concurrency types in conjunction with performBlock: and performBlockAndWait:. You group “standard” messages to send to the context within a block to pass to one of these methods. There are two exceptions:

  • Setter methods on queue-based managed object contexts are thread-safe. You can invoke these methods directly on any thread.

  • If your code is executing on the main thread, you can invoke methods on the main queue style contexts directly instead of using the block based API.

performBlock: and performBlockAndWait: ensure the block operations are executed on the queue specified for the context. The performBlock: method returns immediately and the context executes the block methods on its own thread. With the performBlockAndWait: method, the context still executes the block methods on its own thread, but the method doesn’t return until the block is executed.

It’s important to appreciate that blocks are executed as a distinct body of work. As soon as your block ends, anyone else can enqueue another block, undo changes, reset the context, and so on. Thus blocks may be quite large, and typically end by invoking save:.

__block NSError *error;
__block BOOL savedOK = NO;
[myMOC performBlockAndWait:^{
    // Do lots of things with the context.
    savedOK = [myMOC save:&error];
}];

You can also perform other operations, such as:

NSFetchRequest *fr = [NSFetchRequest fetchRequestWithEntityName:@"Entity"];
__block NSUInteger rCount = 0;
 
[context performBlockAndWait:^() {
    NSError *error;
    rCount = [context countForFetchRequest:fr error:&error];
    if (rCount == NSNotFound) {
        // Handle the error.
    } }];
NSLog(@"Retrieved %d items", (int)rCount);

Subclassing Notes

You are strongly discouraged from subclassing NSManagedObjectContext. The change tracking and undo management mechanisms are highly optimized and hence intricate and delicate. Interposing your own additional logic that might impact processPendingChanges can have unforeseen consequences. In situations such as store migration, Core Data will create instances of NSManagedObjectContext for its own use. Under these circumstances, you cannot rely on any features of your custom subclass. Any NSManagedObject subclass must always be fully compatible with NSManagedObjectContext (that is, it cannot rely on features of a subclass of NSManagedObjectContext).

Symbols

Registering and Fetching Objects

- executeFetchRequest:error:

Returns an array of objects that meet the criteria specified by a given fetch request.

- countForFetchRequest:error:

Returns the number of objects a given fetch request would have returned if it had been passed to executeFetchRequest:error:.

- objectRegisteredForID:

Returns the object for a specified ID, if the object is registered with the receiver.

- objectWithID:

Returns the object for a specified ID.

- existingObjectWithID:error:

Returns the object for the specified ID.

registeredObjects

The set of objects registered with the receiver.

Managed Object Management

- insertObject:

Registers an object to be inserted in the receiver’s persistent store the next time changes are saved.

- deleteObject:

Specifies an object that should be removed from its persistent store when changes are committed.

- assignObject:toPersistentStore:

Specifies the store in which a newly-inserted object will be saved.

- obtainPermanentIDsForObjects:error:

Converts to permanent IDs the object IDs of the objects in a given array.

- detectConflictsForObject:

Marks an object for conflict detection.

- refreshObject:mergeChanges:

Updates the persistent properties of a managed object to use the latest values from the persistent store.

- processPendingChanges

Forces the receiver to process changes to the object graph.

insertedObjects

The set of objects that have been inserted into the receiver but not yet saved in a persistent store.

updatedObjects

The set of objects registered with the receiver that have uncommitted changes.

deletedObjects

The set of objects that will be removed from their persistent store during the next save operation.

Managing Concurrency

- initWithConcurrencyType:

Initializes a context with a given concurrency type.

concurrencyType

The concurrency type for the receiver.

- mergeChangesFromContextDidSaveNotification:

Merges the changes specified in a given notification.

Undo Management

undoManager

The undo manager of the receiver.

- undo

Sends an undo message to the receiver’s undo manager, asking it to reverse the latest uncommitted changes applied to objects in the object graph.

- redo

Sends an redo message to the receiver’s undo manager, asking it to reverse the latest undo operation applied to objects in the object graph.

- reset

Returns the receiver to its base state.

- rollback

Removes everything from the undo stack, discards all insertions and deletions, and restores updated objects to their last committed values.

- save:

Attempts to commit unsaved changes to registered objects to the receiver’s parent store.

hasChanges

A Boolean value that indicates whether the receiver has uncommitted changes.

Managing the Parent Store

persistentStoreCoordinator

The persistent store coordinator of the receiver.

parentContext

The receiver’s parent context.

Locking

- lock

Attempts to acquire a lock on the receiver.

Deprecated
- unlock

Relinquishes a previously acquired lock.

Deprecated
- tryLock

Attempts to acquire a lock.

Deprecated

Delete Propagation

propagatesDeletesAtEndOfEvent

A Boolean that indicates whether the receiver propagates deletes at the end of the event in which a change was made.

Registered Objects

retainsRegisteredObjects

A Boolean value that indicates whether the receiver keeps strong references to all registered managed objects.

Managing the Staleness Interval

stalenessInterval

The maximum length of time that may have elapsed since the store previously fetched data before fulfilling a fault issues a new fetch rather than using the previously-fetched data.

Managing the Merge Policy

mergePolicy

The merge policy of the receiver.

Performing Block Operations

- performBlock:

Asynchronously performs a given block on the receiver’s queue.

- performBlockAndWait:

Synchronously performs a given block on the receiver’s queue.

User Info

userInfo

The receiver’s user info.

Constants

NSManagedObjectContext Change Notification User Info Keys

Core Data uses these string constants as keys in the user info dictionary in a NSManagedObjectContextObjectsDidChangeNotification notification.

NSManagedObjectContextConcurrencyType

Constants to indicate the concurrency pattern with which a context will be used.

Notifications

You should typically not register to receive notifications from all managed object contexts—see Notifications.

NSManagedObjectContextObjectsDidChangeNotification

Posted when values of properties of objects contained in a managed object context are changed.

NSManagedObjectContextDidSaveNotification

Posted whenever a managed object context completes a save operation.

NSManagedObjectContextWillSaveNotification

Posted whenever a managed object context is about to perform a save operation.

Initializers

- initDeprecated

Relationships

Inherits From

Conforms To