| Framework | /System/Library/Frameworks/CoreData.framework |
| Declared in | CoreData/NSFetchedResultsController.h |
| Companion guide |
An instance of NSFetchedResultsController uses methods in this protocol to notify its delegate that the controller’s fetch results have been changed due to an add, remove, move, or update operations.
You should consider carefully whether you want to update the table view as each change is made. If a large number of modifications are made simultaneously—for example, if you are reading data from a background thread—it may be computationally expensive to animate all the changes. Rather than responding to changes individually (as illustrated in “Typical Use”), you could just implement controllerDidChangeContent: (which is sent to the delegate then all pending changes have been processed) to reload the table view.
The fetched results controller reports changes to its section before changes to the fetched objects themselves.
You can use controllerWillChangeContent: and controllerDidChangeContent: to bracket updates to a table view whose content is provided by the fetched results controller as illustrated in the following example:
/* |
Assume self has a property 'tableView' -- as is the case for an instance of a UITableViewController |
subclass -- and a method configureCell:atIndexPath: which updates the contents of a given cell |
with information from a managed object at the given index path in the fetched results controller. |
*/ |
- (void)controllerWillChangeContent:(NSFetchedResultsController *)controller { |
[self.tableView beginUpdates]; |
} |
- (void)controller:(NSFetchedResultsController *)controller didChangeSection:(id <NSFetchedResultsSectionInfo>)sectionInfo |
atIndex:(NSUInteger)sectionIndex forChangeType:(NSFetchedResultsChangeType)type { |
switch(type) { |
case NSFetchedResultsChangeInsert: |
[self.tableView insertSections:[NSIndexSet indexSetWithIndex:sectionIndex] |
withRowAnimation:UITableViewRowAnimationFade]; |
break; |
case NSFetchedResultsChangeDelete: |
[self.tableView deleteSections:[NSIndexSet indexSetWithIndex:sectionIndex] |
withRowAnimation:UITableViewRowAnimationFade]; |
break; |
} |
} |
- (void)controller:(NSFetchedResultsController *)controller didChangeObject:(id)anObject |
atIndexPath:(NSIndexPath *)indexPath forChangeType:(NSFetchedResultsChangeType)type |
newIndexPath:(NSIndexPath *)newIndexPath { |
UITableView *tableView = self.tableView; |
switch(type) { |
case NSFetchedResultsChangeInsert: |
[tableView insertRowsAtIndexPaths:[NSArray arrayWithObject:newIndexPath] |
withRowAnimation:UITableViewRowAnimationFade]; |
break; |
case NSFetchedResultsChangeDelete: |
[tableView deleteRowsAtIndexPaths:[NSArray arrayWithObject:indexPath] |
withRowAnimation:UITableViewRowAnimationFade]; |
break; |
case NSFetchedResultsChangeUpdate: |
[self configureCell:[tableView cellForRowAtIndexPath:indexPath] |
atIndexPath:indexPath]; |
break; |
case NSFetchedResultsChangeMove: |
[tableView deleteRowsAtIndexPaths:[NSArray arrayWithObject:indexPath] |
withRowAnimation:UITableViewRowAnimationFade]; |
[tableView insertRowsAtIndexPaths:[NSArray arrayWithObject:newIndexPath] |
withRowAnimation:UITableViewRowAnimationFade]; |
break; |
} |
} |
- (void)controllerDidChangeContent:(NSFetchedResultsController *)controller { |
[self.tableView endUpdates]; |
} |
In general, NSFetchedResultsController is designed to respond to changes at the model layer. If you allow a user to reorder table rows, then your implementation of the delegate methods must take this into account.
Typically, if you allow the user to reorder table rows, your model object has an attribute that specifies its index. When the user moves a row, you update this attribute accordingly. This, however, has the side effect of causing the controller to notice the change, and so inform its delegate of the update (using controller:didChangeObject:atIndexPath:forChangeType:newIndexPath:). If you simply use the implementation of this method shown in “Typical Use,” then the delegate attempts to update the table view. The table view, however, is already in the appropriate state because of the user’s action.
