Maybe there isn't a way. I'm pretty sure there is a way to customize the UITabBarController layout to work.

My question is about having a header and body in UITabBarController. I have the following code in the TabBarLayout.

TabBarLayout.swift

import UIKit

class TabBarLayout: UITabBarController {
  
  @IBOutlet var viewHeader: UIView!

  override func viewDidLoad() {
    super.viewDidLoad()
    viewHeader.translatesAutoresizingMaskIntoConstraints = false
    view.addSubview(viewHeader)
    let leftSide = NSLayoutConstraint(item: viewHeader!, attribute: .left, relatedBy: .equal, toItem: view, attribute: .left, multiplier: 1.0, constant: 0.0)
    let rightSide = NSLayoutConstraint(item: viewHeader!, attribute: .right, relatedBy: .equal, toItem: view, attribute: .right, multiplier: 1.0, constant: 0.0)
    let topSide = NSLayoutConstraint(item: viewHeader!, attribute: .top, relatedBy: .equal, toItem: view, attribute: .top, multiplier: 1.0, constant: 0.0)
    let height = NSLayoutConstraint(item: viewHeader!, attribute: .height, relatedBy: .equal, toItem: nil, attribute: .notAnAttribute, multiplier: 1, constant: 100)
    view.addConstraint(leftSide)
    view.addConstraint(rightSide)
    view.addConstraint(topSide)
    view.addConstraint(height)
  }
}

TabBar.storyboard

The Storyboard has UITabBarController with Storyboard References.

Buy.storyboard

The Buy Storyboard shows the following screenshot:

BuyController.swift

import UIKit

class BuyController: UIViewController {
  
  override func viewWillAppear(_ animated: Bool) {
    super.viewWillAppear(animated)
    print("BUYCONTROLLER")
    let tbc = tabBarController as! TabBarLayout
    
    // How do I align this view's topAnchor to viewHeader's bottom anchor?
  }
}

How should I align the Buy Storyboard top anchor to viewHeader's bottom anchor so that the Test Label is visible?

Result of iOS Simulator

Try to adjust the additionalSafeAreaInsets in viewWillAppear of BuyController.swift. This ensures that the view respects the space taken up by viewHeader.

BuyController.swift:

override func viewWillAppear(_ animated: Bool) {
    super.viewWillAppear(animated)
    
    let tbc = tabBarController as! TabBarLayout
    
    let headerHeight = tbc.viewHeader.frame.height
    self.additionalSafeAreaInsets.top = headerHeight
}

This method dynamically sets additionalSafeAreaInsets.top based on viewHeader’s height. So BuyController’s content starts below viewHeader, preventing overlap.