我有一个例程在后台获取 RSS 条目，如果还没有，则将它们插入到我的 NSManagedObjectContext 中。
我的问题是该对象找不到重复项或崩溃，具体取决于哪个NSManagedObjectContext I use...请帮助我。
这是简化的.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;
现在这里是 init 方法： 注意如果我在这里设置 _childManagedObjectContext 的父级，程序会崩溃。
-(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 方法：注意：不确定使用哪个 MOC 来确定 localFeed，_managedObjectContext 会使应用程序崩溃。
- (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，我需要这样吗？
- (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;
}
根据转储堆栈，它通常在此方法的某个地方崩溃。如果它没有崩溃，我会将我的帖子添加到（空）类别...
- (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];
}];
}
}
所以，我的问题是：
- 当我有多个 MOC 时，我是否从 main 中获取并写给一个孩子？
- 我应该如何分别使用上述这些？
谢谢！