[KeyboardAvoiding] #protocol #iOS

KeyboardAvoiding.swift

//
//  KeyboardAvoiding.swift
//
//  Created by Jay Lee on 08/14/2019.
//  Copyright © 2019 Jay Lee <jaeho.lee@snu.ac.kr>
//  This work is free. You can redistribute it and/or modify it under the
//  terms of the Do What The Fuck You Want To Public License, Version 2,
//  as published by Sam Hocevar. See http://www.wtfpl.net/ for more details.
//

import UIKit

protocol KeyboardAvoiding: UIViewController {

  var keyboardAvoidingScrollView: UIScrollView { get }
  var footerView: UIView? { get }

  var activeField: UITextField? { get }
  var activeFieldIsSubviewed: Bool { get }

  var notificationCenter: NotificationCenter { get }
  var keyboardObservers: [NSObjectProtocol] { get set }
}


extension KeyboardAvoiding {

  var activeFieldIsSubviewed: Bool { return false }
  var footerView: UIView? { return nil }
  var notificationCenter: NotificationCenter { return .default }


  func registerForKeyboardNotifications() {
    keyboardObservers = [
      notificationCenter.addObserver(
        forName: UIResponder.keyboardWillShowNotification,
        object: nil,
        queue: nil,
        using: keyboardWillShow),

      notificationCenter.addObserver(
        forName: UIResponder.keyboardDidShowNotification,
        object: nil,
        queue: nil,
        using: keyboardDidShow),

      notificationCenter.addObserver(
        forName: UIResponder.keyboardWillHideNotification,
        object: nil,
        queue: nil,
        using: keyboardWillHide),
    ]
  }


  func unregisterForKeyboardNotifications() {
    keyboardObservers.forEach { notificationCenter.removeObserver($0) }
  }


  func keyboardWillShow(_ notification: Notification) {
    guard
      let userInfo = notification.userInfo as NSDictionary?,
      let value = userInfo[UIResponder.keyboardFrameEndUserInfoKey] as? NSValue
      else { return }

    let keyboardHeight = value.cgRectValue.size.height
    let contentInset = UIEdgeInsets(
      top: 0,
      left: 0,
      bottom: keyboardHeight - (footerView?.frame.height ?? 0),
      right: 0)

    keyboardAvoidingScrollView.contentInset = contentInset
    keyboardAvoidingScrollView.scrollIndicatorInsets = contentInset
  }


  func keyboardDidShow(_ notification: Notification) {
    guard
      let userInfo = notification.userInfo as NSDictionary?,
      let value = userInfo[UIResponder.keyboardFrameEndUserInfoKey] as? NSValue,
      let activeField = activeField
      else { return }

    let keyboardHeight = value.cgRectValue.size.height
    // If active text field is hidden by keyboard, scroll it so it's visible
    var frame = view.frame
    frame.size.height -= keyboardHeight - (footerView?.frame.height ?? 0)

    let activeFieldOrigin: CGPoint
    // If activeField is not a direct subview of the `view`, convert the
    // `frame.origin` with respect to the `view`
    if activeFieldIsSubviewed {
      activeFieldOrigin = activeField.superview?
        .convert(activeField.frame.origin, to: view) ?? activeField.frame.origin
    } else {
      activeFieldOrigin = activeField
        .convert(activeField.frame.origin, to: view)
    }

    if !frame.contains(activeFieldOrigin) {
      keyboardAvoidingScrollView
        .scrollRectToVisible(activeField.frame, animated: true)
    }
  }


  func keyboardWillHide(_ notification: Notification) {
    keyboardAvoidingScrollView.contentInset = UIEdgeInsets.zero
    keyboardAvoidingScrollView.scrollIndicatorInsets = UIEdgeInsets.zero
  }
}