This implementation of main() for a Mac application dynamically switch the AppDelegate at runtime based on the presence of the TestingAppDelegate class, which is only available during unit testing.

main.m

#import <Cocoa/Cocoa.h>

int main(int argc, char *argv[])
{
    if (NSClassFromString(@"AppNameTests.TestingAppDelegate") != nil) {
        id appDelegate = [[NSClassFromString(@"AppNameTests.TestingAppDelegate") alloc] init];
        NSApplication * application = [NSApplication sharedApplication];
        [application setDelegate:appDelegate];
        [NSApp run];
    } else {
        return NSApplicationMain(argc,(const char **) argv);
    }
        
}