Threads Using Managed Objects Between Contexts - objective-c

Question

I am at that point where I am losing hair on this so I figured I'd reach out to the great minds here who have had experience using Objective C with Threads and core data. I am having issues with managed objects inserted in on thread in a NSPrivateQueue Context being accessed from the main thread. So at a high level I am using AFNetworking to generate a thread to make requests to retrieve JSON data from a server and then insert the values into my persistent store core data. After this is done I have another thread for downloading some binary data using AFNetworking as well. I have set up 2 managed contexts for this as shown below:
(NSManagedObjectContext *)masterManagedContext {
if (_masterManagedContext != nil) {
return _masterManagedContext;
}
NSPersistentStoreCoordinator *coord = [self coordinator];
if (coord != nil) {
_masterManagedContext = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSMainQueueConcurrencyType];
_masterManagedContext.stalenessInterval = 0.0;
[_masterManagedContext performBlockAndWait:^{
[_masterManagedContext setPersistentStoreCoordinator:coord];
}];
}
return _masterManagedContext;
}
// Return the NSManagedObjectContext to be used in the background during sync
- (NSManagedObjectContext *)backgroundManagedContext {
if (_backgroundManagedContext != nil) {
return _backgroundManagedContext;
}
NSManagedObjectContext *masterContext = [self masterManagedContext];
if (masterContext != nil) {
_backgroundManagedContext = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSPrivateQueueConcurrencyType];
_backgroundManagedContext.stalenessInterval = 0.0;
[_backgroundManagedContext performBlockAndWait:^{
[_backgroundManagedContext setParentContext:masterContext];
}];
}
return _backgroundManagedContext;
}
As is shown above I am using a child context and the parent context. When I make I call to fetch the json data I have something like below:
AFJSONRequestOperation *operation = [AFJSONRequestOperation JSONRequestOperationWithRequest:request success:^(NSURLRequest *request, NSHTTPURLResponse *response, id JSON) {
//Initially delete all records in table. This will change
[[Singleton sharedInstance]removeEntityObjects:className];
for (int x=0; x < [JSON count]; x++) {
NSMutableDictionary *curDict = [JSON objectAtIndex:x];
[[CoreDatam sharedinstance] insertEmployeesWithDictionary:curDict];
}else {
/* do nothing */
}
}
}
} failure:^(NSURLRequest *request, NSHTTPURLResponse *response, NSError *error,id JSON) {
[delegate performSelector:#selector(didNotCompleteSync:) withObject:className];
}];
[operations addObject:operation];
}
[self.AFClient enqueueBatchOfHTTPRequestOperations:operations progressBlock:^(NSUInteger numberOfCompletedOperations, NSUInteger totalNumberOfOperations) {
NSLog(#"Currenlty downloaded table data %d of %d!",numberOfCompletedOperations,totalNumberOfOperations);
} completionBlock:^(NSArray *operations) {
if (_syncInProgress) {
[[CoreDatam sharedInstance]updateEmpForId];
[self downloadAllFiles];
}
}];
}`
for the insert function I have something like below:
insertEmployeesWithDictionary:curDict {
[[self backgroundManagedContext]performBlockAndWait:^{
Employee *emp = [NSEntityDescription insertNewObjectForEntityForName:#"Employee"
inManagedObjectContext:[self backgroundManagedContext]];
/* Issues saving null into core data based on type.*/
[emp setFirst:[dictCopy objectForKey:#"first"]];
[emp setLast:[dictCopy objectForKey:#"last"]];
NSError *error = nil;
BOOL saved;
saved = [[self backgroundManagedContext] save:&error];
if (!saved) {
NSLog(#"Whoops, couldn't save: %#", [error localizedDescription]);
}
[self saveMasterContext];
}];
}
The issue is below where I am trying to access the managed objects in the method that is in the completion block above:
updateEmpId {
[self.backgroundManagedContext performBlockAndWait:^{
NSError *error = nil;
NSFetchRequest *request = [[NSFetchRequest alloc] initWithEntityName:#"Employee"];
[request setSortDescriptors:[NSArray arrayWithObject:
[NSSortDescriptor sortDescriptorWithKey:#"last" ascending:YES]]];
myEmps = [self.backgroundManagedContext executeFetchRequest:request error:nil];
for (Employee *moEmp in myEmps) {
[[self backgroundManagedContext]refreshObject:moEmp mergeChanges:YES];
moEmp.photo = #'default.pic';
}
NSError *saveError = nil;
if (![self.backgroundManagedContext save:&saveError]) {
NSLog(#"Whoops, couldn't save: %#", [error localizedDescription]);
}
[self saveMasterContext];
}
The issue is that I am getting very inconsistent behavior when looking at the managed objects that are modified in the main thread. Is it still necessary to pass managed objectIds when using a parent child context relation? if so how can I do so for the above example? Any help greatly appreciated.

Answer 1

You should pass NSManagedObjectIDs or re-fetch in the main thread context, yeah. If you pass object IDs, get the IDs from the background context after saving the new Employee objects, and use existingObjectWithID:error: in the parent context to instantiate them there. Or just re-do the fetch request from your updateEmpId code block in the masterManagedContext.