Core Data Parent/Child context save fail - multithreading

Question

I setup a background thread with the Parent/Child model. Essentially the context save is failing.
Here is my setup. In the AppDelegate i've setup the _managedObjectContext with the NSMainQueueConcurrencyType:
- (NSManagedObjectContext *)managedObjectContext
{
if (_managedObjectContext != nil) {
return _managedObjectContext;
}
NSPersistentStoreCoordinator *coordinator = [self persistentStoreCoordinator];
if (coordinator != nil) {
_managedObjectContext = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSMainQueueConcurrencyType];//[[NSManagedObjectContext alloc] init];
[_managedObjectContext setPersistentStoreCoordinator:coordinator];
}
return _managedObjectContext;
}
In my data loading class I setup the parent/child mocs here to perform the work on the background thread:
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_async(queue, ^{
NSManagedObjectContext *mainMOC = self.managedObjectContext;
NSManagedObjectContext *moc = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSConfinementConcurrencyType];
[moc setParentContext:mainMOC];
[moc setUndoManager:nil];
When the json data has completed I attempt to peform a save operation with the following macro:
#define SAVE_MOC { NSError *error; \
if (![moc save:&error]) { NSLog(#"Sub MOC Error"); } \
[mainMOC performBlock:^{ NSError *e = nil; if (![mainMOC save:&e]) {
NSLog(#"Main MOC Error %#",error.localizedDescription);}}];}
Also when i've completed the data load I jump back on the main thread like this:
dispatch_async(dispatch_get_main_queue(), ^{
NSLog(#"<---- complete CS sub moc! ---->");
//this fires ok
});
So, from my SAVE_MOC macro i just get a simple error:
Main MOC Error (null)
Let me know if I can provide more info. I'm very new to multi-threading and trying to get a better handle on this approach.
Thanks,
Josh

Answer 1

In my data loading class I setup the parent/child mocs here to perform
the work on the background thread:
dispatch_queue_t queue =
dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_async(queue, ^{
NSManagedObjectContext *mainMOC = self.managedObjectContext;
NSManagedObjectContext *moc = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSConfinementConcurrencyType];
You should not do that. Do this instead.
NSManagedObjectContext *mainMOC = self.managedObjectContext;
NSManagedObjectContext *moc = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSPrivateQueueConcurrencyType];
Make sure you access the MOC in a performBlock. For example,
[moc performBlock:^{
// Anything at all involving this MOC or any of its objects
}];
When the json data has completed I attempt to peform a save operation
with the following macro:
Consider saving with something like this. Your completion block will be called when the save has finished.
- (void)saveMOC:(NSManagedObjectContext*)moc
completion:(void(^)(NSError *error))completion {
[moc performBlock:^{
NSError *error = nil;
if ([moc save:&error]) {
if (moc.parentContext) {
return [self saveMOC:moc.parentContext completion:completion];
}
}
if (completion) {
dispatch_async(dispatch_get_main_queue(), ^{
completion(error);
});
}
}];
}
[self saveMOC:moc completion:^(NSError *error) {
// Completion handler is called from main-thread, after save has finished
if (error) {
// Handle error
} else {
}
}];
EDIT
This code will crash if moc.parentContext is main concurrency type. –
Mundi
There is no inherent reason that the code I posted should cause a crash with a parent MOC of NSMainQueueConcurrencyType. It has supported being a parent context ever since parent/child was added to Core Data.
Maybe I was missing a typo, so I copy/paste saveMOC:completion: straight from this answer, and wrote the following test helper.
- (void)testWithChildConcurrencyType:(NSManagedObjectContextConcurrencyType)childConcurrencyType
parentConcurrencyType:(NSManagedObjectContextConcurrencyType)parentConcurrencyType {
NSAttributeDescription *attr = [[NSAttributeDescription alloc] init];
attr.name = #"attribute";
attr.attributeType = NSStringAttributeType;
NSEntityDescription *entity = [[NSEntityDescription alloc] init];
entity.name = #"Entity";
entity.properties = #[attr];
NSManagedObjectModel *model = [[NSManagedObjectModel alloc] init];
model.entities = #[entity];
NSPersistentStoreCoordinator *psc = [[NSPersistentStoreCoordinator alloc] initWithManagedObjectModel:model];
[psc addPersistentStoreWithType:NSInMemoryStoreType configuration:nil URL:nil options:nil error:NULL];
NSManagedObjectContext *parent = [[NSManagedObjectContext alloc] initWithConcurrencyType:parentConcurrencyType];
parent.persistentStoreCoordinator = psc;
NSManagedObjectContext *child = [[NSManagedObjectContext alloc] initWithConcurrencyType:childConcurrencyType];
child.parentContext = parent;
NSManagedObject *obj = [NSEntityDescription insertNewObjectForEntityForName:#"Entity" inManagedObjectContext:child];
[obj setValue:#"value" forKey:#"attribute"];
XCTestExpectation *expectation = [self expectationWithDescription:[NSString stringWithFormat:#"save from %# to %# finished", concurrencyTypeString(childConcurrencyType), concurrencyTypeString(parentConcurrencyType)]];
[self saveMOC:child completion:^(NSError *error) {
// Verify data saved all the way to the PSC
NSManagedObjectContext *localMoc = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSMainQueueConcurrencyType];
localMoc.persistentStoreCoordinator = psc;
NSFetchRequest *fr = [NSFetchRequest fetchRequestWithEntityName:#"Entity"];
XCTAssertEqualObjects(#"value", [[[localMoc executeFetchRequest:fr error:NULL] firstObject] valueForKey:#"attribute"]);
[expectation fulfill];
}];
[self waitForExpectationsWithTimeout:10 handler:nil];
}
And then, I wrote a test for each possible parent/child relationship.
- (void)testThatDoingRecursiveSaveFromPrivateToPrivateWorks {
[self testWithChildConcurrencyType:NSPrivateQueueConcurrencyType
parentConcurrencyType:NSPrivateQueueConcurrencyType];
}
- (void)testThatDoingRecursiveSaveFromPrivateToMainWorks {
[self testWithChildConcurrencyType:NSPrivateQueueConcurrencyType
parentConcurrencyType:NSMainQueueConcurrencyType];
}
- (void)testThatDoingRecursiveSaveFromMainToPrivateWorks {
[self testWithChildConcurrencyType:NSMainQueueConcurrencyType
parentConcurrencyType:NSPrivateQueueConcurrencyType];
}
- (void)testThatDoingRecursiveSaveFromMainToMainWorks {
[self testWithChildConcurrencyType:NSMainQueueConcurrencyType
parentConcurrencyType:NSMainQueueConcurrencyType];
}
So, what am I missing?
As I write this, I am reminded of a 360iDev presentation where the presenter said that you can't call performBlock on a NSMainQueueConcurrencyType context. At the time, I thought he just misspoke, meaning confinement, but maybe there is some confusion in the community about this.
You can't call performBlock on a NSConfinementConcurrencyType MOC, but performBlock is fully supported for NSMainQueueConcurrencyType.