iOS: Core data performBlock makes UI irresponsive - ios

Question

I just wanted to be clear about core data private context. I am trying to insert 20k record using private context (NSPrivateQueueConcurrencyType). But as soon as 'Insert 20k records' button is tapped UI thread hangs.
- (IBAction)insertRecords:(id)sender {
[[CoreDataStore privateContext] performBlock:^{
NSLog(#"Starting to insert 20k records...");
for (int i = 0 ; i < 20000; i++)
{
NSManagedObject *newManagedObject = [NSEntityDescription insertNewObjectForEntityForName:#"Event" inManagedObjectContext:[CoreDataStore privateContext]];
[newManagedObject setValue:[NSDate date] forKey:#"time"];
}
NSError * error = nil;
NSLog(#"Inserted 20k records to managed object context");
[[CoreDataStore privateContext] save:&error];
NSLog(#"Save context command fired");
}];
NSLog(#"returning from insert method");
}
changes from private context are being merged to main context here
- (void)contextDidSavePrivateQueueContext:(NSNotification *)notification
{
#synchronized(self) {
[self.mainContext performBlock:^{
NSLog(#"merging changes to main context.....");
[self.mainContext mergeChangesFromContextDidSaveNotification:notification];
NSLog(#"merged changes to main context");
}];
}
}
Below is log window output when I hit insert button:
2014-08-04 14:54:12.431 CoreDataDrillDown[11323:90b] returning from insert method
2014-08-04 14:54:12.431 CoreDataDrillDown[11323:1403] Starting to insert 20k records...
2014-08-04 14:54:12.506 CoreDataDrillDown[11323:1403] Inserted 20k records to managed object context
2014-08-04 14:54:12.785 CoreDataDrillDown[11323:90b] merging changes to main context.....
2014-08-04 14:54:12.786 CoreDataDrillDown[11323:1403] Save context command fired
2014-08-04 14:54:27.019 CoreDataDrillDown[11323:90b] merged changes to main context
For more than 15 sec UI was irresponsive. I want to know the reason why?
With below approach suggested by https://stackoverflow.com/users/817182/thom-ek and https://stackoverflow.com/users/2128900/micha%c5%82-ciuba, insertion is quick and does not freeze UI but changes are not getting saved to disk, not sure why.
//writerContext has persistent store coordinator, so it should write data to disk but it
//is not writing any changes to disk.???????
-(NSManagedObjectContext *)writerContext
{
if (!_writerContext)
{
_writerContext = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSPrivateQueueConcurrencyType];
_writerContext.persistentStoreCoordinator = [(AppDelegate *)[[UIApplication sharedApplication] delegate] persistentStoreCoordinator];
}
return _writerContext;
}
//mainContext is being used with NSFetchedResultController
-(NSManagedObjectContext *)mainContext
{
if (!_mainContext)
{
_mainContext = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSMainQueueConcurrencyType];
_mainContext.parentContext = [self writerContext];
}
return _mainContext;
}
//performBlock is being called on privateContext
-(NSManagedObjectContext *)privateContext
{
if (!_privateContext)
{
_privateContext = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSPrivateQueueConcurrencyType];
_privateContext.parentContext = [self mainContext];
}
return _privateContext;
}

Answer 1

Main problem is that mainContext is still tied to mainQueue, so it will wait for this huge merge.
There are good articles about asynchronous saves: on Cocoanetics and ObjC.io.
Other solution is just to reset mainContext and reload all tableViews (you don’t show all of 20k records to user at once).