How IOS uses Tabbarcontroller

I've built a lot of app templates in Xcode, but many times we need more flexibility in initializing projects. Let me briefly explain how to create a Tabbar app from 0.

1. Create a project, because we write the program from the beginning, so theoretically there is no special requirements for the template. Here we choose Single View application template.
2. Once the creation is complete, we can start writing the program. IOS apps are based on a uiwindows, so we just need to create a good one VC and then set the Rootviewcontroller of Windows to the current VC. Here is the code

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-(BOOL) Application: (UIApplication *) application didfinishlaunchingwithoptions: (Nsdictionary *) launchOptions {
WORDLISTVC * ONEVC=[[WORDLISTVC alloc] init];
WORDLISTVC * TWOVC=[[WORDLISTVC alloc] init];
WORDLISTVC * THREEVC=[[WORDLISTVC alloc] init];
OneVC.tabBarItem.title = @Today's plan;
OneVC.tabBarItem.image = [UIImage imagenamed:@Plan-icon];
TwoVC.tabBarItem.title = @Vocabulary List;
TwoVC.tabBarItem.image = [UIImage imagenamed:@List-icon];
Self.placeholderVC.tabBarItem.title = @ ";
ThreeVC.tabBarItem.title = @Unfamiliar vocabulary;
ThreeVC.tabBarItem.image = [UIImage imagenamed:@Unfamiliar-words-icon];
Pvc.tabBarItem.title = @Set up;
Pvc.tabBarItem.image = [UIImage imagenamed:@Settings-icon "];
Uitabbarcontroller *tabbarcontroller = [[Uitabbarcontroller alloc]init];
Tabbarcontroller.delegate = self;
[Tabbarcontroller SETVIEWCONTROLLERS:@[ONEVC,TWOVC,THREEVC]];
[Tabbarcontroller setselectedindex:0];
TabbarController.tabBar.barTintColor = [Uicolor Whitecolor];
Self.tabbarcontroller = Tabbarcontroller;
Self.window.rootviewcontroller=self.tabbarcontroller;
return YES;
}
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How IOS uses Tabbarcontroller