NSManagedObjectContext Class Reference
| Inherits from | |
| Conforms to | |
| Framework | /System/Library/Frameworks/CoreData.framework |
| Availability | Available in Mac OS X v10.4 and later. |
| Declared in | NSManagedObjectContext.h |
| Companion guides | |
| Related sample code |
Overview
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.
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.
Persistent Store Coordinator
A context always has a “parent” persistent store coordinator which provides the model and dispatches requests to the various persistent stores containing the data. Without a coordinator, a context is not fully functional. The context’s coordinator provides the managed object model and handles persistency. 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.
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 (as opposed to any subclass of NSManagedObjectContext).
Tasks
Registering and Fetching Objects
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– executeFetchRequest:error:
-
– countForFetchRequest:error:
-
– objectRegisteredForID:
-
– objectWithID:
-
– existingObjectWithID:error:
-
– registeredObjects
Managed Object Management
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– insertObject:
-
– deleteObject:
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– assignObject:toPersistentStore:
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– obtainPermanentIDsForObjects:error:
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– detectConflictsForObject:
-
– refreshObject:mergeChanges:
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– processPendingChanges
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– insertedObjects
-
– updatedObjects
-
– deletedObjects
Merging Changes from Another Context
Undo Management
Locking
Delete Propagation
Retaining Registered Objects
Managing the Persistent Store Coordinator
Managing the Staleness Interval
Managing the Merge Policy
Instance Methods
assignObject:toPersistentStore:
Specifies the store in which a newly-inserted object will be saved.
- (void)assignObject:(id)object toPersistentStore:(NSPersistentStore *)store
Parameters
- object
A managed object.
- store
A persistent store.
Discussion
You can obtain a store from the persistent store coordinator, using for example persistentStoreForURL:.
Special Considerations
It is only necessary to use this method if the receiver’s persistent store coordinator manages multiple writable stores that have object’s entity in their configuration. Maintaining configurations in the managed object model can eliminate the need for invoking this method directly in many situations. If the receiver’s persistent store coordinator manages only a single writable store, or if only one store has object’s entity in its model, object will automatically be assigned to that store.
Availability
- Available in Mac OS X v10.4 and later.
See Also
Related Sample Code
Declared In
NSManagedObjectContext.hcountForFetchRequest:error:
Returns the number of objects a given fetch request would have returned if it had been passed to executeFetchRequest:error:.
- (NSUInteger)countForFetchRequest:(NSFetchRequest *)request error:(NSError **)error
Parameters
- request
A fetch request that specifies the search criteria for the fetch.
- error
If there is a problem executing the fetch, upon return contains an instance of
NSErrorthat describes the problem.
Return Value
The number of objects a given fetch request would have returned if it had been passed to executeFetchRequest:error:.
Availability
- Available in Mac OS X v10.5 and later.
Declared In
NSManagedObjectContext.hdeletedObjects
Returns the set of objects that will be removed from their persistent store during the next save operation.
- (NSSet *)deletedObjects
Return Value
The set of objects that will be removed from their persistent store during the next save operation.
Discussion
The returned set does not necessarily include all the objects that have been deleted (using deleteObject:)—if an object has been inserted and deleted without an intervening save operation, it is not included in the set.
A managed object context does not post key-value observing notifications when the return value of deletedObjects changes. A context does, however, post a NSManagedObjectContextObjectsDidChangeNotification notification when a change is made, and a NSManagedObjectContextWillSaveNotification notification and a NSManagedObjectContextDidSaveNotification notification before and after changes are committed respectively (although again the set of deleted objects given for a NSManagedObjectContextDidSaveNotification does not include objects that were inserted and deleted without an intervening save operation—that is, they had never been saved to a persistent store).
Availability
- Available in Mac OS X v10.4 and later.
See Also
-
– deleteObject: -
– insertedObjects -
– registeredObjects -
– updatedObjects -
– isDeleted(NSManagedObject)
Declared In
NSManagedObjectContext.hdeleteObject:
Specifies an object that should be removed from its persistent store when changes are committed.
- (void)deleteObject:(NSManagedObject *)object
Parameters
- object
A managed object.
Discussion
When changes are committed, object will be removed from the uniquing tables. If object has not yet been saved to a persistent store, it is simply removed from the receiver.
Availability
- Available in Mac OS X v10.4 and later.
