Core Data locks in background threads

Question

I\'m getting locks in Core Data. I really don\'t understand the reason. Because I\'m creating a background MOC when I\'m processing in a background thread. Below you can see wha

Answer 1

this may not give the full picture but this is the approach I took

Every managed object requires its own thread. On the same thread you can use the same managed object over and over. But not on the same queue. (this was a misconception I leaned about) the background queue can have many different threads and the MOC needs to be unique for each thread.

here is the method I used.

ManagedObjectContextHolder.h

#import 

@interface ManagedObjectContextHolder : NSObject

+ (ManagedObjectContextHolder*) threadContextHolder;

@property (nonatomic, strong) NSManagedObjectContext *context;
@property (nonatomic, strong) NSString *contextThreadMocGuid;
@property (nonatomic, weak) NSThread *contextThread;

@end

ManagedObjectContextHolder.m

#import "ManagedObjectContextHolder.h"

@interface HSContextSaveHandler : NSObject

@property (nonatomic, strong) NSArray *mocArray;

- (void) saveHappened:(NSNotification*) saveNotification;

@end

@implementation HSContextSaveHandler

@synthesize mocArray = _mocArray;

int saveFinished = 0;

- (MyAppDelegate*) appDelegete{
    return (MyAppDelegate*) [UIApplication sharedApplication].delegate;
}


- (void) saveHappened:(NSNotification *)saveNotification{
    if (saveNotification && saveNotification.userInfo){
        NSArray *staticArray = [NSArray arrayWithArray:self.mocArray];

        for (id item in staticArray) {
            if ([item isKindOfClass:[ManagedObjectContextHolder class]]) {
                ManagedObjectContextHolder *holder = item;
                if ([saveNotification object] != holder.context){
                    @try {
                        [holder.context mergeChangesFromContextDidSaveNotification:saveNotification];
                    }
                    @catch (NSException *exception) {
                        HSLogBrute(@"<<<<<<<< MERGE CHANGES FROM CONTEXT DID SAVE NOTIFICATION >>>>>>>>\n%@",saveNotification);
                    }
                }
            }
        }
        saveFinished = 3;
    }

}

@end

@interface NSThread (mocGuid)

- (NSString*) mocGuid;
- (void) setMocGuid:(NSString*) mocGuid;

@end

@implementation NSThread (mocGuid)

- (NSString *)mocGuid{
    return [self.threadDictionary valueForKey:@"mocGuid"];
}
- (void)setMocGuid:(NSString *)mocGuid{
    [self.threadDictionary setValue:mocGuid forKey:@"mocGuid"];
}

@end

@implementation ManagedObjectContextHolder

static NSMutableArray *_mocHolders;
static HSContextSaveHandler *_mocSaveHandler;
+ (NSMutableArray*) mocHolders{
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        _mocHolders = [NSMutableArray arrayWithCapacity:5];
        _mocSaveHandler = [[HSContextSaveHandler alloc] init];
        _mocSaveHandler.mocArray = _mocHolders;

        [[NSNotificationCenter defaultCenter] addObserver:_mocSaveHandler selector:@selector(saveHappened:) name:NSManagedObjectContextDidSaveNotification object:nil];
    });
    return _mocHolders;
}

+ (ManagedObjectContextHolder *)threadContextHolder{
    NSThread *currentThread = [NSThread currentThread];

    NSString *mocGuid = currentThread.mocGuid;

    ManagedObjectContextHolder *result = nil;

    NSMutableArray *removeList = [[NSMutableArray alloc] initWithCapacity:[self mocHolders].count];
    NSLog(@"Context Holders Count %d",[self mocHolders].count);
    for (ManagedObjectContextHolder *item in [self mocHolders]) {
        if (mocGuid != nil && item.contextThread == currentThread && item.contextThreadMocGuid == currentThread.mocGuid){
            result = item;
        }

        if (item.contextThread == nil) {
            [removeList addObject:item];
        }
    }

    if (removeList.count > 0){
        NSLog(@"Removing %d Context Holders for Nil Threads",removeList.count);
        [[self mocHolders] removeObjectsInArray:removeList];
    }
    if (result == nil){
        result = [[ManagedObjectContextHolder alloc] init];
        result.contextThread = currentThread;

        if (mocGuid == nil){
            mocGuid = [HSStaticContainer uuidAsShortString];
            currentThread.mocGuid = mocGuid;
        }

        result.contextThreadMocGuid = mocGuid;
        result.context = [[NSManagedObjectContext alloc] init];
        [result.contextcontext setPersistentStoreCoordinator:[self appDelegate].persistentStoreCoordinator];
        [result.context setMergePolicy:NSMergeByPropertyObjectTrumpMergePolicy];
        [[self mocHolders] addObject:result];
    }
    return result;

}

@synthesize context = _context;
@synthesize contextThreadMocGuid = _contextThreadMocGuid;
@synthesize contextThread = _contextThread;

- (id)init{
    self = [super init];
    if (self) {
        NSLog(@"Creating a Managed Object Context Holder. Here is the Stack Trace.\r\r%@",[NSThread callStackSymbols]);
    }
    return self;
}

@end