Custom inputView performing. Custom inputView doesn't appear unless default keyboard is opened

I have a problem that I cannot perform custom inputView unless keyboard is opened. I have 2 UITextField one of them has custom inputView. Let's call them tf1 and tf2. tf2 has custom inputView. if I tap tf2 first nothing happening. If I tap tf1 first and default keyboard appears and then when I click tf2 custom inputView appears too. If there is no keyboard on the screen custom inputView doesn't appear. If there is a keyboard on the screen custom inputView can appear. Why?

How I assign inputview is shown below :

let numPad = KeyboardViewController(nibName:"KeyboardView",bundle: NSBundle.mainBundle())
let numPadFrame = CGRectMake(0, 0, 1024, 352)

override func viewDidLoad() {
    super.viewDidLoad()

    customKeyboard = numPad.view
    customKeyboard.frame = numPadFrame
    tf2.inputView = customKeyboard

Finally I made it. I've deleted the codes about loading nib etc. from viewDidLoad and created a function for it.

Here the function

func loadInterface() {

    // load the nib file
    let calculatorNib = UINib(nibName: "KeyboardView", bundle: NSBundle.mainBundle())
    // instantiate the view
    let calculatorView = calculatorNib.instantiateWithOwner(self, options: nil)[0] as! UIView

    self.view.frame = CGRectMake(0, 0, 1024, 352)
    let objects = calculatorNib.instantiateWithOwner(self, options: nil)
    self.view = objects[0] as! UIView
    self.inputView?.addSubview(calculatorView)


}

and call this function in viewDidLoad.

    override func viewDidLoad() {
        super.viewDidLoad()

        loadInterface()

        self.view.autoresizingMask = UIViewAutoresizing.FlexibleHeight
        self.view.translatesAutoresizingMaskIntoConstraints = false


//        let nib = UINib(nibName: "KeyboardView", bundle: NSBundle.mainBundle())
//        self.view.frame = CGRectMake(0, 0, 1024, 352)
//        let objects = nib.instantiateWithOwner(self, options: nil)
//        view = objects[0] as! UIView;
        var counter = 0
        for view in self.view.subviews {
            if let btn = view as? UIButton {
                btn.layer.cornerRadius = 5.0
                btn.translatesAutoresizingMaskIntoConstraints = false
                btn.tag = counter
                counter++

            }
        }
    }

And it solved everything.