Change NSFetchedResultsController on SegmentedControl change

Question

I've got a UITableView which is filled using a Core Data NSFetchedResultsController. I've now added a UISegmentedControl to the view and when you change the current segment, I'd like the contents of the tableview to change.

I've read around online that it would be wise to use two different NSFetchedResultsControllers as then I can benefit from the built-in caching. Only problem is I can't seem to find any sample code for doing this and don't know where to begin.

Can anyone explain where to begin in creating a second NSFetchedResultsController and changing which sources the tableview based on the segmented control?

View header code:

#import <CoreData/CoreData.h>

@interface DomainViewController : UIViewController <NSFetchedResultsControllerDelegate, UITableViewDataSource, UITableViewDelegate> {

    UITableView *domainView;
    UISegmentedControl *segmentedControl;
    NSString *domain;

}

@property (nonatomic, retain) NSFetchedResultsController *fetchedResultsController;
@property (nonatomic, retain) NSManagedObjectContext *managedObjectContext;
@property (nonatomic, retain) NSString *domain;

@property (nonatomic, retain) IBOutlet UITableView *domainView;
@property (nonatomic, retain) IBOutlet UISegmentedControl *segmentedControl;

- (IBAction)segmentedControlIndexChanged;

View implementation code:

@interface DomainViewController ()
- (void)configureCell:(UITableViewCell *)cell atIndexPath:(NSIndexPath *)indexPath;
@end

@implementation DomainViewController

@synthesize fetchedResultsController = __fetchedResultsController;
@synthesize managedObjectContext = __managedObjectContext;
@synthesize domain;
@synthesize domainView;
@synthesize segmentedControl;

- (void)viewDidLoad
{
    [super viewDidLoad];

    if (self.managedObjectContext == nil) 
    { 
        self.managedObjectContext = [(GARankingsAppDelegate *)[[UIApplication sharedApplication] delegate] managedObjectContext]; 
    }

}

- (IBAction)segmentedControlIndexChanged
{
    switch(self.segmentedControl.selectedSegmentIndex){
        case 0:
            break;
        case 1:
            break;
        default:
            break;
    }
}

- (void)configureCell:(UITableViewCell *)cell atIndexPath:(NSIndexPath *)indexPath
{
    NSManagedObject *managedObject = [self.fetchedResultsController objectAtIndexPath:indexPath];
    cell.textLabel.text = [[managedObject valueForKey:@"Keyphrase"] description];
}

#pragma mark - Table view data source

- (NSInteger)numberOfSectionsInTableView:(UITableView *)tableView
{
    return [[self.fetchedResultsController sections] count];
}

- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section
{
    id <NSFetchedResultsSectionInfo> sectionInfo = [[self.fetchedResultsController sections] objectAtIndex:section];
    return [sectionInfo numberOfObjects];
}

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
    static NSString *CellIdentifier = @"Cell";

    UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier];
    if (cell == nil) {
        cell = [[[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:CellIdentifier] autorelease];
    }

    // Configure the cell.
    [self configureCell:cell atIndexPath:indexPath];
    return cell;
}

- (NSFetchedResultsController *)fetchedResultsController
{
    if (__fetchedResultsController != nil)
    {
        return __fetchedResultsController;
    }

    /*
     Set up the fetched results controller.
     */
    // Create the fetch request for the entity.
    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
    // Edit the entity name as appropriate.
    NSEntityDescription *entity = [NSEntityDescription entityForName:@"Result" inManagedObjectContext:self.managedObjectContext];
    [fetchRequest setEntity:entity];

    // Set the batch size to a suitable number.
    [fetchRequest setFetchBatchSize:20];

    // Edit the sort key as appropriate.
    NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"Keyphrase" ascending:NO];
    NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:sortDescriptor, nil];

    [fetchRequest setSortDescriptors:sortDescriptors];

    // Edit the section name key path and cache name if appropriate.
    // nil for section name key path means "no sections".
    NSFetchedResultsController *aFetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:self.managedObjectContext sectionNameKeyPath:nil cacheName:@"Root"];
    aFetchedResultsController.delegate = self;
    self.fetchedResultsController = aFetchedResultsController;

    [aFetchedResultsController release];
    [fetchRequest release];
    [sortDescriptor release];
    [sortDescriptors release];

    NSError *error = nil;
    if (![self.fetchedResultsController performFetch:&error])
    {
        /*
         Replace this implementation with code to handle the error appropriately.

         abort() causes the application to generate a crash log and terminate. You should not use this function in a shipping application, although it may be useful during development. If it is not possible to recover from the error, display an alert panel that instructs the user to quit the application by pressing the Home button.
         */
        NSLog(@"Unresolved error %@, %@", error, [error userInfo]);
        abort();
    }

    return __fetchedResultsController;
}

Any help is much appreciated. Thanks.

