Can't understand why my tableview with GCD is crashing - iphone

Question

I've just finished re-writing this, and covered every conceivable angle I can think of. I don't know why this is crashing. Perhaps somebody could help me figure it out.
This is the cellForRowAtIndexPath code:
- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
static NSString *CellIdentifier = #"Cell";
JHomeViewCell *cell = (JHomeViewCell *)[tableView dequeueReusableCellWithIdentifier:CellIdentifier];
if (cell == nil) {
cell = [[JHomeViewCell alloc] initWithStyle:UITableViewCellStyleDefault
reuseIdentifier:CellIdentifier];
cell.delegate = self;
}
cell.cellContent.cellInfo = [self cellInfoForCellAtIndexPath:indexPath];
if (cell.cellContent.cellInfo.thumbnailsComplete == YES || cell.cellContent.cellInfo.thumbnailsBeingCreated == YES) {
[cell.cellContent setNeedsDisplay];
}
else {
[cell.cellContent setup];
}
return cell;
}
And in cellContent, there's this setup method:
-(void)setup {
[self setNeedsDisplay];
self.cellInfo.thumbnailsBeingCreated = YES;
NSManagedObjectID *entryID = self.cellInfo.objectID;
dispatch_queue_t cellSetupQueue = dispatch_queue_create("com.Journalized.SetupCell", NULL);
dispatch_async(cellSetupQueue, ^{
NSManagedObjectContext *newMoc = [[NSManagedObjectContext alloc] init];
NSPersistentStoreCoordinator *coordinator = [[CoreDataStore mainStore] context].persistentStoreCoordinator;
[newMoc setPersistentStoreCoordinator:coordinator];
NSNotificationCenter *notify = [NSNotificationCenter defaultCenter];
[notify addObserver:self
selector:#selector(mergeChanges:)
name:NSManagedObjectContextDidSaveNotification
object:newMoc];
Entry *entry = (Entry *)[newMoc objectWithID:entryID];
[newMoc save:nil];
int i = 0;
while (i < self.cellInfo.numberOfThumbnailsToDraw) {
NSLog(#"number of thumbnails: %i %i %i", self.cellInfo.numberOfThumbnailsToDraw, entry.media.count, i);
Media *media = [entry.media objectAtIndex:i];
UIImage *image = [media getThumbnail];
BOOL success = [newMoc save:nil];
//NSLog(#"time: %# success: %i", entry.entryTableInfo.creationTimeString, success);
[self.cellInfo.thumbnails setObject:image forKey:[NSNumber numberWithInt:i]];
i++;
}
[[NSNotificationCenter defaultCenter] removeObserver:self];
dispatch_async(dispatch_get_main_queue(), ^{
self.cellInfo.thumbnailsComplete = YES;
[self setNeedsDisplay];
});
});
dispatch_release(cellSetupQueue);
It crashes on the line:
Media *media = [entry.media objectAtIndex:i];
With the error:
index 1 beyond bounds [0 .. 0]
The NSLog above that...
NSLog(#"number of thumbnails: %i %i %i", self.cellInfo.numberOfThumbnailsToDraw, entry.media.count, i);
Gives the result:
number of thumbnails: 2 1 1
Which sort of explains the crash, except that value is set in the [cellInfoForCellAtIndexPath:]; method, like so:
cellInfo.numberOfMediaItems = entry.media.count;
cellInfo.numberOfThumbnailsToDraw = MIN(cellInfo.numberOfMediaItems, 3);
I really don't know where the problem is occurring, or why it's occurring, but I can't move on with my app until this part is fixed.

Answer 1

Well numberOfThumbnailsToDraw is 2 meaning the while loop will do 0, 1, but the count of your entry.media is only 1 so it only has a 0 index so of course it'll crash.

Answer 2

[managedObjectContext obtainPermanentIDsForObjects:self.cellInfo error:nil];
[managedObjectContext save:...];
NSManagedObjectID *entryID = self.cellInfo.objectID;
You need to make sure that
1. You have a permanent object ID; not a temporary one
2. The object is persisted so that it appears on the new MOC.

Answer 3

It looks like you are querying entry.media.count where entry is a pointer into one MOC, and then you are querying it from another. You are asking using the objectID, which is reasonable.
However, when the new MOC gets the object, it does not see the same values as you saw in the other MOC. Most likely, this means that you have not properly saved the other MOC.
What happens if you execute a fetch for the object on the new MOC?
Also, I would enable core data debugging (-com.apple.CoreData.SQLDebug 1) in command line options. This will log to the console what's going on underneath. You should see the SQL statements for the underlying database logged to the console.
Also, you are saving your MOC without ever making any changes to it, which leads me to believe you are a bit confused on how your many MOCs are working together.