KeyboardInsettable.swift

//
//  KeyboardInsettable.swift
//
//  Created by Josh Avant on 9/11/18.
//

import Foundation
import UIKit

protocol KeyboardInsettable {
    var keyboardInsettableScrollView: UIScrollView { get }
    
    static var defaultScrollViewContentInset: UIEdgeInsets { get }
    
    func registerForKeyboardNotifications(willChangeFrameSelector selector: Selector)
    
    func handleKeyboardWillChangeFrame(notification: Notification)
}

// NOTE: Does not support adjustable iPad keyboard.

extension KeyboardInsettable {
    func registerForKeyboardNotifications(willChangeFrameSelector selector: Selector) {
        NotificationCenter.default.addObserver(self, selector: selector, name: NSNotification.Name.UIKeyboardWillChangeFrame, object: nil)
    }
    
    func handleKeyboardWillChangeFrame(notification: Notification) {
        guard let keyboardFrame = (notification.userInfo?[UIKeyboardFrameEndUserInfoKey] as? NSValue)?.cgRectValue,
            let animationDuration = notification.userInfo?[UIKeyboardAnimationDurationUserInfoKey] as? NSNumber,
            let animationCurve = notification.userInfo?[UIKeyboardAnimationCurveUserInfoKey] as? NSNumber,
            let window = UIApplication.shared.keyWindow,
            let superview = self.keyboardInsettableScrollView.superview else { return }
        
        let scrollViewYMinimum = window.convert(self.keyboardInsettableScrollView.frame, from: superview).minY
        let scrollViewYMaximum = scrollViewYMinimum + self.keyboardInsettableScrollView.frame.height
        let scrollViewYCoordinateRange = scrollViewYMinimum..<scrollViewYMaximum
        
        let keyboardYMinimum = keyboardFrame.origin.y
        let keyboardYMaximum = keyboardYMinimum + keyboardFrame.size.height
        let keyboardYCoordinateRange = keyboardYMinimum..<keyboardYMaximum
        
        let overlappingYCoordinateRange = scrollViewYCoordinateRange.clamped(to: keyboardYCoordinateRange)
        
        let overlapAmount = overlappingYCoordinateRange.upperBound - overlappingYCoordinateRange.lowerBound
        
        let newContentInsets = UIEdgeInsets(top: Self.defaultScrollViewContentInset.top,
                                            left: Self.defaultScrollViewContentInset.left,
                                            bottom: Self.defaultScrollViewContentInset.bottom + overlapAmount,
                                            right: Self.defaultScrollViewContentInset.right)
        
        let newIndicatorInsets = UIEdgeInsets(top: 0.0, left: 0.0, bottom: overlapAmount, right: 0.0)
        
        let curve = UIViewAnimationOptions(rawValue: animationCurve.uintValue)
        UIView.animate(withDuration: animationDuration.doubleValue, delay: 0, options: curve, animations: {
            self.keyboardInsettableScrollView.contentInset = newContentInsets
            self.keyboardInsettableScrollView.scrollIndicatorInsets = newIndicatorInsets
        }, completion: nil)
    }
}