See Also
-
– deletedObjects -
– isDeleted(NSManagedObject)
Related Sample Code
Declared In
NSManagedObjectContext.hdetectConflictsForObject:
Marks an object for conflict detection.
- (void)detectConflictsForObject:(NSManagedObject *)object
Parameters
- object
A managed object.
Discussion
If on the next invocation of save: object has been modified in its persistent store, the save fails. This allows optimistic locking for unchanged objects. Conflict detection is always performed on changed or deleted objects.
Availability
- Available in Mac OS X v10.4 and later.
Declared In
NSManagedObjectContext.hexecuteFetchRequest:error:
Returns an array of objects that meet the criteria specified by a given fetch request.
- (NSArray *)executeFetchRequest:(NSFetchRequest *)request error:(NSError **)error
Parameters
- request
A fetch request that specifies the search criteria for the fetch.
- error
If there is a problem executing the fetch, upon return contains an instance of
NSErrorthat describes the problem.
Return Value
An array of objects that meet the criteria specified by request fetched from the receiver and from the persistent stores associated with the receiver’s persistent store coordinator. If an error occurs, returns nil. If no objects match the criteria specified by request, returns an empty array.
Discussion
Returned objects are registered with the receiver.
The following points are important to consider:
If the fetch request has no predicate, then all instances of the specified entity are retrieved, modulo other criteria below.
An object that meets the criteria specified by request (it is an instance of the entity specified by the request, and it matches the request’s predicate if there is one) and that has been inserted into a context but which is not yet saved to a persistent store, is retrieved if the fetch request is executed on that context.
If an object in a context has been modified, a predicate is evaluated against its modified state, not against the current state in the persistent store. Therefore, if an object in a context has been modified such that it meets the fetch request’s criteria, the request retrieves it even if changes have not been saved to the store and the values in the store are such that it does not meet the criteria. Conversely, if an object in a context has been modified such that it does not match the fetch request, the fetch request will not retrieve it even if the version in the store does match.
If an object has been deleted from the context, the fetch request does not retrieve it even if that deletion has not been saved to a store.
Objects that have been realized (populated, faults fired, “read from”, and so on) as well as pending updated, inserted, or deleted, are never changed by a fetch operation without developer intervention. If you fetch some objects, work with them, and then execute a new fetch that includes a superset of those objects, you do not get new instances or update data for the existing objects—you get the existing objects with their current in-memory state.
Availability
- Available in Mac OS X v10.4 and later.
Related Sample Code
Declared In
NSManagedObjectContext.hexistingObjectWithID:error:
Returns the object for the specified ID.
- (NSManagedObject *)existingObjectWithID:(NSManagedObjectID *)objectIDerror:(NSError **)error
Parameters
- objectID
The object ID for the requested object.
- error
If there is a problem in retrieving the object specified by objectID, upon return contains an error that describes the problem.
Return Value
The object specified by objectID. If the object cannot be fetched, or does not exist, or cannot be faulted, it returns nil.
Discussion
If there is a managed object with the given ID already registered in the context, that object is returned directly; otherwise the corresponding object is faulted into the context.
This method might perform I/O if the data is uncached.
Unlike objectWithID:, this method never returns a fault.
Availability
- Available in Mac OS X v10.6 and later.
See Also
Declared In
NSManagedObjectContext.hhasChanges
Returns a Boolean value that indicates whether the receiver has uncommitted changes.
- (BOOL)hasChanges
Return Value
YES if the receiver has uncommitted changes, otherwise NO.
Discussion
On Mac OS X v10.6 and later, this property is key-value observing compliant (see Key-Value Observing Programming Guide).
Prior to Mac OS X v10.6, this property is not key-value observing compliant—for example, if you are using Cocoa bindings, you cannot bind to the hasChanges property of a managed object context.
Special Considerations
If you are observing this property using key-value observing (KVO) you should not touch the context or its objects within your implementation of observeValueForKeyPath:ofObject:change:context: for this notification. (This is because of the intricacy of the locations of the KVO notifications—for example, the context may be in the middle of an undo operation, or repairing a merge conflict.) If you need to send messages to the context of change any of its managed objects as a result of a change to the value of hasChanges, you must do so after the call stack unwinds (typically using performSelector:withObject:afterDelay: or a similar method).
Availability
- Available in Mac OS X v10.4 and later.
Declared In
NSManagedObjectContext.hinsertedObjects
Returns the set of objects that have been inserted into the receiver but not yet saved in a persistent store.
