Xcode7 貌似要指定viewController ,
_window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];
_window.backgroundColor = [UIColor blueColor];
_window.rootViewController = [[UIViewController alloc] init];
[_window makeKeyAndVisible];
_window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];
_window.backgroundColor = [UIColor blueColor];
_window.rootViewController = [[UIViewController alloc] init];
[_window makeKeyAndVisible];
2016-04-09