In general, therefore, if you support user-driven updates, you should set a flag if a move is initiated by the user. In the implementation of your delegate methods, if the flag is set, you bypass main method implementations; for example:
- (void)controller:(NSFetchedResultsController *)controller didChangeObject:(id)anObject |
atIndexPath:(NSIndexPath *)indexPath forChangeType:(NSFetchedResultsChangeType)type |
newIndexPath:(NSIndexPath *)newIndexPath { |
if (!changeIsUserDriven) { |
UITableView *tableView = self.tableView; |
// Implementation continues... |
NSFetchedResultsController does not support sections being deleted as a result of a UI-driven change.Notifies the receiver that a fetched object has been changed due to an add, remove, move, or update.
- (void)controller:(NSFetchedResultsController *)controller didChangeObject:(id)anObject atIndexPath:(NSIndexPath *)indexPath forChangeType:(NSFetchedResultsChangeType)type newIndexPath:(NSIndexPath *)newIndexPath
The fetched results controller that sent the message.
The object in controller’s fetched results that changed.
The index path of the changed object (this value is nil for insertions).
The type of change. For valid values see “Results Object Change Types.”
The destination path for the object for insertions or moves (this value is nil for a deletion).
The fetched results controller reports changes to its section before changes to the fetch result objects.
Changes are reported with the following heuristics:
On add and remove operations, only the added/removed object is reported.
It’s assumed that all objects that come after the affected object are also moved, but these moves are not reported.
A move is reported when the changed attribute on the object is one of the sort descriptors used in the fetch request.
An update of the object is assumed in this case, but no separate update message is sent to the delegate.
An update is reported when an object’s state changes, but the changed attributes aren’t part of the sort keys.
This method may be invoked many times during an update event (for example, if you are importing data on a background thread and adding them to the context in a batch). You should consider carefully whether you want to update the table view on receipt of each message.
Notifies the receiver of the addition or removal of a section.
- (void)controller:(NSFetchedResultsController *)controller didChangeSection:(id <NSFetchedResultsSectionInfo>)sectionInfo atIndex:(NSUInteger)sectionIndex forChangeType:(NSFetchedResultsChangeType)type
The fetched results controller that sent the message.
The section that changed.
The index of the changed section.
The type of change (insert or delete). Valid values are NSFetchedResultsChangeInsert and NSFetchedResultsChangeDelete.
The fetched results controller reports changes to its section before changes to the fetched result objects.
This method may be invoked many times during an update event (for example, if you are importing data on a background thread and adding them to the context in a batch). You should consider carefully whether you want to update the table view on receipt of each message.
Notifies the receiver that the fetched results controller has been completed processing of one or more changes due to an add, remove, move, or update.
- (void)controllerDidChangeContent:(NSFetchedResultsController *)controller
The fetched results controller that sent the message.
This method is invoked after all invocations of controller:didChangeObject:atIndexPath:forChangeType:newIndexPath: and controller:didChangeSection:atIndexPath:forChangeType: have been sent for a given change event (such as the controller receiving a NSManagedObjectContextDidSaveNotification notification).
Notifies the receiver that the fetched results controller is about to start processing of one or more changes due to an add, remove, move, or update.
- (void)controllerWillChangeContent:(NSFetchedResultsController *)controller
The fetched results controller that sent the message.
This method is invoked before all invocations of controller:didChangeObject:atIndexPath:forChangeType:newIndexPath: and controller:didChangeSection:atIndexPath:forChangeType: have been sent for a given change event (such as the controller receiving a NSManagedObjectContextDidSaveNotification notification).
Specify types of change.
enum {
NSFetchedResultsChangeInsert = 1,
NSFetchedResultsChangeDelete = 2,
NSFetchedResultsChangeMove = 3,
NSFetchedResultsChangeUpdate = 4
};
NSFetchedResultsChangeInsertSpecifies that an object was inserted.
NSFetchedResultsChangeDeleteSpecifies that an object was deleted.
NSFetchedResultsChangeMoveSpecifies that an object was moved.
NSFetchedResultsChangeUpdateSpecifies that an object was changed.
A type to specify types of change.
typedef NSUInteger NSFetchedResultsChangeType;
For valid values, see “Results Object Change Types.”
Last updated: 2009-11-17
iPhone OS Reference Library Data Management NSFetchedResultsControllerDelegate Protocol Reference