Present Modal ViewController when tap a TabBarItem (like camera VC in Instagram)

gistfile1.swift

// 1. First Create a Dummy UIViewController that has a root relation with the UITabBarController
// 2. Make this controller implement a UITabBarControllerDelegate
// 3. In ViewDidLoad:
  override func viewDidLoad() {
    super.viewDidLoad()
    self.tabBarController?.delegate = self
  }
// 4. Implement this delegate method:
  func tabBarController(tabBarController: UITabBarController, didSelectViewController viewController: UIViewController) {
    let isModalTab = viewController == self
    
    if isModalTab {
      let cameraVC = UIStoryboard.cameraVC() // 5. HERE YOU SPECIFY THE MODAL VIEW CONTROLLER!!!
      self.presentViewController(cameraVC, animated: true, completion: nil)
    }
  }
// 5. That's all

For this to work properly, the delegate method has to be implemented in the first viewController that would show on the UITabBarController.

Additionally, after stopping receiving tapGesture for your popup tab item you have to implement custom presentation style of popup viewcontroller inside the tabBarController didSelect func, otherwise it does not seem to work.