- (NSSet *)insertedObjects
Return Value
The set of objects that have been inserted into the receiver but not yet saved in a persistent store.
Discussion
A managed object context does not post key-value observing notifications when the return value of insertedObjects changes—it does, however, post a NSManagedObjectContextObjectsDidChangeNotification notification when a change is made, and a NSManagedObjectContextWillSaveNotification and a NSManagedObjectContextDidSaveNotification notification before and after changes are committed respectively.
Availability
- Available in Mac OS X v10.4 and later.
See Also
Declared In
NSManagedObjectContext.hinsertObject:
Registers an object to be inserted in the receiver’s persistent store the next time changes are saved.
- (void)insertObject:(NSManagedObject *)object
Parameters
- object
A managed object.
Discussion
The managed object (object) is registered in the receiver with a temporary global ID. It is assigned a permanent global ID when changes are committed. If the current transaction is rolled back (for example, if the receiver is sent a rollback message) before a save operation, the object is unregistered from the receiver.
Availability
- Available in Mac OS X v10.4 and later.
See Also
Declared In
NSManagedObjectContext.hlock
Attempts to acquire a lock on the receiver.
- (void)lock
Discussion
This method blocks a thread’s execution until the lock can be acquired. An application protects a critical section of code by requiring a thread to acquire a lock before executing the code. Once the critical section is past, the thread relinquishes the lock by invoking unlock.
Sending this message to a managed object context helps the framework to understand the scope of a transaction in a multi-threaded environment. It is preferable to use the NSManagedObjectContext’s implementation of NSLocking instead using of a separate mutex object.
If you lock (or successfully tryLock) a managed object context, the thread in which the lock call is made must have a retain until it invokes unlock. If you do not properly retain a context in a multi-threaded environment, this will result in deadlock.
Availability
- Available in Mac OS X v10.4 and later.
See Also
Declared In
NSManagedObjectContext.hmergeChangesFromContextDidSaveNotification:
Merges the changes specified in a given notification.
- (void)mergeChangesFromContextDidSaveNotification:(NSNotification *)notification
Parameters
- notification
An instance of an
NSManagedObjectContextWillSaveNotificationnotification posted by another context.
Discussion
This method refreshes any objects which have been updated in the other context, faults in any newly-inserted objects, and invokes deleteObject:: on those which have been deleted.
Availability
- Available in Mac OS X v10.5 and later.
Declared In
NSManagedObjectContext.hmergePolicy
Returns the merge policy of the receiver.
- (id)mergePolicy
Return Value
The receiver’s merge policy.
Discussion
The default is NSErrorMergePolicy.
Availability
- Available in Mac OS X v10.4 and later.
Declared In
NSManagedObjectContext.hobjectRegisteredForID:
Returns the object for a specified ID, if the object is registered with the receiver.
- (NSManagedObject *)objectRegisteredForID:(NSManagedObjectID *)objectID
Parameters
- objectID
An object ID.
Return Value
The object for the specified ID if it is registered with the receiver, otherwise nil.
Availability
- Available in Mac OS X v10.4 and later.
See Also
Related Sample Code
Declared In
NSManagedObjectContext.hobjectWithID:
Returns the object for a specified ID.
- (NSManagedObject *)objectWithID:(NSManagedObjectID *)objectID
Parameters
- objectID
An object ID.
Return Value
The object for the specified ID.
Discussion
If the object is not registered in the context, it may be fetched or returned as a fault. This method always returns an object. The data in the persistent store represented by objectID is assumed to exist—if it does not, the returned object throws an exception when you access any property (that is, when the fault is fired). The benefit of this behavior is that it allows you to create and use faults, then create the underlying rows later or in a separate context.
Availability
- Available in Mac OS X v10.4 and later.
See Also
-
– objectRegisteredForID: -
– existingObjectWithID:error: -
– managedObjectIDForURIRepresentation: -
– URIRepresentation
Related Sample Code
Declared In
NSManagedObjectContext.hobtainPermanentIDsForObjects:error:
Converts to permanent IDs the object IDs of the objects in a given array.
- (BOOL)obtainPermanentIDsForObjects:(NSArray *)objects error:(NSError **)error
Parameters
- objects
An array of managed objects.
- error
If an error occurs, upon return contains an
NSErrorobject that describes the problem.
