How to use NSFetchedResultsController and UISearchDisplayController

Question

I've been creating an iPhone App using Core Data.

First of all, does it make sense to use an NSFetchedResultsController and a UISearchDisplayController together to fetch the result? Would you recommend something else?

I've been trying quite long combining an NSFetchedResultController and a UISearchDisplayController. I've been thinking of setting an NSPredicate in the (BOOL)searchDisplayController:(UISearchDisplayController *)controller shouldReloadTableForSearchScope:(NSInteger)searchOption method to the NSFetchedResultController of the UIViewController. But this isn't working that well.

So, have you any idea how to implement a solution for my problem? Thank you already for posting answers or links to good tutorials.

EDIT

Here is my code. The UISearchDisplayDelegate methods call the method (void)filterContentForSearchText:(NSString*)searchText scope:(NSString*)scope that should set the predicate in the NSFetchedResultController. I've also added the code of the NSFetchedResultController.

- (BOOL)searchDisplayController:(UISearchDisplayController *)controller shouldReloadTableForSearchString:(NSString *)searchString
{
    [self filterContentForSearchText:searchString scope:
     [[self.searchDisplayController.searchBar scopeButtonTitles] objectAtIndex:[self.searchDisplayController.searchBar selectedScopeButtonIndex]]];

    // Return YES to cause the search result table view to be reloaded.
    return YES;
}


- (BOOL)searchDisplayController:(UISearchDisplayController *)controller shouldReloadTableForSearchScope:(NSInteger)searchOption
{
    [self filterContentForSearchText:[self.searchDisplayController.searchBar text] scope:
     [[self.searchDisplayController.searchBar scopeButtonTitles] objectAtIndex:searchOption]];

    // Return YES to cause the search result table view to be reloaded.
    return YES;
}


- (void)filterContentForSearchText:(NSString*)searchText scope:(NSString*)scope
{
    NSString *query = self.searchDisplayController.searchBar.text;
    if (query && query.length) {
        NSPredicate *predicate = [NSPredicate predicateWithFormat:@"Name contains[cd] %@", query];
        [fetchedResultsController.fetchRequest setPredicate:predicate];
    }

    NSError *error = nil;
    if (![[self fetchedResultsController] performFetch:&error]) {
        // Handle error
        NSLog(@"Unresolved error %@, %@", error, [error userInfo]);
        exit(-1);  // Fail
    }  

}


- (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:@"Event" inManagedObjectContext: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:@"timeStamp" 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:managedObjectContext sectionNameKeyPath:nil cacheName:@"Root"];
    aFetchedResultsController.delegate = self;
    self.fetchedResultsController = aFetchedResultsController;

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

    return fetchedResultsController;
} 

Answer 1

Glancing at your code, its right there. I ran into issues using this method for changing the search. The fix: Clear your cache!

Say you initiated your fetched result controller as follows. Note the cacheName property.

    NSFetchedResultsController *aFetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:[self fetchRequest] managedObjectContext:managedObjectContext sectionNameKeyPath:nil cacheName:@"UserSearch"];

As you change your predicate, simply add:

 [NSFetchedResultsController deleteCacheWithName:@"UserSearch"];

Like magic, it will work nicely.

You will also want to monitor when the search display controller will end its search, and wipe your predicate.

Although this thread looks dead, I found it on google, so perhaps someone out there will get use of this. :)

Answer 2

According to the docs, you'll either want to filter the results into an array using a predicate or reinitialize the fetchedResultsController.

Important: You must not modify the fetch request after you have initialized the controller. For example, you must not change the predicate or the sort orderings.

-- NSFetchedResultsController

(I'd comment on idle's answer, but I'm karma-less.)

Answer 3

It sounds like you are just interested in fetching and displaying objects. In this case, you do NOT use a search display controller. The search display controller is used with a search bar to help with allowing user-entered text searches of your content.

Typically you use a fetched results controller to aid in your implementation of the table view datasource and delegate methods. Also, you create and use a fetch request for your fetched results controller. This is all boiler-plate (see the template code provided by Apple when you create a new project with the "Use Core Data for storage" option selected).

It is in the fetch request that you can create and specify a predicate. This will allow you to filter the objects that your table displays. For example:

// only fetch objects with myAttribute set to someValue
NSPredicate *pred = [NSPredicate predicateWithFormat:@"myAttribute == %@",someValue];
[fetchRequest setPredicate:pred];