NSManagedObjectContext+DebugSwizzling.h

#if DEBUG
/**
 Toggles assertion of Core Data managed object contexts. Only available when DEBUG is defined.

 When enabled, your application will throw an exception when accessing or modifying entities in a context other than their own.
 */
void TCBToggleAssertingContextQueues();
#endif

NSManagedObjectContext+DebugSwizzling.m

#import "NSManagedObjectContext+DebugSwizzling.h"

#if DEBUG
#import <CoreData/CoreData.h>
#import <objc/runtime.h>
#import "TCBRuntimeHelpers.h"

static NSUInteger TCBManagedObjectContextDisableChecksCount = 0;

@implementation NSManagedObjectContext (DebugSwizzling)

- (BOOL)tcb_shouldPerformContextQueueCheck
{
    return (TCBManagedObjectContextDisableChecksCount == 0);
}

- (void)tcb_stopContextQueueCheck
{
    TCBManagedObjectContextDisableChecksCount++;
}

- (void)tcb_startContextQueueCheck
{
    TCBManagedObjectContextDisableChecksCount--;
}

- (id)tcb_dispatchQueue
{
    id dispatchQueue = object_getIvar(self, class_getInstanceVariable([NSManagedObjectContext class], "_dispatchQueue"));
    NSParameterAssert(dispatchQueue);
    return dispatchQueue;
}

- (void)tcb_validateCurrentQueue
{
    if (NO == [self tcb_shouldPerformContextQueueCheck]) {
        return;
    }

#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wdeprecated-declarations"
    NSAssert(dispatch_get_current_queue() == self.tcb_dispatchQueue, @"Accessing managed object from a different queue than its context uses: %@", self);
#pragma clang diagnostic pop
}

- (void)tcb_mergeChangesFromContextDidSaveNotification:(NSNotification *)notification
{
    // Temporarily disable thread checks when merging changes from a did save notification — they are not always performed on the expected queue (although it's your responsibility to ensure that they are started on the correct queue for the context you are merging into!
    [self tcb_stopContextQueueCheck];
    [self tcb_mergeChangesFromContextDidSaveNotification:notification];
    [self tcb_startContextQueueCheck];
}

@end

@implementation NSManagedObject (DebugSwizzling)

- (id)tcb_initWithEntity:(NSEntityDescription*)entity insertIntoManagedObjectContext:(NSManagedObjectContext*)context
{
    [context tcb_validateCurrentQueue];
    return [self tcb_initWithEntity:entity insertIntoManagedObjectContext:context];
}

- (void)tcb_willAccessValueForKey:(NSString*)key
{
    [[self managedObjectContext] tcb_validateCurrentQueue];
    [self tcb_willAccessValueForKey:key];
}

- (void)tcb_willChangeValueForKey:(NSString*)key
{
    [[self managedObjectContext] tcb_validateCurrentQueue];
    [self tcb_willChangeValueForKey:key];
}

- (void)tcb_willChangeValueForKey:(NSString*)inKey withSetMutation:(NSKeyValueSetMutationKind)inMutationKind usingObjects:(NSSet*)inObjects
{
    [[self managedObjectContext] tcb_validateCurrentQueue];
    [self tcb_willChangeValueForKey:inKey withSetMutation:inMutationKind usingObjects:inObjects];
}

@end

@implementation NSEntityDescription (DebugSwizzling)

+ (id)tcb_insertNewObjectForEntityForName:(NSString*)entityName inManagedObjectContext:(NSManagedObjectContext*)context
{
    [context tcb_validateCurrentQueue];
    return [self tcb_insertNewObjectForEntityForName:entityName inManagedObjectContext:context];
}

+ (NSEntityDescription*)tcb_entityForName:(NSString*)entityName inManagedObjectContext:(NSManagedObjectContext*)context
{
    [context tcb_validateCurrentQueue];
    return [self tcb_entityForName:entityName inManagedObjectContext:context];
}

@end

void TCBToggleAssertingContextQueues()
{
    // NSManagedObjectContext
    TCBSwapInstanceMethods([NSManagedObjectContext class], @selector(mergeChangesFromContextDidSaveNotification:), @selector(tcb_mergeChangesFromContextDidSaveNotification:));

    // NSEntityDescription
    TCBSwapClassMethods([NSEntityDescription class], @selector(insertNewObjectForEntityForName:inManagedObjectContext:), @selector(tcb_insertNewObjectForEntityForName:inManagedObjectContext:));
    TCBSwapClassMethods([NSEntityDescription class], @selector(entityForName:inManagedObjectContext:), @selector(tcb_entityForName:inManagedObjectContext:));

    // NSManagedObject
    TCBSwapInstanceMethods([NSManagedObject class], @selector(initWithEntity:insertIntoManagedObjectContext:), @selector(tcb_initWithEntity:insertIntoManagedObjectContext:));
    TCBSwapInstanceMethods([NSManagedObject class], @selector(willAccessValueForKey:), @selector(tcb_willAccessValueForKey:));
    TCBSwapInstanceMethods([NSManagedObject class], @selector(willChangeValueForKey:), @selector(tcb_willChangeValueForKey:));
    TCBSwapInstanceMethods([NSManagedObject class], @selector(willChangeValueForKey:withSetMutation:usingObjects:), @selector(tcb_willChangeValueForKey:withSetMutation:usingObjects:));
}
#endif /* if DEBUG */

TCBRuntimeHelpers.h

//
//  Created by Tony Arnold on 19/05/2014.
//  Copyright (c) 2014 The CocoaBots. All rights reserved.
//

#import <Cocoa/Cocoa.h>

void TCBSwapInstanceMethods(Class cls, SEL originalSel, SEL newSel);
void TCBSwapClassMethods(Class cls, SEL originalSel, SEL newSel);

TCBRuntimeHelpers.m

//
//  Created by Tony Arnold on 19/05/2014.
//  Copyright (c) 2014 The CocoaBots. All rights reserved.
//

#import "TCBRuntimeHelpers.h"
#import <objc/runtime.h>

void TCBSwapInstanceMethods(Class cls, SEL originalSel, SEL newSel)
{
    Method originalMethod = class_getInstanceMethod(cls, originalSel);
    Method newMethod = class_getInstanceMethod(cls, newSel);

    NSCAssert(originalMethod != NULL, @"Method cannot be NULL");
    NSCAssert(newMethod != NULL, @"Method cannot be NULL");

    method_exchangeImplementations(originalMethod, newMethod);
}

void TCBSwapClassMethods(Class cls, SEL originalSel, SEL newSel)
{
    Method originalMethod = class_getClassMethod(cls, originalSel);
    Method newMethod = class_getClassMethod(cls, newSel);

    NSCAssert(originalMethod != NULL, @"Method cannot be NULL");
    NSCAssert(newMethod != NULL, @"Method cannot be NULL");

    method_exchangeImplementations(originalMethod, newMethod);
}