Custom UIViewController that takes care of repositioning its view above a raised window. Written for Swift 3.0.

KeyboardAwareUIViewController.swift

//
// The MIT License (MIT)
//
// Copyright (c) 2017 Noor Dawod (noor@fine47.com)
//
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in all
// copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
// SOFTWARE.
//
// Inspired by Pavle Mijatovic: https://stackoverflow.com/a/45323660/4202478
//

import UIKit

/// A custom UIViewController that automatically panes its underlying view when a keyboard is shown
/// and a text field is focused.
open class KeyboardAwareUIViewController: UIViewController {

  // MARK: - Overridden methods.

  override open func viewDidAppear(_ animated: Bool) {
    super.viewDidAppear(animated)
    self.addKeyboardObservers()
  }

  override open func viewDidDisappear(_ animated: Bool) {
    super.viewDidAppear(animated)
    self.removeKeyboardObservers()
  }

  override open func touchesBegan(_ touches: Set<UITouch>, with event: UIEvent?) {
    self.resignAllFirstResponders()
  }

  // MARK: - Custom implementations.

  /// Add observers to be notified when the keyboard is shown, changed or hidden. If you override
  /// this method and don't provide your own observers, remember to call 
  /// super.addKeyboardObservers().
  func addKeyboardObservers() {
    NotificationCenter.default.addObserver(
      self,
      selector: #selector(self.keyboardWillChangeFrame),
      name: NSNotification.Name.UIKeyboardWillChangeFrame,
      object: nil
    )
    NotificationCenter.default.addObserver(
      self,
      selector: #selector(self.keyboardWillHide),
      name: NSNotification.Name.UIKeyboardWillHide,
      object: nil
    )
  }

  /// Remove observers that were added previously. If you override this method and don't provide 
  /// your own observers, remember to call removeKeyboardObservers().
  func removeKeyboardObservers() {
    NotificationCenter.default.removeObserver(
      self,
      name: NSNotification.Name.UIKeyboardWillChangeFrame,
      object: self.view.window
    )
    NotificationCenter.default.removeObserver(
      self,
      name: NSNotification.Name.UIKeyboardWillHide,
      object: self.view.window
    )
  }

  // MARK: - Observers implementations.

  /// Method's notified when the keyboard is about to be shown or change its size.
  ///
  /// - Parameter notification: System keyboard notification
  func keyboardWillChangeFrame(notification: NSNotification) {
    if
      let window = self.view.window,
      let responder = KeyboardAwareUIViewController.firstResponder(in: self.view),
      let userInfo = notification.userInfo,
      let keyboardFrameValue = userInfo[UIKeyboardFrameBeginUserInfoKey] as? NSValue,
      let keyboardAnimationDuration = userInfo[UIKeyboardAnimationDurationUserInfoKey] as? NSNumber
    {
      let keyboardHeight = keyboardFrameValue.cgRectValue.height
      UIView.animate(
        withDuration: Double(keyboardAnimationDuration),
        animations: {
          window.frame.origin.y = min(
            0,
            -min(
              keyboardHeight,
              responder.layer.position.y
                - (window.bounds.height - keyboardHeight - responder.bounds.height)
                * self.spacingPercentageFromTop()
            )
          )
          self.view.layoutIfNeeded()
        }
      )
    }
  }

  /// Method's notified when the keyboard is about to be dismissed.
  ///
  /// - Parameter notification: System keyboard notification
  func keyboardWillHide(notification: NSNotification) {
    if
      let window = self.view.window,
      let userInfo = notification.userInfo,
      let animationDuration = userInfo[UIKeyboardAnimationDurationUserInfoKey] as? NSNumber
    {
      UIView.animate(
        withDuration: Double(animationDuration),
        animations: {
          window.frame.origin.y = 0
          self.view.layoutIfNeeded()
        }
      )
    }
  }

  // MARK: - Utility methods.

  /// How much space (in percentage of remaining available space) to designate under the focused
  /// text field. By default, text fields will be positioned 2/3th the space from top. Override
  /// if you want to change this, for example 0.5 if you want to center-position the focused field.
  func spacingPercentageFromTop() -> CGFloat {
    return 2 / 3
  }

  /// Asks the system to resign all first responders (usually input fields), which effectively
  /// causes the keyboard to dismiss itself.
  func resignAllFirstResponders() {
    self.view.endEditing(true)
  }
}

extension KeyboardAwareUIViewController {

  /// Recursively traverses a view until a focused text field is found. May return nil if none is
  /// found and all views and subviews were exhausted.
  ///
  /// - Parameter subview: Subview to look in
  /// - Returns: Focused text field if found, nil otherwise
  static func firstResponder(in subview: UIView) -> UITextField? {
    var textField: UITextField?

    // Recursively look for the focused text field.
    KeyboardAwareUIViewController.firstResponderImpl([subview], textField: &textField)

    return textField
  }

  private static func firstResponderImpl(_ subviews: [UIView], textField: inout UITextField?) {
    for view in subviews {
      if let tf = view as? UITextField, view.isFirstResponder {
        textField = tf
        return
      }
      if !view.subviews.isEmpty {
        KeyboardAwareUIViewController.firstResponderImpl(view.subviews, textField: &textField)
      }
    }
  }
}