Async core data from multiple class and threads - ios

Question

After trying to figure out in my previous question what is the exact problem:
fetchedObjects (NSArray) count return 0 when it's full of objects
I'm pretty sure I need my core data to be async from multiple classes and threads.
I tried multiple calls to my core data in a row, one by one and I had no problem.
But obviously I need it to be read/write from multiple classes and threads.
I trued using #synchronized and still nothing, I've an 0 records in fetchedObjects array from core data but there's are data in there.
What is the correct approach to do it?
EDIT 1:
The code above works only once if I'm trying to schedule it using NSTimer:
TrapService.mm:
self.managedObjectContext = appDelegate.managedObjectContext;
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:kCORE_DATA_ALL_TRAPS_ENTITY inManagedObjectContext:self.managedObjectContext];
[fetchRequest setEntity:entity];
NSError *error = nil;
NSArray *fetchedObjects = [self.managedObjectContext executeFetchRequest:fetchRequest error:&error];
NSLog(#"fetchedObjects.count: %d", fetchedObjects.count);
EDIT 2:
Another example of codes i'm using with core data, execute once, then all of the operations regarding core data doing nothing or giving me back array with 0 records.
TrapService.mm:
- (void)initializeQuadTree
{
self.qTree = [[QuadTree alloc] init];
self.qTree = [dbat addCoordinatesToQuadTree:self.qTree];
}
- (Traps*)getCloseTrapFromTree:(CLLocation*)location
{
return [dbat getCloseTrapFromTree:self.qTree andLocation:location];
}
DataBaseAllTraps.m:
- (QuadTree*)addCoordinatesToQuadTree:(QuadTree*)quadTree
{
if (quadTree == nil) {
quadTree = [[QuadTree alloc] init];
}
BOOL success = YES;
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:kCORE_DATA_ALL_TRAPS_ENTITY inManagedObjectContext:self.managedObjectContext];
[fetchRequest setEntity:entity];
NSError *error = nil;
NSArray *fetchedObjects = [self.managedObjectContext executeFetchRequest:fetchRequest error:&error];
if (fetchedObjects == nil || fetchedObjects.count == 0)
{
NSLog(#"addCoordinatesToQuadTree - localizedDescription: %#, userInfo: %#", error.localizedDescription, error.userInfo);
success = NO;
}
NSLog(#"addCoordinatesToQuadTree - fetchedObjects.count: %d", fetchedObjects.count);
if (success)
{
for (CoreDataAllTraps *trap in fetchedObjects)
{
double latitude = trap.lat.doubleValue;
double longitude = trap.lon.doubleValue;
double closePointLat = trap.close_point_lat.doubleValue;
double closePointLon = trap.close_point_lon.doubleValue;
DummyAnnotation *trapAnnotation = [[DummyAnnotation alloc] init];
if (closePointLat != 0.0 || closePointLon != 0.0) trapAnnotation.coordinate = CLLocationCoordinate2DMake(closePointLat, closePointLon);
else trapAnnotation.coordinate = CLLocationCoordinate2DMake(latitude, longitude);
[quadTree insertObject:trapAnnotation];
}
}
else
{
for (Traps *trap in kNETROADS_CONTEXT.arrayOfAllTraps)
{
double latitude = trap.lat;
double longitude = trap.lon;
double closePointLat = trap.closePointLat;
double closePointLon = trap.closePointLon;
DummyAnnotation *trapAnnotation = [[DummyAnnotation alloc] init];
if (closePointLat != 0.0 || closePointLon != 0.0) trapAnnotation.coordinate = CLLocationCoordinate2DMake(closePointLat, closePointLon);
else trapAnnotation.coordinate = CLLocationCoordinate2DMake(latitude, longitude);
[quadTree insertObject:trapAnnotation];
}
}
NSLog(#"TOTAL NUMBER OF TRAPS (%s): %i", __PRETTY_FUNCTION__, success?fetchedObjects.count:[Netroads sharedInstance].arrayOfAllTraps.count);
return quadTree;
}
- (Traps*)getCloseTrapFromTree:(QuadTree*)quadTree andLocation:(CLLocation*)location
{
NSLog(#"%s", __PRETTY_FUNCTION__);
NSArray *closeTraps = [quadTree neighboursForLocation:location.coordinate limitCount:1];
if (closeTraps.count == 0) { return nil; }
// NSAssert(closeTraps.count > 0, #"closeTraps.count == 0, get close trap from quad tree.");
int trapID = 0;
DummyAnnotation *trapLocation = closeTraps.firstObject;
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:kCORE_DATA_ALL_TRAPS_ENTITY inManagedObjectContext:self.managedObjectContext];
NSPredicate *predicate = [NSPredicate predicateWithFormat:#"%# == %f AND %# == %f", CLOSE_POINT_LAT, trapLocation.coordinate.latitude, CLOSE_POINT_LON, trapLocation.coordinate.longitude];
[fetchRequest setEntity:entity];
[fetchRequest setPredicate:predicate];
[fetchRequest setFetchLimit:1];
NSError *error = nil;
NSArray *fetchedObjects = [self.managedObjectContext executeFetchRequest:fetchRequest error:&error];
if (fetchedObjects != nil && fetchedObjects.count > 0) { // We have close point
CoreDataAllTraps *trap = fetchedObjects.firstObject;
trapID = trap.trapID.intValue;
}
else { // We do not have close point, use normal coordinates (lat, lon)
NSLog(#"%s error: %#\n%#", __PRETTY_FUNCTION__, error.localizedDescription, error.userInfo);
fetchRequest = [[NSFetchRequest alloc] init];
entity = [NSEntityDescription entityForName:kCORE_DATA_ALL_TRAPS_ENTITY inManagedObjectContext:self.managedObjectContext];
predicate = [NSPredicate predicateWithFormat:#"%# == %f AND %# == %f", LAT, trapLocation.coordinate.latitude, LON, trapLocation.coordinate.longitude];
[fetchRequest setEntity:entity];
[fetchRequest setPredicate:predicate];
[fetchRequest setFetchLimit:1];
error = nil;
fetchedObjects = [self.managedObjectContext executeFetchRequest:fetchRequest error:&error];
if (fetchedObjects != nil && fetchedObjects.count > 0) {
CoreDataAllTraps *trap = fetchedObjects.firstObject;
trapID = trap.trapID.intValue;
}
else {
NSLog(#"%s error: %#\n%#", __PRETTY_FUNCTION__, error.localizedDescription, error.userInfo);
}
}
if (trapID > 0) {
return [self getTrap_trapID:trapID];
}
else {
return nil;
}
}
EDIT 3:
I'm creating a new MOC and still nothing, same problems:
NSManagedObjectContext *context = [[NSManagedObjectContext alloc] init];
[context setPersistentStoreCoordinator:appDelegate.persistentStoreCoordinator];

