一般APP首次啟動(dòng)，會(huì)有登錄界面，需要在AppDelegate里設(shè)置rootViewController

self.window.rootViewController  = loginVC;

//登錄成功后，切換根視圖，上一個(gè)根視圖自動(dòng)釋放
UITabBarController *tabbar = [[UITabBarController alloc] init];
[UIApplication sharedApplication].keyWindow.rootViewController = tabbar;

如果，根視圖為A，A --> loginVC是經(jīng)過 present 模態(tài)跳轉(zhuǎn)過來的

self.window.rootViewController  = A;
......
//模態(tài)跳轉(zhuǎn)
[A presentViewController:nav animated:YES completion:nil];

//登錄成功后，切換根視圖，需要dismiss釋放上一個(gè)根視圖
[self dismissViewControllerAnimated:NO completion:^{
        UITabBarController *tabbar = [[UITabBarController alloc] init];
        [UIApplication sharedApplication].keyWindow.rootViewController = tabbar;
}];