Replacing data in multithreaded Core Data - ios

Question

I'm using three-leveled multithreaded Core Data introduced by Marcus Zarra in his book. tempMOC for dealing with difficult tasks, mainMOC for UI management and writerMOC for writing data to persistant store.
I'm trying to integrate this model with my UITableView.
Everytime user pull-to-refresh I process downloading, parsing and loading this data. In this process there is one extra step - deleting previos entries for entit. I want this to be smooth so, the current UITableView (MOC as well) will wait to be cleaned until the last moment so this gap between deleting and loading new data wouldn't last for c.a 4 sec.
Here is my method which is called everytime I pull-to-refresh (I've removed parsing to keep the code cleaner):
- (void)loadTimetableToCoreData:(id)timetable
{
[self.pullToRefresh finishLoading];
// Initializing temporary context
NSManagedObjectContext *tempContext = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSPrivateQueueConcurrencyType];
tempContext.parentContext = self.moc;
[tempContext performBlock:^{
// Parsing JSON data
}];
[self deleteAllObjects:#"Timetable"];
NSLog(#"Finished loading to temp MOC");
[tempContext performBlock:^{
// Saving procedure with multithreading
NSError *error;
if (![tempContext save:&error]) {
NSLog(#"Couldn't save: %#", [error localizedDescription]);
}
NSLog(#"Finished saving to temp MOC");
[self.moc performBlock:^{
// Save groups to presistant store
NSError *error;
if (![self.moc save:&error]) {
NSLog(#"Couldn't save: %#", [error localizedDescription]);
}
NSLog(#"Finished saving to main MOC");
[self.writer performBlock:^{
// Save groups to presistant store
NSError *error;
if (![self.writer save:&error]) {
NSLog(#"Couldn't save: %#", [error localizedDescription]);
}
NSLog(#"Finished saving to writer MOC");
}];
}];
}];
}
I've also put some logs and if you run this code it's like:
2014-04-08 09:55:28.349 devPlan[21125:1803] Finished loading to temp MOC
2014-04-08 09:55:33.145 devPlan[21125:1803] Finished saving to temp MOC
2014-04-08 09:55:33.650 devPlan[21125:60b] Finished saving to main MOC
2014-04-08 09:55:33.652 devPlan[21125:60b] Finished saving to writer MOC
So as you can see there is this gap between loading and saving to temp MOC. It's okey because there is a lot of work going on but I would like to wait with [self deleteAllObjects:#"Timetable"]; until this work is done. When this is executed it wipes all data and when it's realoaded again it's displayed in UITableView - but still with time gap in which UITableView is empty...
What should I do to resolve thing thing? Below is the list of what I've tried so far:
Putting this delete method in various places across download method.
Messing around with MOCs performBlock: and performBlockAndWait:.
Fetching and initializing deleteMOC with NSFetchRequest, waiting for data being processed and later call delete method.
I must tell you, I'm stuck with this one... And it bothers me so much but I think there must be a logical explanation to this!
Edit
Here is the code of the method responsible for deleting:
- (void)deleteAllObjects:(NSString *)entityDescription
{
// Initializing temporary context
NSManagedObjectContext *tempContext = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSPrivateQueueConcurrencyType];
tempContext.parentContext = self.moc;
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:entityDescription inManagedObjectContext:tempContext];
[fetchRequest setEntity:entity];
NSError *error;
NSArray *items = [tempContext executeFetchRequest:fetchRequest error:&error];
[tempContext performBlockAndWait:^{
for (NSManagedObject *managedObject in items) {
[tempContext deleteObject:managedObject];
}
NSError *error;
if (![tempContext save:&error]) {
NSLog(#"Error deleting %# - error:%#", entityDescription, [error localizedDescription]);
}
}];
}

Answer 1

First the easy answer; I would put the -deleteAllObjects after the save of the tempContext.
But I would question the idea of deleting everything from the table. Could you not just remove the objects that need to be removed, add the objects that need to be added and update the ones that need to be updated? By capturing the notification from NSManagedObjectContextDidSaveNotification you can resolve that data and present (imho) the data more cleanly.
Update
I am not concerned with the parsing (insert, update, delete) part of it as who cares how long it takes. It is human perceivable no matter what so we are not going to do magic there. I am thinking more of the UX of the table view disappearing and then reappearing as opposed to individual cells updating and resorting themselves. I personally feel that is a far cleaner, slicker looking experience.
The doubling up sounds like there is something wonky in your NSFetchedResultsController delegate methods or in your -deleteAllObjects. Care to post that?
Update
Ok so you are literally deleting everything from that table. Is there a reason you are deleting vs. merging?
Now that I understand a bit more, I would do this order of events:
Process the data
Block the UITableView from updating with a -beginUpdates
Delete all data
Save both contexts
Unblock the UITableView with a -endUpdates
That should give you a more "instantaneous" refresh of the tableview.
I am not a fan of the delete/insert way of handling data but it may be appropriate in your case.

Answer 2

With a little help of my older code I've came onto this:
NSFetchRequest* r = [NSFetchRequest fetchRequestWithEntityName:#"Timetable"];
[r setIncludesPendingChanges:NO];
NSArray *existingTimetables = [tempContext executeFetchRequest:r error:nil];
for (Timetable *table in existingTimetables) {
[tempContext deleteObject:table];
}
So I'm fetching Timetable and not messing it with pending changes inside tempContext. With this code everything works like it should but is blocking main thread! But I guess this qualifies to be another question.