Setting up an in-memory Core Data stack for unit testing. This example uses Specta, but can be easily be adapted for XCTest.

CoreDataInMemorySetupExampleSpec.m

#import <Specta.h>
#define EXP_SHORTHAND
#import <Expecta.h>
#import <CoreData/CoreData.h>

SpecBegin(CoreDataInMemorySetupExample)
    __block NSManagedObjectContext *context;

    beforeAll(^{
        // ObjectModel from any models in app bundle
        NSManagedObjectModel *managedObjectModel = [NSManagedObjectModel mergedModelFromBundles:nil];

        // Coordinator with in-mem store type
        NSPersistentStoreCoordinator *coordinator = [[NSPersistentStoreCoordinator alloc] initWithManagedObjectModel:managedObjectModel];
        expect([coordinator addPersistentStoreWithType:NSInMemoryStoreType configuration:nil URL:nil options:nil error:nil]).notTo.beNil;

        // Context with private queue
        context = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSPrivateQueueConcurrencyType]; // Choose your concurrency type, or leave it off entirely
        context.persistentStoreCoordinator = coordinator;

        // TODO: At this point you would pass the context var to your code...
    });

    afterAll(^{
        context = nil;
    });
    
    it(@"some test here", ^AsyncBlock {
        // TODO: write some expectations here which use the in-mem context set up above
    });
SpecEnd

A nicety would be to factor this code out into a category for convenience.
An example could be (NSManagedObjectContext+Testing.m):

@implementation NSManagedObjectContext (Testing)

+ (instancetype)testing_inMemoryContext:(NSManagedObjectContextConcurrencyType)concurrencyType error:(NSError **)error
{
    // ObjectModel from any models in app bundle
    NSManagedObjectModel *managedObjectModel = [NSManagedObjectModel mergedModelFromBundles:nil];

    // Coordinator with in-mem store type
    NSPersistentStoreCoordinator *coordinator = [[NSPersistentStoreCoordinator alloc] initWithManagedObjectModel:managedObjectModel];
    [coordinator addPersistentStoreWithType:NSInMemoryStoreType configuration:nil URL:nil options:nil error:error];

    // Context with private queue
    NSManagedObjectContext *context = [[NSManagedObjectContext alloc] initWithConcurrencyType:concurrencyType];
    context.persistentStoreCoordinator = coordinator;

    return context;
}

@end

Hi @aceontech, this is a really good example for me to using Core data when I try to Unit test with managedObject.

But I still have some problem,

First, I didn't use Specta , I just using XCTestCase, and I move the code go to + (void)setUp and + (void)tearDown, is it correct?
(In official document, it said these two method just like Specta's beforeAll block and afterAll block.)

Second, I put the context = nil; in +(void)tearDown, sometimes cause crash, is it correct in ARC?

My code like the following attachment:

NSManagedObjectContext *inMemoryContext;

@interface ViewModelTest : XCTestCase

@end

@implementation ViewModelTest

#pragma mark - Setup

+ (void)setUp {
    inMemoryContext = [NSManagedObjectContext testing_inMemoryContext:NSPrivateQueueConcurrencyType error:nil];
}

+ (void)tearDown {
//    I comment this, crash go away 

//    if (inMemoryContext) {
//        inMemoryContext = nil;
//    }
}