MagicalRecord: saving context after deleting is too long - ios

Question

I have next problem: I have database with about 7000 of entities, when I need to update them (I have XML file which I parse) for first I delete all entities, after it I parse XML file, later I create new entities and save context. Earlier all worked perfect: no freezes, no crashes - all was fast on iOS 7.
But with release iOS 8 there were problems:
I resolved this problem by providing one context for all operations: deleting, creating and saving.
BUT! What I've got:
When I just install app on my device all goes well: there are no deleting, only creating entities, 7000 terms and 7 groups are parsed so fast (about 4 seconds on iPhone 6), saving goes fast too.
When I changing version of DB in my plist file (increase) my parser start this algorthm:
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
[MagicalRecord saveWithBlock:^(NSManagedObjectContext *localContext) {
[Term MR_truncateAllInContext:localContext];
[Group MR_truncateAllInContext:localContext];
} completion:^(BOOL success, NSError *error) {
[self parseTermsInContext:[NSManagedObjectContext contextForCurrentThread] from:self.count];
}];
});
"saveWithBlock" method blocks Thread 1 (in profiler), my CPU loaded on 99-108(error apparently) percents (with every next update saving operation takes more and more seconds, from 20 and more, more than 120 seconds).
I've tried this way (I gathered all operations in one method for you):
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
NSManagedObjectContext *localContext = [NSManagedObjectContext contextForCurrentThread];
NSMutableArray *objects = [NSMutableArray arrayWithArray:[Group MR_findAllInContext:localContext]];
[objects addObjectsFromArray:[Term MR_findAllInContext:localContext]];
if (objects && [objects count] > 0) {
for (NSManagedObject *object in objects) {
[object MR_deleteInContext:localContext];
}
[localContext MR_saveToPersistentStoreAndWait];
}
[self parseTermsInContext:localContext from:self.count];
});
Here operation "MR_saveToPersistentStoreAndWait" take a long time too like "saveWithBlock".
I tried way without saving context after deleting, that is line "[localContext MR_saveToPersistentStoreAndWait];" does not exists. In this way Groups and Terms was deleted so fast too, later they was parsed so fast too but saving context was so long.
And I don't know why but even if I start deleting and saving processes in background thread saving operation froze UI thread (in UI thread I show progress from 0 to 100). When I parse XML in this thread I send message to view that one term is parsed and setting progress in percents, delegate calls method for setting progress in ProgressView in main queue.
I have not another threads that can operate core data objects.
There is link with work of app: http://rghost.ru/60274051
After 6 seconds: for test purposes I start NSTimer that updated progress every 0.3 second with fake data to fill progress for 50% before starting deleting and saving operation (updating progress goes in main queue). Timer fires several times then saving process starts in background thread but blocks main thread (as I understand) and moves setting progress operation to end (if I understand correctly).
1:08 : then after saving ends I start parsing xml-file. This is thread where I saved context after deleting. You can see progress updating. In this video it works with bugs because of a lot of manipulations, but You can believe me that it works and looks fine. After parsing 7000 objects I save context AGAIN and saving operation does not block UI thread.
Additional info:
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Answer 1

} completion:^(BOOL success, NSError *error) {
[self parseTermsInContext:[NSManagedObjectContext contextForCurrentThread] from:self.count];
}];
block above called in Main Thread (block UI), so contextForCurrentThread == UI Thread
instead of iterate an delete all objects you can use TruncateAll
My suggestion is:
// saveWithBlock - already perform block in background thread
[MagicalRecord saveWithBlock:^(NSManagedObjectContext *localContext) {
[Term MR_truncateAllInContext:localContext];
[Group MR_truncateAllInContext:localContext];
} completion:^(BOOL success, NSError *error) {
[MagicalRecord saveWithBlock:^(NSManagedObjectContext *localContext) {
// self must be weak!
[self parseTermsInContext:localContext from:self.count];
} completion:^(BOOL success, NSError *error) {
// Update UI
}];
}];