I\'m having some trouble getting my Core Data entities to play nice and order when using an UITableView.
I\'ve been through a number of tutorials and other questions
Here a full solution how to manage an indexed table with core data. Your attribute is called displayOrder
, I call it index
.
First of all, you better separate view controller and model. For this I use a model controller, which is the interface between the view and the model.
There are 3 cases you need to manage that the user can influence via the view controller.
The first two cases Adding and Deleting are pretty straightforward. Delete calls a routine called renewObjectIndicesUpwardsFromIndex
in order to update the indices after the deleted object.
- (void)createObjectWithTitle:(NSString*)title {
FFObject* object = [FFObject insertIntoContext:self.managedObjectContext];
object.title = title;
object.index = [NSNumber numberWithInteger:[self numberTotalObjects]];
[self saveContext];
}
- (void)deleteObject:(FFObject*)anObject {
NSInteger objectIndex = [anObject.index integerValue];
[anObject deleteObject];
[self renewObjectIndicesUpwardsFromIndex:objectIndex];
[self saveContext];
}
- (void)renewObjectIndicesUpwardsFromIndex:(NSInteger)fromIndex {
NSFetchRequest* fetchRequest = [[NSFetchRequest alloc] init];
[fetchRequest setEntity:[NSEntityDescription entityForName:@"Object" inManagedObjectContext:self.managedObjectContext]];
NSPredicate* predicate = [NSPredicate predicateWithFormat:@"(index > %d)", fromIndex];
[fetchRequest setPredicate:predicate];
NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"index" ascending:YES];
NSArray *sortDescriptors = [NSArray arrayWithObjects:sortDescriptor, nil];
[fetchRequest setSortDescriptors:sortDescriptors];
NSError* fetchError = nil;
NSArray* objects = [self.managedObjectContext executeFetchRequest:fetchRequest error:&fetchError];
NSInteger index = fromIndex;
for (FFObject* object in objects) {
object.index = [NSNumber numberWithInteger:index];
index += 1;
}
[self saveContext];
}
Before I come to the controller routines for the re-order, here the part in the view controller. I use a bool isModifyingOrder
similar to this answer. Notice that the view controller calls two functions in the controller moveObjectOrderUp
and moveObjectOrderDown
. Depending on how you display the objects in the table view - newest first or newest last - you can switch them.
- (void)tableView:(UITableView *)tableView moveRowAtIndexPath:(NSIndexPath *)sourceIndexPath toIndexPath:(NSIndexPath *)destinationIndexPath {
isModifyingOrder = YES;
NSUInteger fromIndex = sourceIndexPath.row;
NSUInteger toIndex = destinationIndexPath.row;
if (fromIndex == toIndex) {
return;
}
FFObject *affectedObject = [self.fetchedResultsController.fetchedObjects objectAtIndex:fromIndex];
NSInteger delta;
if (fromIndex < toIndex) {
delta = toIndex - fromIndex;
NSLog(@"Moved down by %lu cells", delta);
[self.objectController moveObjectOrderUp:affectedObject by:delta];
} else {
delta = fromIndex - toIndex;
NSLog(@"Moved up by %lu cells", delta);
[self.objectController moveObjectOrderDown:affectedObject by:delta];
}
isModifyingOrder = NO;
}
And here the part in the controller. This can be written nicer, but for understanding this is maybe best.