Return Value
YES if permanent IDs are obtained for all the objects in objects, otherwise NO.
Discussion
This method converts the object ID of each managed object in objects to a permanent ID. Although the object will have a permanent ID, it will still respond positively to isInserted until it is saved. Any object that already has a permanent ID is ignored.
Any object not already assigned to a store is assigned based on the same rules Core Data uses for assignment during a save operation (first writable store supporting the entity, and appropriate for the instance and its related items).
Special Considerations
This method results in a transaction with the underlying store which changes the file’s modification date.
This results an additional consideration if you invoke this method on the managed object context associated with an instance of NSPersistentDocument. Instances of NSDocument need to know that they are in sync with the underlying content. To avoid problems, after invoking this method you must therefore update the document’s modification date (using setFileModificationDate:).
Availability
- Available in Mac OS X v10.5 and later.
Declared In
NSManagedObjectContext.hpersistentStoreCoordinator
Returns the persistent store coordinator of the receiver.
- (NSPersistentStoreCoordinator *)persistentStoreCoordinator
Return Value
The persistent store coordinator of the receiver.
Availability
- Available in Mac OS X v10.4 and later.
Related Sample Code
Declared In
NSManagedObjectContext.hprocessPendingChanges
Forces the receiver to process changes to the object graph.
- (void)processPendingChanges
Discussion
This method causes changes to registered managed objects to be recorded with the undo manager.
In AppKit-based applications, this method is invoked automatically at least once during the event loop (at the end of the loop)—it may be called more often than that if the framework needs to coalesce your changes before doing something else. You can also invoke it manually to coalesce any pending unprocessed changes.
Availability
- Available in Mac OS X v10.4 and later.
See Also
Related Sample Code
Declared In
NSManagedObjectContext.hpropagatesDeletesAtEndOfEvent
Returns a Boolean that indicates whether the receiver propagates deletes at the end of the event in which a change was made.
- (BOOL)propagatesDeletesAtEndOfEvent
Return Value
YES if the receiver propagates deletes at the end of the event in which a change was made, NO if it propagates deletes only immediately before saving changes.
Availability
- Available in Mac OS X v10.4 and later.
See Also
Declared In
NSManagedObjectContext.hredo
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.
- (void)redo
Availability
- Available in Mac OS X v10.4 and later.
See Also
Declared In
NSManagedObjectContext.hrefreshObject:mergeChanges:
Updates the persistent properties of a managed object to use the latest values from the persistent store.
- (void)refreshObject:(NSManagedObject *)object mergeChanges:(BOOL)flag
Parameters
- object
A managed object.
- flag
A Boolean value.
If flag is
NO, then object is turned into a fault and any pending changes are lost. The object remains a fault until it is accessed again, at which time its property values will be reloaded from the store or last cached state.If flag is
YES, then object’s property values are reloaded from the values from the store or the last cached state then any changes that were made (in the local context) are re-applied over those (now newly updated) values. (If flag isYESthe merge of the values into object will always succeed—in this case there is therefore no such thing as a “merge conflict” or a merge that is not possible.)
Discussion
If the staleness interval (see stalenessInterval) has not been exceeded, any available cached data is reused instead of executing a new fetch. If flag is YES, this method does not affect any transient properties; if flag is NO, transient properties are released.
You typically use this method to ensure data freshness if more than one managed object context may use the same persistent store simultaneously, in particular if you get an optimistic locking failure when attempting to save.
It is important to note that turning object into a fault (flag is NO) also causes related managed objects (that is, those to which object has a reference) to be released, so you can also use this method to trim a portion of your object graph you want to constrain memory usage.
Availability
- Available in Mac OS X v10.4 and later.
See Also
Related Sample Code
Declared In
NSManagedObjectContext.hregisteredObjects
Returns the set of objects registered with the receiver.
- (NSSet *)registeredObjects
Return Value
The set of objects registered with the receiver.
Discussion
A managed object context does not post key-value observing notifications when the return value of registeredObjects changes.
Availability
- Available in Mac OS X v10.4 and later.
See Also
Declared In
NSManagedObjectContext.hreset
Returns the receiver to its base state.
- (void)reset
Discussion
All the receiver's managed objects are “forgotten.” If you use this method, you should ensure that you also discard references to any managed objects fetched using the receiver, since they will be invalid afterwards.
Availability
- Available in Mac OS X v10.4 and later.
