iOS - Thread does not going back to the main thread - iphone

Question

I having trouble with my threads.
After i segue a couple of times between 2 screen when the thread is busy. The thread don't perform every line.., The breakpoint just disappear when it has to return to the main thread.
Can somebody please help me ?
I release the thread when the view is unload.
Thanks,
- (void)fetchFeedDataIntoDocument
{
NSString * labelString = [NSString stringWithFormat:#"Feed Fetcher %#", self.pageTitle];
const char *label = [labelString UTF8String];
self.fetchF = dispatch_queue_create(label, NULL);
dispatch_async(self.fetchF, ^{
NSArray *feeds = [FeedFetcher getDataForJson:self.pageTitle downloadBy:#"up"];
NSDictionary *lastfeed;
AppDelegate *appDelegate = [[UIApplication sharedApplication] delegate];
NSManagedObjectContext *context = [appDelegate getManagedObjectContext];
if ([feeds count] > 0)
{
lastfeed = [feeds objectAtIndex:0];
[FeedFetcher setLastUpdateIdToCatgorie:self.pageTitle WithId:[lastfeed objectForKey:#"id"] AndPulishDate:[lastfeed objectForKey:#"publish_up"]];
}
for (NSDictionary *feedInfo in feeds) {
[Feed FeedWithInfo:feedInfo InManageObject:context];
}
NSError *error = nil;
[context save:&error];
if (error){
NSLog(#"Error save : %#", error);}
dispatch_async(dispatch_get_main_queue(), ^{
[self setupFetchedResultsController];
[self.tableView reloadData];
[self downloadImagesForFeeds:feeds];
});
});

Answer 1

You are accessing the managedObjectContext from a different thread from where it was created. This is Core Data Threading Rule #1.
You are getting the MOC from the app delegate. If it's the normal Xcode-generated MOC, then it is created with thread-confinement concurrency. You can't even call performBlock with it. You can only access that MOC from the main thread. Period. Anything else is playing with fire, at best.
If you want to do all the work in a separate thread, you need a separate MOC as well. Like this (just typed - not compiled)...
NSManagedObjectContext *moc = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSPrivateQueueConcurrencyType];
moc.parentContext = appDelegate.managedObjectContext;
[moc performBlock:^{
// Go get your remote data and whatever you want to do
// Calling save on this MOC will push the data up into the "main" MOC
// (though it is now in the main MOC it has not been saved to the store).
[moc save:&error];
}];
Which would translate into something like this...
- (void)fetchFeedDataIntoDocument
{
NSString * labelString = [NSString stringWithFormat:#"Feed Fetcher %#", self.pageTitle];
const char *label = [labelString UTF8String];
AppDelegate *appDelegate = [[UIApplication sharedApplication] delegate];
NSManagedObjectContext *mainContext = [appDelegate getManagedObjectContext];
NSManagedObjectContext *context = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSPrivateQueueConcurrencyType];
context.parentContext = mainContext;
[context performBlock:^{
NSArray *feeds = [FeedFetcher getDataForJson:self.pageTitle downloadBy:#"up"];
NSDictionary *lastfeed;
if ([feeds count] > 0)
{
lastfeed = [feeds objectAtIndex:0];
[FeedFetcher setLastUpdateIdToCatgorie:self.pageTitle WithId:[lastfeed objectForKey:#"id"] AndPulishDate:[lastfeed objectForKey:#"publish_up"]];
}
for (NSDictionary *feedInfo in feeds) {
[Feed FeedWithInfo:feedInfo InManageObject:context];
}
NSError *error = nil;
[context save:&error];
if (error){
NSLog(#"Error save : %#", error);}
DO you really want to continue on error?
dispatch_async(dispatch_get_main_queue(), ^{
// Data has been pushed into main context from the background
// but it still needs to be saved to store...
// Do not forget to perform error handling...
NSError *error = nil;
[mainContext save:&error];
[self setupFetchedResultsController];
[self.tableView reloadData];
[self downloadImagesForFeeds:feeds];
});
});
EDIT
The code generated by Xcode for creating the MOC uses init, which uses NSConfinementConcurrencyType. You can replace it with MainConcurrency, without any problems, but get several benefits.
In your app delegate file, replace...
__managedObjectContext = [[NSManagedObjectContext alloc] init];
with this...
__managedObjectContext = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSMainQueueConcurrencyType];
Now, your main MOC can be "parented" and you can also call performBlock on it as well.

Answer 2

what about doing this...
-(void)functionToCallFetch {
[self performSelectorInBackground:#selector(fetchFeedDataIntoDocument) withObject:nil];
}
- (void)fetchFeedDataIntoDocument
{
NSArray *feeds = [FeedFetcher getDataForJson:self.pageTitle downloadBy:#"up"];
NSDictionary *lastfeed;
AppDelegate *appDelegate = [[UIApplication sharedApplication] delegate];
NSManagedObjectContext *context = [appDelegate getManagedObjectContext];
if ([feeds count] > 0)
{
lastfeed = [feeds objectAtIndex:0];
[FeedFetcher setLastUpdateIdToCatgorie:self.pageTitle WithId:[lastfeed objectForKey:#"id"] AndPulishDate:[lastfeed objectForKey:#"publish_up"]];
}
for (NSDictionary *feedInfo in feeds) {
[Feed FeedWithInfo:feedInfo InManageObject:context];
}
NSError *error = nil;
[context save:&error];
if (error){
NSLog(#"Error save : %#", error);}
//dispatch_async(dispatch_get_main_queue(), ^{
// [self setupFetchedResultsController];
// [self.tableView reloadData];
// [self downloadImagesForFeeds:feeds];
//});
[self performSelectorOnMainThread:#selector(setupFetchedResultsController) withObject:nil waitUntilDone:NO];
[self.tableView performSelectorOnMainThread:#selector(reloadData) withObject:nil waitUntilDone:NO];
[self performSelectorOnMainThread:#selector(downloadImagesForFeeds:) withObject:feeds waitUntilDone:NO];
}
Maybe that would work better?

Answer 3

Change following code..
dispatch_queue_t queue1 = dispatch_queue_create("com.MyApp.AppTask",NULL);
dispatch_queue_t main = dispatch_get_main_queue();
dispatch_async(queue1,
^{
dispatch_async(main,
^{
[self setupFetchedResultsController];
[self.tableView reloadData];
[self downloadImagesForFeeds:feeds];
});
});
Hope, this will help you

Answer 4

Did you try building a method like :
- (void)methodToBePerformedOnMainThread{
[self setupFetchedResultsController];
[self.tableView reloadData];
[self downloadImagesForFeeds:feeds];
}
and call it with
[self performSelectorOnMainThread:#selector(methodToBePerformedOnMainThread) withObject:nil waitUntilDone:NO];
at the end of fetchFeedDataIntoDocument
Edit :
Did you try to wrap it with NSOperationQueue in place of dispatch_queue ?
Like :
NSOperationQueue *operationQueue = [NSOperationQueue new];
NSInvocationOperation *operation = [[NSInvocationOperation alloc] initWithTarget:self selector:#selector(fetchFeedDataIntoDocument) object:nil];
if(operation != nil){
[operationQueue addOperation:operation];