一般APP首次启动，会有登录界面，需要在AppDelegate里设置rootViewController
self.window.rootViewController = loginVC;
//登录成功后，切换根视图，上一个根视图自动释放
UITabBarController *tabbar = [[UITabBarController alloc] init];
[UIApplication sharedApplication].keyWindow.rootViewController = tabbar;
如果，根视图为A，A --> loginVC是经过 present 模态跳转过来的
self.window.rootViewController = A;
......
//模态跳转
[A presentViewController:nav animated:YES completion:nil];
//登录成功后，切换根视图，需要dismiss释放上一个根视图
[self dismissViewControllerAnimated:NO completion:^{
UITabBarController *tabbar = [[UITabBarController alloc] init];
[UIApplication sharedApplication].keyWindow.rootViewController = tabbar;
}];