See Also
Related Sample Code
Declared In
NSManagedObjectContext.hretainsRegisteredObjects
Returns a Boolean that indicates whether the receiver sends a retain message to objects upon registration.
- (BOOL)retainsRegisteredObjects
Return Value
YES if the receiver sends a retain message to objects upon registration, otherwise NO.
Availability
- Available in Mac OS X v10.4 and later.
See Also
Declared In
NSManagedObjectContext.hrollback
Removes everything from the undo stack, discards all insertions and deletions, and restores updated objects to their last committed values.
- (void)rollback
Discussion
This method does not refetch data from the persistent store or stores.
Availability
- Available in Mac OS X v10.4 and later.
See Also
Declared In
NSManagedObjectContext.hsave:
Attempts to commit unsaved changes to registered objects to their persistent store.
- (BOOL)save:(NSError **)error
Parameters
- error
A pointer to an
NSErrorobject. You do not need to create anNSErrorobject. The save operation aborts after the first failure if you passNULL.
Return Value
YES if the save succeeds, otherwise NO.
Discussion
If there were multiple errors (for example several edited objects had validation failures) the description of NSError returned indicates that there were multiple errors, and its userInfo dictionary contains the key NSDetailedErrors. The value associated with the NSDetailedErrors key is an array that contains the individual NSError objects.
Availability
- Available in Mac OS X v10.4 and later.
Related Sample Code
Declared In
NSManagedObjectContext.hsetMergePolicy:
Sets the merge policy of the receiver.
- (void)setMergePolicy:(id)mergePolicy
Parameters
- mergePolicy
The merge policy of the receiver. For possible values, see
“Merge Policies”.
Availability
- Available in Mac OS X v10.4 and later.
Declared In
NSManagedObjectContext.hsetPersistentStoreCoordinator:
Sets the persistent store coordinator of the receiver.
- (void)setPersistentStoreCoordinator:(NSPersistentStoreCoordinator *)coordinator
Parameters
- coordinator
The persistent store coordinator of the receiver.
Discussion
The coordinator provides the managed object model and handles persistency. Note that multiple contexts can share a coordinator.
This method raises an exception if coordinator is nil. If you want to “disconnect" a context from its persistent store coordinator, you should simply release all references to the context and allow it to be deallocated normally.
Availability
- Available in Mac OS X v10.4 and later.
Related Sample Code
Declared In
NSManagedObjectContext.hsetPropagatesDeletesAtEndOfEvent:
Sets whether the context propagates deletes to related objects at the end of the event.
- (void)setPropagatesDeletesAtEndOfEvent:(BOOL)flag
Parameters
- Flag
A Boolean value that indicates whether the context propagates deletes to related objects at the end of the event (
YES) or not (NO).
Discussion
The default is YES. If the value is NO, then deletes are propagated during a save operation.
Availability
- Available in Mac OS X v10.4 and later.
See Also
Declared In
NSManagedObjectContext.hsetRetainsRegisteredObjects:
Sets whether or not the receiver retains all registered objects, or only objects necessary for a pending save (those that are inserted, updated, deleted, or locked).
- (void)setRetainsRegisteredObjects:(BOOL)flag
Parameters
- flag
A Boolean value.
If flag is
NO, then registered objects are retained only when they are inserted, updated, deleted, or locked.If flag is
YES, then all registered objects are retained.
Discussion
The default is NO.
Availability
- Available in Mac OS X v10.4 and later.
See Also
Declared In
NSManagedObjectContext.hsetStalenessInterval:
Sets 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.
- (void)setStalenessInterval:(NSTimeInterval)expiration
Parameters
- expiration
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.
A negative value represents an infinite value;
0.0represents “no staleness acceptable”.
Discussion
The staleness interval controls whether fulfilling a fault uses data previously fetched by the application, or issues a new fetch (see also refreshObject:mergeChanges:). The staleness interval does not affect objects currently in use (that is, it is not used to automatically update property values from a persistent store after a certain period of time).
The expiration value is applied on a per object basis. It is the relative time until cached data (snapshots) should be considered stale. For example, a value of 300.0 informs the context to utilize cached information for no more than 5 minutes after an object was originally fetched.
Note that the staleness interval is a hint and may not be supported by all persistent store types. It is not used by XML and binary stores, since these stores maintain all current values in memory.
Availability
- Available in Mac OS X v10.4 and later.
