IOS How to sync multithreading NSManagedObjectContext? - objective-c

Question

Application must update data from WebService in loop each 10 sec in background and display data to user by his request in the main thread. Also I need update and delete records by user request.
Updates done with runloop.
I have registered notification in the AppDelegate
[[NSNotificationCenter defaultCenter] addObserver:self selector:#selector(contextChanged:) name:NSManagedObjectContextDidSaveNotification object:nil];
- (void)contextChanged:(NSNotification*)notification
{
if ([notification object] == [self managedObjectContext]) return;
if (![NSThread isMainThread]) {
[self performSelectorOnMainThread:#selector(contextChanged:) withObject:notification waitUntilDone:YES];
return;
}
[[self managedObjectContext] mergeChangesFromContextDidSaveNotification:notification];
[self saveContext]; //do I need this here or marge save data too?
}
I have Storage sharedInstance class with
NSOperationQueue* operationQueue;
then inserts,updates,selects operators added this way from any thread:
-(void)do_something
{
[self.operationQueue addOperationWithBlock:^{
NSManagedObjectContext*moc; //creating new NSManagedObjectContext with AppDelegate.persistentStoreCoordinator
//do my staff
[moc save:&error]
}]
}
The problem is when I try update entities with #"my_id=%#", #(my_id)
[moc countForFetchRequest:fetchRequest error:&error]
return 0 and cause inserting of duplicate exists entity
The problem is with synchronization.
Advice please.
should I use instance of dispatch_queue_create("com.my.", 0); instead for each CoreData operation?
I did try remove operationQuiue
-(void)query:(void(^)(NSManagedObjectContext *context))queryBlock
{
NSLog(#"query CALL");
__block NSManagedObjectContext *context;
//if remove dispatch_sync and/or run in main thread result the same
dispatch_sync( dispatch_queue_create("com.myapp.db-queue", 0), ^{
AppDelegate*app = AppDelegate();
//same result if I use
//app.persistentStoreCoordinator or
//[app.managedObjectContext persistentStoreCoordinator]
NSPersistentStoreCoordinator *persistentStoreCoordinator= [app.managedObjectContext persistentStoreCoordinator];
context = [NSManagedObjectContext new];
[context setPersistentStoreCoordinator:persistentStoreCoordinator];
[context setMergePolicy:NSMergeByPropertyStoreTrumpMergePolicy];
queryBlock(context);
if ([context hasChanges])
{
NSError*err;
[context save:&err];
if (err) {
NSLog(#"context save: %#",[err localizedDescription]);
}
}
});
}
and call it as :
CoreStorage* cs = [CoreStorage sharedInstance];
NSArray* list = [ws GetSections]; //array of NSDictionaries
//if add this to operationQuiue resunt the same
[cs query:^(NSManagedObjectContext *moc) {
NSLog(#"START");
for (NSDictionary *section in list) {
NSNumber* Id= #([[section objectForKey:#"section_id"] integerValue]);
NSFetchRequest * fetchRequest = [NSFetchRequest new];
NSEntityDescription *entity = [NSEntityDescription entityForName:#"Section" inManagedObjectContext: moc];
[fetchRequest setEntity:entity];
[fetchRequest setFetchLimit:1];
[fetchRequest setIncludesSubentities:NO];
[fetchRequest setPredicate: [NSPredicate predicateWithFormat:#"section_id=%#",Id]];
NSError *error =nil;
Section *entry;
if ([moc countForFetchRequest:fetchRequest error:&error] >0)
{
entry = [moc executeFetchRequest:fetchRequest error:nil][0];
NSLog(#"exist"); //this never call
}
else
{
entry = [NSEntityDescription insertNewObjectForEntityForName:#"Section" inManagedObjectContext:moc];
NSLog(#"NEW");
}
entry.section_id = Id;
entry.timeStamp = [NSDate date];
}
}];
Any sugastions please?

Answer 1

The problem is probably in the operation queue. You haven't configured it with max concurrent operations to be 1, right? In this case it is not serial, and operations that you add to it run concurrently. So here what happens. First operation fetches for the count of object with some ID, doesn't find it and creates one. At some point before it saves, another operation is added. This second operation fetches for the object with the same ID, doesn't find it and creates one. Then the first operation saves, then the second one saves, and you have a duplicate.
So try to make your operation queue serial [operationQueue maxConcurrentOperationCount:1];.
And no, you don't have to save after calling merge method of the managed object context.