Get height of iOS keyboard without displaying keyboard

This Swift class provides a turn-key solution that manages all the necessary notifications and initializations, letting you simply call a class method and have returned the keyboard size or height.

Calling from Swift:

let keyboardHeight = KeyboardService.keyboardHeight()
let keyboardSize = KeyboardService.keyboardSize()

Calling from Objective-C:

 CGFloat keyboardHeight = [KeyboardService keyboardHeight];
 CGRect keyboardSize = [KeyboardService keyboardSize];

If wanting to use this for initial view layout, call this from the viewWillAppear method of a class where you want the keyboard height or size before the keyboard appears. It should not be called in viewDidLoad, as a correct value relies on your views having been laid out. You can then set an autolayout constraint constant with the value returned from the KeyboardService, or use the value in other ways. For instance, you might want to obtain the keyboard height in prepareForSegue to assist in setting a value associated with the contents of a containerView being populated via an embed segue.

Note re safe area, keyboard height, and iPhone X:
The value for keyboard height returns the full height of the keyboard, which on the iPhone X extends to the edge of the screen itself, not just to the safe area inset. Therefore, if setting an auto layout constraint value with the returned value, you should attach that constraint to the superview bottom edge, not to the safe area.

Note re hardware keyboard in Simulator:
When a hardware keyboard is attached, this code will provide the on-screen height of that hardware keyboard, that is, no height. This state does need to be accounted for, of course, as this simulates what will occur if you have a hardware keyboard attached to an actual device. Therefore, your layout that is expecting a keyboard height needs to respond appropriately to a keyboard height of zero.

KeyboardService class:
As usual, if calling from Objective-C, you simply need to import the app's Swift bridging header MyApp-Swift.h in your Objective-C class.

import UIKit

class KeyboardService: NSObject {
    static var serviceSingleton = KeyboardService()
    var measuredSize: CGRect = CGRect.zero

    @objc class func keyboardHeight() -> CGFloat {
        let keyboardSize = KeyboardService.keyboardSize()
        return keyboardSize.size.height
    }

    @objc class func keyboardSize() -> CGRect {
        return serviceSingleton.measuredSize
    }

    private func observeKeyboardNotifications() {
        let center = NotificationCenter.default
        center.addObserver(self, selector: #selector(self.keyboardChange), name: .UIKeyboardDidShow, object: nil)
    }

    private func observeKeyboard() {
        let field = UITextField()
        UIApplication.shared.windows.first?.addSubview(field)
        field.becomeFirstResponder()
        field.resignFirstResponder()
        field.removeFromSuperview()
    }

    @objc private func keyboardChange(_ notification: Notification) {
        guard measuredSize == CGRect.zero, let info = notification.userInfo,
            let value = info[UIKeyboardFrameEndUserInfoKey] as? NSValue
            else { return }

        measuredSize = value.cgRectValue
    }

    override init() {
        super.init()
        observeKeyboardNotifications()
        observeKeyboard()
    }

    deinit {
        NotificationCenter.default.removeObserver(self)
    }    
}

Head nod:
The observeKeyboard method here based on the original approach outlined by Peres in the Objective-C answer to this question.

A quick solution that you could use, is the same one used when you want to cache the keyboard (the first time you show it, you get a slight delay...). The library is here. The interesting bits:

[[[[UIApplication sharedApplication] windows] lastObject] addSubview:field];
[field becomeFirstResponder];
[field resignFirstResponder];
[field removeFromSuperview];

So basically is showing it and then hiding it. You could listen for notifications and just get the height without actually seeing it. Bonus: you get to cache it. :)