[ios] Adding a view controller as a subview in another view controller

I have found few posts for this problem but none of them solved my issue.

Say like I've..

1. ViewControllerA
2. ViewControllerB

I tried to add ViewControllerB as a subview in ViewControllerA but, it's throwing an error like "fatal error: unexpectedly found nil while unwrapping an Optional value".

Below is the code...

ViewControllerA

var testVC: ViewControllerB = ViewControllerB();

override func viewDidLoad()
{
    super.viewDidLoad()
    self.testVC.view.frame = CGRectMake(0, 0, 350, 450);
    self.view.addSubview(testVC.view);
    // Do any additional setup after loading the view.
}

ViewControllerB is just a simple screen with a label in it.

ViewControllerB

 @IBOutlet weak var test: UILabel!

override func viewDidLoad() {
    super.viewDidLoad()
    test.text = "Success" // Throws ERROR here "fatal error: unexpectedly found nil while unwrapping an Optional value"
}

EDIT

With the suggested solution from the user answers, ViewControllerB in ViewControllerA is going off the screen. Grey border is the frame I have created for the subview.

Thanks to Rob, Updated Swift 4.2 syntax

let controller:WalletView = self.storyboard!.instantiateViewController(withIdentifier: "MyView") as! WalletView
controller.view.frame = self.view.bounds
self.view.addSubview(controller.view)
self.addChild(controller)
controller.didMove(toParent: self)

Thanks to Rob. Adding detailed syntax for your second observation :

let controller:MyView = self.storyboard!.instantiateViewControllerWithIdentifier("MyView") as! MyView
controller.ANYPROPERTY=THEVALUE // If you want to pass value
controller.view.frame = self.view.bounds
self.view.addSubview(controller.view)
self.addChildViewController(controller)
controller.didMoveToParentViewController(self)

And to remove the viewcontroller :

self.willMoveToParentViewController(nil)
self.view.removeFromSuperview()
self.removeFromParentViewController() 

func callForMenuView() {

    if(!isOpen)

    {
        isOpen = true

        let menuVC : MenuViewController = self.storyboard!.instantiateViewController(withIdentifier: "menu") as! MenuViewController
        self.view.addSubview(menuVC.view)
        self.addChildViewController(menuVC)
        menuVC.view.layoutIfNeeded()

        menuVC.view.frame=CGRect(x: 0 - UIScreen.main.bounds.size.width, y: 0, width: UIScreen.main.bounds.size.width-90, height: UIScreen.main.bounds.size.height);

        UIView.animate(withDuration: 0.3, animations: { () -> Void in
            menuVC.view.frame=CGRect(x: 0, y: 0, width: UIScreen.main.bounds.size.width-90, height: UIScreen.main.bounds.size.height);
    }, completion:nil)

    }else if(isOpen)
    {
        isOpen = false
      let viewMenuBack : UIView = view.subviews.last!

        UIView.animate(withDuration: 0.3, animations: { () -> Void in
            var frameMenu : CGRect = viewMenuBack.frame
            frameMenu.origin.x = -1 * UIScreen.main.bounds.size.width
            viewMenuBack.frame = frameMenu
            viewMenuBack.layoutIfNeeded()
            viewMenuBack.backgroundColor = UIColor.clear
        }, completion: { (finished) -> Void in
            viewMenuBack.removeFromSuperview()

        })
    }

Please also check the official documentation on implementing a custom container view controller:

https://developer.apple.com/library/content/featuredarticles/ViewControllerPGforiPhoneOS/ImplementingaContainerViewController.html#//apple_ref/doc/uid/TP40007457-CH11-SW1

This documentation has much more detailed information for every instruction and also describes how to do add transitions.

Translated to Swift 3:

func cycleFromViewController(oldVC: UIViewController,
               newVC: UIViewController) {
   // Prepare the two view controllers for the change.
   oldVC.willMove(toParentViewController: nil)
   addChildViewController(newVC)

   // Get the start frame of the new view controller and the end frame
   // for the old view controller. Both rectangles are offscreen.r
   newVC.view.frame = view.frame.offsetBy(dx: view.frame.width, dy: 0)
   let endFrame = view.frame.offsetBy(dx: -view.frame.width, dy: 0)

   // Queue up the transition animation.
   self.transition(from: oldVC, to: newVC, duration: 0.25, animations: { 
        newVC.view.frame = oldVC.view.frame
        oldVC.view.frame = endFrame
    }) { (_: Bool) in
        oldVC.removeFromParentViewController()
        newVC.didMove(toParentViewController: self)
    }
}

For Add and Remove ViewController

 var secondViewController :SecondViewController?

  // Adding 
 func add_ViewController() {
    let controller  = self.storyboard?.instantiateViewController(withIdentifier: "secondViewController")as! SecondViewController
    controller.view.frame = self.view.bounds
    self.view.addSubview(controller.view)
    self.addChild(controller)
    controller.didMove(toParent: self)
    self.secondViewController = controller
}

// Removing
func remove_ViewController(secondViewController:SecondViewController?) {
    if secondViewController != nil {
        if self.view.subviews.contains(secondViewController!.view) {
             secondViewController!.view.removeFromSuperview()
        }
        
    }
}

This code will work for Swift 4.2.

let controller:SecondViewController = 
self.storyboard!.instantiateViewController(withIdentifier: "secondViewController") as! 
SecondViewController
controller.view.frame = self.view.bounds;
self.view.addSubview(controller.view)
self.addChild(controller)
controller.didMove(toParent: self)