KeyboardAwareScrollViewCoordinator.swift

import Foundation
import UIKit

/**
 * Coordinates contentInset on scroll view to always show full content on keyboard appearing.
 */
class KeyboardAwareScrollViewCoordinator: NSObject {
    
    let scrollView: UIScrollView
    private let originalContentInset: UIEdgeInsets
    
    init(scrollView: UIScrollView) {
        self.scrollView = scrollView
        originalContentInset = scrollView.contentInset
        super.init()
        
        NotificationCenter.default.addObserver(self, selector: #selector(keyboardWillHide(notification:)), name: NSNotification.Name.UIKeyboardWillHide, object: nil)
        NotificationCenter.default.addObserver(self, selector: #selector(keyboardWillShow(notification:)), name: NSNotification.Name.UIKeyboardWillShow, object: nil)
    }
    
    deinit {
        NotificationCenter.default.removeObserver(self, name: NSNotification.Name.UIKeyboardWillHide, object: nil)
        NotificationCenter.default.removeObserver(self, name: NSNotification.Name.UIKeyboardWillShow, object: nil)
    }
    
    func keyboardWillHide(notification: NSNotification) {
        self.scrollView.contentInset = originalContentInset
    }
    
    func keyboardWillShow(notification: NSNotification) {
        guard let keyboardFrameEnd = notification.userInfo?[UIKeyboardFrameEndUserInfoKey] as? CGRect,
            // Better solution to get a window? scrollView.window might not be in place yet
            let scrollViewRect = UIApplication.shared.keyWindow?.convert(scrollView.bounds, from: scrollView) else {
                return
        }
        
        let intersection = scrollViewRect.intersection(keyboardFrameEnd)
        // increase content inset with a height covered by a keyboard
        scrollView.contentInset = originalContentInset
        scrollView.contentInset.bottom += intersection.height > 0 ? intersection.height : 0
        scrollView.scrollIndicatorInsets = scrollView.contentInset
    }
}