Other class view cannot be loaded into TabBar Controller

Question

Hi,

I am making a project with 3 child views that are switched by a TabBar.

What I did

1. Create a project (Window-Based)
2. Add 3 child classes (Window-Based too)
3. Add a Tab Bar Controller to MainWindow.xib
4. For each view under Tab Bar Controller, the 'Class', 'Name', 'Title' and 'NIB Name' are all set to connect to the corresponding class
5. Add @class, IBOutlet are done in AppDelegate.h and AppDelegate.m
6. In didFinishLaunchingWithOptions, I called [window addSubView: tabBarController.view];
7. Save and Run --> Tab Bar with 3 items can be seen
8. But ... the 3 child views ARE NOT LOADED and SHOWN ... :(

Anything that I missed ?

Million Thanks