Unit Testing in Xcode, does it run the app?

I'm running into a strange problem that I haven't run into before.

When you do cmd+U to run your Unit Tests (OCUnit for example) does it actually call the main.m, new up the appDelegate and run the app as if your had pressed cmd+R?

I only ask because I'm using CoreData behind this DataLayer. I'm mocking out the DataLayer successfully in my tests, but once I implemented a getAll method that is actually calling CoreData, the app/xcode is throwing an exception about the managed object model can't be nil. Which I understand, but I'm not meaning to actually new up the DataLayer class, and I've put a break point in my mainviewcontroller loadView method where it is calling the DataLayer getAll method. It shouldn't matter with tests because this is a mock object, but it's apparently calling the real instance.

So back to my question, when pressing cmd+U does it also run the app first then run the tests?

The application is actually run but there is a trick you can use to prevent it from running.

int main(int argc, char* argv[]) {
    int returnValue;

    @autoreleasepool {
        BOOL inTests = (NSClassFromString(@"SenTestCase") != nil
                     || NSClassFromString(@"XCTest") != nil);    

        if (inTests) {
            //use a special empty delegate when we are inside the tests
            returnValue = UIApplicationMain(argc, argv, nil, @"TestsAppDelegate");
        }
        else {
            //use the normal delegate 
            returnValue = UIApplicationMain(argc, argv, nil, @"AppDelegate");
        }
    }

    return returnValue;
}

Here's a variation of Sulthan's answer that uses XCTest, which is the default for test classes generated by XCode 5.

int main(int argc, char * argv[])
{
    @autoreleasepool {
        BOOL runningTests = NSClassFromString(@"XCTestCase") != nil;
        if(!runningTests)
        {
            return UIApplicationMain(argc, argv, nil, NSStringFromClass([AppDelegate class]));
        }
        else
        {
            return UIApplicationMain(argc, argv, nil, @"TestAppDelegate");
        }
    }
}

This goes into main.m, which should be under Supporting Files in a standard project layout.

Then in your tests directory add:

TestAppDelegate.h

#import <Foundation/Foundation.h>

@interface TestAppDelegate : NSObject<UIApplicationDelegate>
@end

TestAppDelegate.m

#import "TestAppDelegate.h"

@implementation TestAppDelegate
@end

In Swift, I prefere to bypass a normal execution path inside application: didFinishLaunchingWithOptions:

func application(application: UIApplication, didFinishLaunchingWithOptions launchOptions: [NSObject: AnyObject]?) -> Bool {
    guard normalExecutionPath() else {
        window = nil
        return false
    }

    // regular setup

    return true
}

private func normalExecutionPath() -> Bool {
    return NSClassFromString("XCTestCase") == nil
}

Code inside guard will remove any views created from storyboard.