- (void)moveObjectOrderUp:(FFObject*)affectedObject by:(NSInteger)delta {
NSInteger fromIndex = [affectedObject.index integerValue] - delta;
NSInteger toIndex = [affectedObject.index integerValue];
if (fromIndex < 1) {
return;
}
NSFetchRequest* fetchRequest = [[NSFetchRequest alloc] init];
[fetchRequest setEntity:[NSEntityDescription entityForName:@"Object" inManagedObjectContext:self.managedObjectContext]];
NSPredicate* predicate = [NSPredicate predicateWithFormat:@"(index >= %d) AND (index < %d)", fromIndex, toIndex];
[fetchRequest setPredicate:predicate];
NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"index" ascending:YES];
NSArray *sortDescriptors = [NSArray arrayWithObjects:sortDescriptor, nil];
[fetchRequest setSortDescriptors:sortDescriptors];
NSError* fetchError = nil;
NSArray* objects = [self.managedObjectContext executeFetchRequest:fetchRequest error:&fetchError];
for (FFObject* object in objects) {
NSInteger newIndex = [object.index integerValue] + 1;
object.index = [NSNumber numberWithInteger:newIndex];
}
affectedObject.index = [NSNumber numberWithInteger:fromIndex];
[self saveContext];
}
- (void)moveObjectOrderDown:(FFObject*)affectedObject by:(NSInteger)delta {
NSInteger fromIndex = [affectedObject.index integerValue];
NSInteger toIndex = [affectedObject.index integerValue] + delta;
NSFetchRequest* fetchRequest = [[NSFetchRequest alloc] init];
[fetchRequest setEntity:[NSEntityDescription entityForName:@"Object" inManagedObjectContext:self.managedObjectContext]];
NSPredicate* predicate = [NSPredicate predicateWithFormat:@"(index > %d) AND (index <= %d)", fromIndex, toIndex];
[fetchRequest setPredicate:predicate];
NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"index" ascending:YES];
NSArray *sortDescriptors = [NSArray arrayWithObjects:sortDescriptor, nil];
[fetchRequest setSortDescriptors:sortDescriptors];
NSError* fetchError = nil;
NSArray* objects = [self.managedObjectContext executeFetchRequest:fetchRequest error:&fetchError];
for (FFObject* object in objects)
{
NSInteger newIndex = [object.index integerValue] - 1;
object.index = [NSNumber numberWithInteger:newIndex];
}
affectedObject.index = [NSNumber numberWithInteger:toIndex];
[self saveContext];
}
Don't forget to use a second BOOL
in your view controller for the delete action to prevent the move notification to do anything. I call it isDeleting
and put it here.
- (void)controller:(NSFetchedResultsController *)controller didChangeObject:(id)anObject
atIndexPath:(NSIndexPath *)indexPath forChangeType:(NSFetchedResultsChangeType)type
newIndexPath:(NSIndexPath *)newIndexPath {
if (isModifyingOrder) return;
...
switch(type) {
...
case NSFetchedResultsChangeMove:
if (isDeleting == false) {
[self.tableView deleteRowsAtIndexPaths:[NSArray arrayWithObject:localIndexPath] withRowAnimation:UITableViewRowAnimationFade];
[self.tableView insertRowsAtIndexPaths:[NSArray arrayWithObject:localNewIndexPath]withRowAnimation:UITableViewRowAnimationFade];
}
break;
...
}
}
I think that:
affectedObject.displayOrderValue = toIndex;
must be placed after:
for (NSUInteger i = start; i <= end; i++) {
FFObject *otherObject = [self.fetchedResultsController.fetchedObjects objectAtIndex:i];
NSLog(@"Updated %@ / %@ from %i to %i", otherObject.name, otherObject.state, otherObject.displayOrderValue, otherObject.displayOrderValue + delta);
otherObject.displayOrderValue += delta;
}
and before:
[self FF_fetchResults];
I took a look at your code and this might work better:
- (void)tableView:(UITableView *)tableView moveRowAtIndexPath:(NSIndexPath *)fromIndexPath toIndexPath:(NSIndexPath *)toIndexPath {
NSUInteger fromIndex = fromIndexPath.row;
NSUInteger toIndex = toIndexPath.row;
if (fromIndex == toIndex) {
return;
}
FFObject *affectedObject = [self.fetchedResultsController.fetchedObjects objectAtIndex:fromIndex];
affectedObject.displayOrderValue = toIndex;
NSUInteger start, end;
int delta;
if (fromIndex < toIndex) {
// move was down, need to shift up
delta = -1;
start = fromIndex + 1;
end = toIndex;
} else { // fromIndex > toIndex
// move was up, need to shift down
delta = 1;
start = toIndex;
end = fromIndex - 1;
}
for (NSUInteger i = start; i <= end; i++) {
FFObject *otherObject = [self.fetchedResultsController.fetchedObjects objectAtIndex:i];
NSLog(@"Updated %@ / %@ from %i to %i", otherObject.name, otherObject.state, otherObject.displayOrderValue, otherObject.displayOrderValue + delta);
otherObject.displayOrderValue += delta;
}
[self FF_fetchResults];
}
(This is intended as as comment on gerry3's answer above, but I am not yet able to comment on other users' questions and answers.)
A small improvement for gerry3's - very elegant - solution. If I'm not mistaken, the line
otherObject.displayOrderValue += delta;
will actually perform pointer arithmetic if displayOrderValue
is not of primitive type. Which may not be what you want. Instead, to set the value of the entity, I propose:
otherObject.displayOrderValue = [NSNumber numberWithInt:[otherObject.displayOrderValue intValue] + delta];
This should update your entity property correctly and avoid any EXC_BAD_ACCESS crashes.