iOS 버전의 evernote에서는 이러한 구현이 있겠지요. 어떻게 하고 있다고 조사하면서 구현해 보았습니다. 구현 자체는 툴바를 inputaccessory 하는 것과 같은 노리로 상당히 안이하게 움직입니다. 비망록을 위해 써 남겨 둡니다.

참조

reference.swift

// Presented when object becomes first responder.  If set to nil, reverts to following responder chain.  If
    // set while first responder, will not take effect until reloadInputViews is called.
    open var inputView: UIView?

    open var inputAccessoryView: UIView?

구현

ViewController.swift

import UIKit
let TestView = UIView.init(frame: CGRect.init(x: 0, y: 0, width: 250, height: 250))


class ViewController: UIViewController, UITextViewDelegate {

    @IBOutlet weak var textView: UITextView!

    override func viewDidLoad() {
        super.viewDidLoad()

        textView.delegate = self
        let bgColor = UIColor.blue
        TestView.backgroundColor = bgColor
    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
    }

    func keyboardToolbar(textView: UITextView) {

        let toolbar: UIToolbar = UIToolbar(frame: CGRect(x: 0, y: 0, width: 1, height: 1))
        toolbar.barStyle = UIBarStyle.default
        toolbar.bounds.size.height = 28

        let flexSpace = UIBarButtonItem(barButtonSystemItem: UIBarButtonSystemItem.flexibleSpace, target: nil, action: nil)


        let done: UIBarButtonItem = UIBarButtonItem(title: "keyboard", style: UIBarButtonItemStyle.done, target: self, action: #selector(self.doneButtonActionn))
        done.tintColor = UIColor.red

        let clear: UIBarButtonItem = UIBarButtonItem(title: "UIView", style: UIBarButtonItemStyle.plain, target: self, action: #selector(self.doneButtonAction))
        clear.tintColor = UIColor.black


        var items = [UIBarButtonItem]()

        items.append(clear)
        items.append(flexSpace)
        items.append(done)
        toolbar.items = items
        toolbar.sizeToFit()
        textView.inputAccessoryView = toolbar
    }

    func doneButtonAction() {
        textView.inputView = TestView
        textView.resignFirstResponder()
        textView.becomeFirstResponder()
    }

    func doneButtonActionn() {
        textView.inputView = nil
        textView.reloadInputViews()
    }

    func textViewShouldBeginEditing(_ textView: UITextView) -> Bool {
        self.keyboardToolbar(textView: textView)
        return true
    }
}

참고

switching inputview for uitextfield
How to refresh keyboard/returnKey while editing textField in Swift?