See Also
Declared In
NSManagedObjectContext.hsetUndoManager:
Sets the undo manager of the receiver.
- (void)setUndoManager:(NSUndoManager *)undoManager
Parameters
- undoManager
The undo manager of the receiver.
Discussion
You can set the undo manager to nil to disable undo support. This provides a performance benefit if you do not want to support undo for a particular context, for example in a large import process—see Core Data Programming Guide.
If a context does not have an undo manager, you can enable undo support by setting one. You may also replace a context’s undo manager if you want to integrate the context’s undo operations with another undo manager in your application.
Important: On Mac OS X, a context provides an undo manager by default; on iPhone OS, the undo manager is nil by default.
Availability
- Available in Mac OS X v10.4 and later.
Declared In
NSManagedObjectContext.hstalenessInterval
Returns 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.
- (NSTimeInterval)stalenessInterval
Return Value
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.
Discussion
The default is infinite staleness, represented by an interval of -1 (although any negative value represents an infinite value); 0.0 represents “no staleness acceptable”.
For a full discussion, see setStalenessInterval:.
Availability
- Available in Mac OS X v10.4 and later.
See Also
Declared In
NSManagedObjectContext.htryLock
Attempts to acquire a lock.
- (BOOL)tryLock
Return Value
YES if a lock was acquired, NO otherwise.
Discussion
This method returns immediately after the attempt to acquire a lock.
Availability
- Available in Mac OS X v10.4 and later.
See Also
Declared In
NSManagedObjectContext.hundo
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.
- (void)undo
Availability
- Available in Mac OS X v10.4 and later.
See Also
-
– reset -
– rollback -
– undoManager -
– processPendingChanges
Declared In
NSManagedObjectContext.hundoManager
Returns the undo manager of the receiver.
- (NSUndoManager *)undoManager
Return Value
The undo manager of the receiver.
Discussion
For a discussion, see setUndoManager:.
Important: On Mac OS X, a context provides an undo manager by default; on iPhone OS, the undo manager is nil by default.
Availability
- Available in Mac OS X v10.4 and later.
Related Sample Code
Declared In
NSManagedObjectContext.hunlock
Relinquishes a previously acquired lock.
- (void)unlock
Availability
- Available in Mac OS X v10.4 and later.
See Also
Declared In
NSManagedObjectContext.hupdatedObjects
Returns the set of objects registered with the receiver that have uncommitted changes.
- (NSSet *)updatedObjects
Return Value
The set of objects registered with the receiver that have uncommitted changes.
Discussion
A managed object context does not post key-value observing notifications when the return value of updatedObjects changes. A context does, however, post a NSManagedObjectContextObjectsDidChangeNotification notification when a change is made, and a NSManagedObjectContextWillSaveNotification notification and a NSManagedObjectContextDidSaveNotification notification before and after changes are committed respectively.
Availability
- Available in Mac OS X v10.4 and later.
See Also
Declared In
NSManagedObjectContext.hConstants
NSManagedObjectContext Change Notification User Info Keys
Core Data uses these string constants as keys in the user info dictionary in aNSManagedObjectContextObjectsDidChangeNotification notification.
NSString * const NSInsertedObjectsKey; NSString * const NSUpdatedObjectsKey; NSString * const NSDeletedObjectsKey; NSString * const NSRefreshedObjectsKey; NSString * const NSInvalidatedObjectsKey; NSString * const NSInvalidatedAllObjectsKey;
Constants
NSInsertedObjectsKeyKey for the set of objects that were inserted into the context.
Available in Mac OS X v10.4 and later.
Declared in
NSManagedObjectContext.h.NSUpdatedObjectsKeyKey for the set of objects that were updated.
Available in Mac OS X v10.4 and later.
Declared in
NSManagedObjectContext.h.NSDeletedObjectsKeyKey for the set of objects that were marked for deletion during the previous event.
Available in Mac OS X v10.4 and later.
Declared in
NSManagedObjectContext.h.NSRefreshedObjectsKeyKey for the set of objects that were refreshed.
Available in Mac OS X v10.5 and later.
Declared in
NSManagedObjectContext.h.NSInvalidatedObjectsKeyKey for the set of objects that were invalidated.
Available in Mac OS X v10.5 and later.
Declared in
NSManagedObjectContext.h.NSInvalidatedAllObjectsKeyKey that specifies that all objects in the context have been invalidated.
Available in Mac OS X v10.5 and later.
