How to update input accessory view to handle iPhone X with external keyboard

ViewController.swift

//
//  ViewController.swift
//  TEST_InputViewX
//
//  Created by Greg Pierce on 10/5/17.
//  Copyright © 2017 Agile Tortoise. All rights reserved.
//
//  Demonstrates how to update an input accessory view
//  to work on iPhone X with external keyboard attached
//  using safe area insets.
//
//  This adds the necessary padding under the input accessory
//  So it's not clipped by home button area.

import UIKit

class ViewController: UIViewController, UITextViewDelegate {
    
    struct Constants {
        static let AccessoryViewHeight: CGFloat = 44.0
    }
    
    @IBOutlet weak var textView: UITextView!
    var accessoryView: UIView?
    
    override func viewDidLoad() {
        super.viewDidLoad()

        // observe keyboard events
        let nc = NotificationCenter.default
        nc.addObserver(self, selector: #selector(keyboardDidShow(_:)), name: NSNotification.Name.UIKeyboardDidShow, object: nil)
        nc.addObserver(self, selector: #selector(keyboardWillHide(_:)), name: NSNotification.Name.UIKeyboardWillHide, object: nil)
        nc.addObserver(self, selector: #selector(keyboardWillChangeFrame(_:)), name: NSNotification.Name.UIKeyboardWillChangeFrame, object: nil)
    }
    
    func textViewShouldBeginEditing(_ textView: UITextView) -> Bool {
        buildInputView()
        textView.inputAccessoryView = accessoryView
        return true
    }
    
    func buildInputView() {
        // create input accessory view
        guard accessoryView == nil else { return }
        
        let accView = UIView(frame: CGRect(x: 0.0, y: 0.0, width: self.view.bounds.size.width, height: Constants.AccessoryViewHeight))
        accView.autoresizingMask = [.flexibleHeight] // important! allows it to resize
        accView.backgroundColor = .red
        
        // create a nested content view for your actual input controls
        let contentView = UIView(frame: .zero)
        contentView.backgroundColor = .green
        
        // constrain content view to safe area of parent view
        contentView.translatesAutoresizingMaskIntoConstraints = false
        accView.addSubview(contentView)
        contentView.leadingAnchor.constraint(equalTo: accView.safeAreaLayoutGuide.leadingAnchor).isActive = true
        contentView.trailingAnchor.constraint(equalTo: accView.safeAreaLayoutGuide.trailingAnchor).isActive = true
        contentView.topAnchor.constraint(equalTo: accView.safeAreaLayoutGuide.topAnchor).isActive = true
        contentView.heightAnchor.constraint(equalToConstant: Constants.AccessoryViewHeight).isActive = true
        
        accessoryView = accView
    }
    
    func updateInputViewFrame() {
        guard let accView = accessoryView else { return }
        
        // calculate the accessory view height based on safe insets
        let newHeight = accView.safeAreaInsets.bottom + Constants.AccessoryViewHeight
        if newHeight != accView.frame.size.height {
            accView.frame = CGRect(x: 0.0, y: 0.0, width: self.view.bounds.size.width, height: newHeight)
            textView.reloadInputViews()
        }
    }
    
    @objc func keyboardDidShow(_ note: NSNotification) {
        // update height of accessory view if needed
        updateInputViewFrame()
    }
    
    @objc func keyboardWillChangeFrame(_ note: NSNotification) {
        // update height of accessory view if needed
        updateInputViewFrame()
    }
    
    @objc func keyboardWillHide(_ note: NSNotification) {

    }
    
    @IBAction func hideKeyboard(_ sender: Any) {
        textView.resignFirstResponder()
    }

}

Thanks! The if newHeight != accView.frame.size.height { seems to be pretty important, otherwise the device will get stuck in an endless cycle of keyboardDidShow getting called.