UPDATE: updated code

View header code:

@property (nonatomic, retain) NSFetchedResultsController *currentFetchedResultsController;
@property (nonatomic, retain) NSFetchedResultsController *competitorFetchedResultsController;
@property (nonatomic, retain) NSFetchedResultsController *keyphraseFetchedResultsController;

View implementation code:

@synthesize currentFetchedResultsController = __fetchedResultsController;
@synthesize competitorFetchedResultsController;
@synthesize keyphraseFetchedResultsController;

- (NSFetchedResultsController *)fetchedResultsController
{
    if (__fetchedResultsController != nil)
    {
        return __fetchedResultsController;
    }

    /*
     Set up the fetched results controller.
     */
    // Create the fetch request for the entity.
    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
    // Edit the entity name as appropriate.
    NSEntityDescription *entity = [NSEntityDescription entityForName:@"Result" inManagedObjectContext:self.managedObjectContext];
    [fetchRequest setEntity:entity];

    // Set the batch size to a suitable number.
    [fetchRequest setFetchBatchSize:20];

    // Edit the sort key as appropriate.
    NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"Keyphrase" ascending:NO];
    NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:sortDescriptor, nil];

    [fetchRequest setSortDescriptors:sortDescriptors];

    // Edit the section name key path and cache name if appropriate.
    // nil for section name key path means "no sections".
    NSFetchedResultsController *aFetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:self.managedObjectContext sectionNameKeyPath:nil cacheName:@"Root"];
    aFetchedResultsController.delegate = self;
    //self.fetchedResultsController = aFetchedResultsController;
    self.currentFetchedResultsController = aFetchedResultsController;

    [aFetchedResultsController release];
    [fetchRequest release];
    [sortDescriptor release];
    [sortDescriptors release];

    NSError *error = nil;
    //if (![self.fetchedResultsController performFetch:&error])
    if (![self.currentFetchedResultsController performFetch:&error])
    {
        /*
         Replace this implementation with code to handle the error appropriately.

         abort() causes the application to generate a crash log and terminate. You should not use this function in a shipping application, although it may be useful during development. If it is not possible to recover from the error, display an alert panel that instructs the user to quit the application by pressing the Home button.
         */
        NSLog(@"Unresolved error %@, %@", error, [error userInfo]);
        abort();
    }

    return __fetchedResultsController;
} 

Answer 1

You could add another instance variable that represents the currently selected NSFetchedResultsController. And when the UISegmentedControl changes update this ivar.

this could be the action that is triggered by the valuechange event of the segment

- (IBAction *)segmentChanged:(UISegmentedControl *)sender {
    if ([sender selectedSegmentIndex] == 0) {
        self.currentFetchedResultsController = self.nsfrc1;
        [self.tableView reloadData];
    }
    else if ([sender selectedSegmentIndex] == 1) {
        self.currentFetchedResultsController = self.nsfrc2;     
        [self.tableView reloadData];
    }
}

one UITableViewDataSource method as example:

- (NSInteger)numberOfSectionsInTableView:(UITableView *)tableView {
    return [[self.currentFetchedResultsController sections] count];
}

and you have to make sure that only the current nsfrc triggers a tableview update in the NSFetchedResultsControllerDelegate methods. So you have to change all of them too.

- (void)controllerWillChangeContent:(NSFetchedResultsController *)controller {
    if (controller == self.currentFetchedResultsController) {
        [self.tableView beginUpdates];
    } 
}

---

EDIT: Nope, you are doing it wrong. the currentFetchedResultsController is just an ivar, without a lazy loading getter. It's "just" a pointer to the currently used controller.

But the two other fetchedResultsControllers should have such a lazy loading getter.

- (NSFetchedResultsController *)competitorFetchedResultsController {
    if (!myCompetitorFetchedResultsController) {
        // create competitorFetchedResultsController
    }
    return myCompetitorFetchedResultsController;
}

- (NSFetchedResultsController *)keyphraseFetchedResultsController {
    if (!myKeyphraseFetchedResultsController) {
        // create keyphraseFetchedResultsController
    }
    return myKeyphraseFetchedResultsController;
}

and then switch with:

self.currentFetchedResultsController = self.keyphraseFetchedResultsController;

or

self.currentFetchedResultsController = self.competitorFetchedResultsController;