Declared in
NSManagedObjectContext.h.
Declared In
NSManagedObjectContext.hMerge Policies
Merge policy constants define the way conflicts are handled during a save operation.
id NSErrorMergePolicy; id NSMergeByPropertyStoreTrumpMergePolicy; id NSMergeByPropertyObjectTrumpMergePolicy; id NSOverwriteMergePolicy; id NSRollbackMergePolicy;
Constants
NSErrorMergePolicyThis policy causes a save to fail if there are any merge conflicts.
In the case of failure, the save method returns with an error with a userInfo dictionary that contains the key
@"conflictList"; the corresponding value is an array of conflict records.Available in Mac OS X v10.4 and later.
Declared in
NSManagedObjectContext.h.NSMergeByPropertyStoreTrumpMergePolicyThis policy merges conflicts between the persistent store’s version of the object and the current in-memory version, giving priority to external changes.
The merge occurs by individual property. For properties that have been changed in both the external source and in memory, the external changes trump the in-memory ones.
Available in Mac OS X v10.4 and later.
Declared in
NSManagedObjectContext.h.NSMergeByPropertyObjectTrumpMergePolicyThis policy merges conflicts between the persistent store’s version of the object and the current in-memory version, giving priority to in-memory changes.
The merge occurs by individual property. For properties that have been changed in both the external source and in memory, the in-memory changes trump the external ones.
Available in Mac OS X v10.4 and later.
Declared in
NSManagedObjectContext.h.NSOverwriteMergePolicyThis policy overwrites state in the persistent store for the changed objects in conflict.
Changed objects’ current state is forced upon the persistent store.
Available in Mac OS X v10.4 and later.
Declared in
NSManagedObjectContext.h.NSRollbackMergePolicyThis policy discards in-memory state changes for objects in conflict.
The persistent store’s version of the objects’ state is used.
Available in Mac OS X v10.4 and later.
Declared in
NSManagedObjectContext.h.
Discussion
The default policy is the NSErrorMergePolicy. It is the only policy that requires action to correct any conflicts; the other policies make a save go through silently by making changes following their rules.
Declared In
NSManagedObjectContext.hThe following constants, defined in CoreDataErrors.h, relate to errors returned following validation failures or problems encountered during a save operation.
NSValidationObjectErrorKey | Key for the object that failed to validate for a validation error. |
NSAffectedStoresErrorKey | The key for stores prompting an error. |
NSAffectedObjectsErrorKey | The key for objects prompting an error. |
Each conflict record in the @"conflictList" array in the userInfo dictionary for an error from the NSErrorMergePolicy is a dictionary containing some of the keys described in the following table. Of the cachedRow, databaseRow, and snapshot keys, only two will be present depending on whether the conflict is between the managed object context and the persistent store coordinator (snapshot and cachedRow) or between the persistent store coordinator and the persistent store (cachedRow and databaseRow).
Notifications
NSManagedObjectContextObjectsDidChangeNotification
Posted when values of properties of objects contained in a managed object context are changed.
The notification is posted during processPendingChanges, after the changes have been processed, but before it is safe to call save: again (if you try, you will generate an infinite loop).
The notification object is the managed object context. The userInfo dictionary contains the following keys: NSInsertedObjectsKey, NSUpdatedObjectsKey, and NSDeletedObjectsKey.
Note that this notification is posted only when managed objects are changed; it is not posted when managed objects are added to a context as the result of a fetch.
Availability
- Available in Mac OS X v10.4 and later.
Declared In
NSManagedObjectContext.hNSManagedObjectContextDidSaveNotification
Posted whenever a managed object context completes a save operation.
The notification object is the managed object context. The userInfo dictionary contains the following keys: NSInsertedObjectsKey, NSUpdatedObjectsKey, and NSDeletedObjectsKey.
Availability
- Available in Mac OS X v10.4 and later.
Declared In
NSManagedObjectContext.hNSManagedObjectContextWillSaveNotification
Posted whenever a managed object context is about to perform a save operation.
The notification object is the managed object context. The userInfo dictionary contains the following keys: NSInsertedObjectsKey, NSUpdatedObjectsKey, and NSDeletedObjectsKey.
Availability
- Available in Mac OS X v10.6 and later.
Declared In
NSManagedObjectContext.h© 2009 Apple Inc. All Rights Reserved. (Last updated: 2009-10-06)
