Example of mocking [NSNotificationCenter defaultCenter]

TestingWithNotificationCenter.m

@interface DTClassToTest : NSObject{
    id notificationObserver;
}
@property (strong, nonatomic) NSNotificationCenter *notificationCenter;
@property (assign, nonatomic) BOOL receivedNotification;
@end

@implementation DTClassToTest
-(id)init{
    return [self initWithNotificationCenter:[NSNotificationCenter defaultCenter]];
}
-(id)initWithNotificationCenter:(NSNotificationCenter *)notificationCenter{
    if (!(self = [super init])) return self;
    _notificationCenter = notificationCenter;
    _receivedNotification = NO;
    return self;
}

- (void)registerForNotifications{
    id __weak wself = self;
    notificationObserver = [self.notificationCenter addObserverForName:@"CONTRIVED_NOTIFICATION"
                                                                object:nil
                                                                 queue:[NSOperationQueue mainQueue]
                                                            usingBlock:^(NSNotification *n){
                                                                DTClassToTest *self = wself;
                                                                self.receivedNotification = YES;
                                                            }];
}
- (void)dealloc{
    [_notificationCenter removeObserver:notificationObserver];
}
@end

@interface DTClassToTestTests : XCTestCase{
    DTClassToTest *objectUnderTest;
    NSNotificationCenter *testNotificationCenter;
}
@end

// If you can inject notification center into a class for tests, then this is the best way to do this
@implementation DTClassToTestTests

- (void)setUp{
    [super setUp];
    testNotificationCenter = [NSNotificationCenter new];
    objectUnderTest = [[DTClassToTest alloc] initWithNotificationCenter:testNotificationCenter];
}

- (void)test_responds_to_notification_correctly{
    [objectUnderTest registerForNotifications];
    [testNotificationCenter postNotificationName:@"CONTRIVED_NOTIFICATION" object:nil];
    XCTAssertTrue(objectUnderTest.receivedNotification);
}
@end

@interface DTClassToTestTestsTheClassMockWay : XCTestCase{
    DTClassToTest *objectUnderTest;
    NSNotificationCenter *testNotificationCenter;
    id mockCenter;
}
@end

// if there is no injection point, you can use OCMock to mock the defaultCenter class method. but you need to explicitly call
// stopMocking in your test tearDown.
@implementation DTClassToTestTestsTheClassMockWay

- (void)setUp{
    [super setUp];
    testNotificationCenter = [NSNotificationCenter new];
    mockCenter = [OCMockObject niceMockForClass:NSNotificationCenter.class];
    [[[[mockCenter stub] classMethod] andReturn:testNotificationCenter] defaultCenter];
    objectUnderTest = [DTClassToTest new]; // Will use [NSNotificationCenter defaultCenter];
}

- (void)tearDown{
    [mockCenter stopMocking]; //if you omit this line in your tear down, your tests will not finish.
    [super tearDown];
}

- (void)test_responds_to_notification_correctly{
    [objectUnderTest registerForNotifications];
    [testNotificationCenter postNotificationName:@"CONTRIVED_NOTIFICATION" object:nil];
    XCTAssertTrue(objectUnderTest.receivedNotification);
}                  
@end