Answer 1

I did not analyze your code. (I'm too lazy. :-)) But when I did a search for a single save I found none.
Please remember, what is going on: In a standard set-up you have one SQL-DB as backend. You have different contexts for each thread/queue that (partially) takes out the data of the SQL-DB on a fetch request and (partially) saves it on a save request.
No context pushes its changes (including insertions and deletions) automatically to the DB or to another context. No context pulls changes pushed by another context automatically from the DB or another context. So transmitting data from context to another one has to be done a way "manually".
As long as you do not have deletions you can simply store the data when one context is done using save and listen to the did save notification on the other thread.

Answer 2

Read up on Apples documentation on how to use CoreData in a concurrent fashion.
Basically it is highly important to use separate NSManagedObjectContext per thread and not to pass objects between these threads, but only reference these by their NSManagedObjectID.
Your code example above needs more information on where you sue that code. But what makes me wonder immediately is
self.managedObjectContext = appDelegate.managedObjectContext;
If not run on main thread, this is exactly contrary to what the concurrency guide tells to do. With that line you only create a pointer that points to appDelegate.managedObjectContext. This is not a new object!
There is usually no need to synchronize or add locks and such, if done the right way.
To give a good answer, though your question is too vague and it would need a rather lengthy answer. But maybe after reading Apple's documentation you may be able to partially solve your problem and come back with questions on problems. that can be answered satisfactorily more easily.