I have a routine that fetches RSS entries in the background and insert these in my NSManagedObjectContext if not already there.
My problem is that this object doesn't find duplicates or crashes, depending on which NSManagedObjectContext I use... Help me, please.
Here's the simplified .h
@interface AsyncFetchEngine : NSObject <NSXMLParserDelegate,NSFetchedResultsControllerDelegate>
@property (strong, nonatomic) dispatch_queue_t rssParserQueue;
@property (strong, nonatomic) NSMutableArray *uRLsToFetch;
@property (strong, nonatomic) NSManagedObjectContext *managedObjectContext;
@property (strong, nonatomic) NSManagedObjectContext *childManagedObjectContext;
@property (strong, nonatomic) NSFetchedResultsController *fetchedResultsController;
- (Boolean) isAlreadyInTheFetchQueue:(Feed *)feed;
- (void) fetchPosts:(Feed *)feed;
- (void) createPostInFeed:(Feed*)feed withTitle:(NSString *)title withContent:(NSString *)content withURL:(NSString *)url withDate:(NSDate *)date;
Now here's the init method: Note if I set the _childManagedObjectContext's parent here, the program crashes.
-(AsyncFetchEngine *)init
{
_rssParserQueue = dispatch_queue_create("com.example.MyQueue", NULL);
_uRLsToFetch = [[NSMutableArray alloc] initWithCapacity:32];
_childManagedObjectContext = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSPrivateQueueConcurrencyType];
[_childManagedObjectContext setPersistentStoreCoordinator:[_managedObjectContext persistentStoreCoordinator]];
NSNotificationCenter *center = [NSNotificationCenter defaultCenter];
[center addObserver:self
selector:@selector(contextDidSave:)
name:NSManagedObjectContextDidSaveNotification
object:_childManagedObjectContext];
return self;
}
- (void)contextDidSave:(NSNotification*)notification
{
void (^mergeChanges) (void) = ^ {
[_managedObjectContext mergeChangesFromContextDidSaveNotification:notification];
};
if ([NSThread isMainThread]) {
mergeChanges();
} else {
dispatch_sync(dispatch_get_main_queue(), mergeChanges);
}
}
Fetch method: Note: not sure which MOC to use to determine localFeed, _managedObjectContext crashes the app.
- (void) fetchPosts:(Feed *)feed
{
if (!_childManagedObjectContext.parentContext) {
[_childManagedObjectContext setParentContext:self.managedObjectContext];
}
if ([self isAlreadyInTheFetchQueue:feed]) {
NSLog(@"AsyncFetchEngine::fetchPosts> \"%@\" is already in the fetch queue", feed.name);
return;
}
[_uRLsToFetch addObject:feed];
NSURL *url=[NSURL URLWithString:feed.rss];
NSURLRequest *req = [[NSURLRequest alloc] initWithURL:url];
if (![NSURLConnection canHandleRequest:req]) {
return;
}
Feed *localFeed = ((Feed *)[_childManagedObjectContext existingObjectWithID:[feed objectID] error:nil]);
dispatch_async(_rssParserQueue, ^{
NSLog(@"AsyncFetchEngine::fetchPosts> Opening %@", feed.rss);
[RSSParser parseRSSFeedForRequest:req success:^(NSArray *feedItems)
{
for(RSSItem *i in feedItems)
{
[self createPostInFeed:localFeed withTitle:i.title withContent:(i.content?i.content:i.itemDescription) withURL:[i.link absoluteString] withDate:(i.pubDate?i.pubDate:[NSDate date])];
}
NSLog(@"AsyncFetchEngine::fetchPosts> Found %d items", [feedItems count]);
[_uRLsToFetch removeObject:feed];
}
failure:^(NSError *error)
{
NSLog(@"AsyncFetchEngine::fetchPosts> RSSParser lost it: %@", [error localizedDescription]);
}];
});
}
- (Boolean) isAlreadyInTheFetchQueue:(Feed *)feed
{
Feed *f=nil;
for (f in _uRLsToFetch) {
if ([f isEqual:feed]){
return YES;
}
}
return NO;
}
NSFecthedResultsController, do I need it that way?
- (NSFetchedResultsController *)fetchedResultsController
{
if (_fetchedResultsController != nil) {
return _fetchedResultsController;
}
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
// Edit the entity name as appropriate.
// Note: I originally tried to use the main MOC here, but it also used to crash the app.
NSEntityDescription *entity = [NSEntityDescription entityForName:@"Post" inManagedObjectContext:_childManagedObjectContext];
[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:@"date" ascending:NO];
NSArray *sortDescriptors = @[sortDescriptor];
[fetchRequest setSortDescriptors:sortDescriptors];
/* NSPredicate *predicate =[NSPredicate predicateWithFormat:@"feed.rss LIKE '%@'", _detailItem.rss];
[fetchRequest setPredicate:predicate]; */
// 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:nil];
aFetchedResultsController.delegate = self;
_fetchedResultsController = aFetchedResultsController;
return _fetchedResultsController;
}
It usually crashes somewhere in this method, according to the dump stack. If it doesn't crash, I have my post added to a (null) Category...
- (void)createPostInFeed:(Feed*)feed withTitle:(NSString *)title withContent:(NSString *)content withURL:(NSString *)url withDate:(NSDate *)date
{
[_childManagedObjectContext performBlockAndWait:^{
NSError *error = nil;
if (![self.fetchedResultsController performFetch:&error]) {
NSLog(@"Error in refetch: %@",[error localizedDescription]);
abort();
}
}];
NSLog(@"AsyncFetchEngine.h: Searching similar posts among %d", [[self.fetchedResultsController fetchedObjects] count]);
Boolean found=NO;
NSPredicate *predicate=[NSPredicate predicateWithFormat:@"title == %@ AND url == %@ AND feed == %@", title, url, feed];
NSArray *similarPosts = [_fetchedResultsController.fetchedObjects filteredArrayUsingPredicate:predicate];
if ([similarPosts count] > 0)
{
NSLog(@"\n\n\n\t\tAsyncFetchEngine::fetchPosts> Skipping %@ (%@)", title, url);
} else {
NSLog(@"\n\n\n\t\tAsyncFetchEngine::fetchPosts> Putting new post in %@", feed.name);
NSEntityDescription *postEntityDescription = [NSEntityDescription entityForName:@"Post"
inManagedObjectContext:_childManagedObjectContext];
[_childManagedObjectContext performBlock:^{
Post *initPost = (Post *)[[NSManagedObject alloc]
initWithEntity:postEntityDescription
insertIntoManagedObjectContext:_childManagedObjectContext];
initPost.title = title;
initPost.url = url;
initPost.excerpt = content;
initPost.date = date;
initPost.read = nil;
initPost.feed = feed;
NSError *error;
if (![_childManagedObjectContext save:&error])
{
NSLog(@"[createPost] Error saving context: %@", error);
}
NSLog(@"Created: %@ (%@)", title, url);
//[[NSNotificationCenter defaultCenter] postNotificationName:@"NewPostAdded" object:self];
}];
}
}
So, my questions are:
Thanks!
Answering my own question for the sake of eventually helping another newbie.
Check NSOperationQueue with Coredata. There's a very good example here.