NSRangeException when using Core Data Asynchronous UISearchDisplayController

Question

I'm asynchronously fetching data, and I've used this as a guide: http://deeperdesign.wordpress.com/2011/05/30/cancellable-asynchronous-searching-with-uisearchdisplaycontroller/

In

- (BOOL)searchDisplayController:(UISearchDisplayController *)controller shouldReloadTableForSearchString:(NSString *)searchString{    
//setup request / predicate etc...            
[self.searchQueue addOperationWithBlock:^{
             NSError *error;             
             self.matchingObjects = [self.managedObjectContext executeFetchRequest:request error:&error];
             [request release];

             [[NSOperationQueue mainQueue] addOperationWithBlock:^
              {
                  [self.searchDisplayController.searchResultsTableView reloadData];
              }];

     }];            
    // Return YES to cause the search result table view to be reloaded.
    return NO;
}

- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section{    
        // Return the number of rows in the section.
        return [self.matchingObjects count];
}

Every now and then I'll get something to the effect of:

*** Terminating app due to uncaught exception 'NSRangeException', reason: '*** -[NSArray objectAtIndex:]: index 0 beyond bounds for empty array'

This is thrown at the matchingObjects ivar when accessing it to construct the table cell in:

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath

The crash doesn't occur all the time, just seems to happen on random occasions. I'm guessing that somewhere the count on the matchingObjects array is returning a certain value, which changes and is not being updated.

I'm not entirely sure of how to deal with this - been looking over this for hours, is there something I'm missing?

Answer 1

I figured out what it was - took me a while, but I looked again at the example that I just linked. I was updating the self.matchingObjects iVar in the background thread, which on some occasions caused a mismatch between the range of the array available in the main thread and the background thread. So for example, the variable may have been updated in the background thread, and the main thread may still be accessing a part of the range that no longer exists in the variable since it was updated.

Fixed it by amending my code as follows:

 [self.searchQueue addOperationWithBlock:^
 {
    NSError *error;

    NSArray *results = [self.managedObjectContext executeFetchRequest:request error:&error];
    [request release];

    [[NSOperationQueue mainQueue] addOperationWithBlock:^
    {
       self.matchingObjects = results;
       [self.searchDisplayController.searchResultsTableView reloadData];
    }];

}];

Now the results of the search are loaded into a temporary holding array named "results", and the matchingObjects iVar is first updated in the main thread and then the tableView is reloaded. This way, the tableView always is referring to an array that is never changed whilst it is being accessed, since the tableView relies on matchingObjects to get the number of rows and data.