KeyboardScrollController.swift

//
//  KeyboardScrollController.swift
//
//  Created by Daniel Tartaglia on 9/16/15.
//  Copyright © 2015. MIT License.
//

import UIKit


final class KeyboardScrollController: NSObject {

    /// `view` is the primary view of the screen
    /// `bottomConstraint` is the constraint that connects the bottom of the scroll view to the bottom of the main view.
    init(view: UIView, bottomConstraint: NSLayoutConstraint) {
        self.view = view
        self.scrollView = view.findDeepChildSatisfying { $0 is UIScrollView } as! UIScrollView
        self.bottomConstraint = bottomConstraint
        savedConstant = bottomConstraint.constant
        super.init()
        NotificationCenter.default.addObserver(self, selector: #selector(keyboardWillShowNotification), name: UIResponder.keyboardWillShowNotification, object: nil)
        NotificationCenter.default.addObserver(self, selector: #selector(keyboardWillHideNotification), name: UIResponder.keyboardWillHideNotification, object: nil)
        NotificationCenter.default.addObserver(self, selector: #selector(keyboardWillChangeFrameNotification), name: UIResponder.keyboardWillChangeFrameNotification, object: nil)
    }

    deinit {
        NotificationCenter.default.removeObserver(self)
    }

    @objc func keyboardWillShowNotification(notification: Notification) {
        guard let keyboardInfo = KeyboardInfo(notification) else { return }
        view.layoutIfNeeded()
        UIView.animate(withDuration: keyboardInfo.animationDuration, animations: {
            [unowned self] in
            self.bottomConstraint.constant = max(keyboardInfo.frameEnd.height, self.savedConstant)
            self.view.layoutIfNeeded()
        })

        if let firstResponder = self.view.findDeepChildSatisfying(criteria: { $0.isFirstResponder }), firstResponder is UITextView {
            let frame = self.scrollView.convert(firstResponder.bounds, from: firstResponder)
            self.scrollView.scrollRectToVisible(frame, animated: false)
        }
    }

    @objc func keyboardWillHideNotification(notification: Notification) {
        guard let keyboardInfo = KeyboardInfo(notification) else { return }
        view.layoutIfNeeded()
        UIView.animate(withDuration: keyboardInfo.animationDuration, animations: {
            [unowned self] in
            self.bottomConstraint.constant = self.savedConstant
            self.view.layoutIfNeeded()
        })
    }

    @objc func keyboardWillChangeFrameNotification(notification: Notification) {
        guard let keyboardInfo = KeyboardInfo(notification) else { return }
        view.layoutIfNeeded()
        UIView.animate(withDuration: keyboardInfo.animationDuration, animations: {
            [unowned self] in
            self.bottomConstraint.constant = max(keyboardInfo.frameEnd.height, self.savedConstant)
            self.view.layoutIfNeeded()
        })
    }

    private let view: UIView
    private let scrollView: UIScrollView
    private let bottomConstraint: NSLayoutConstraint
    private let savedConstant: CGFloat

}

struct KeyboardInfo {
    var animationCurve: UIView.AnimationCurve
    var animationDuration: Double
    var isLocal: Bool
    var frameBegin: CGRect
    var frameEnd: CGRect
}

extension KeyboardInfo {
    init?(_ notification: Notification) {
        guard notification.name == UIResponder.keyboardWillShowNotification || notification.name == UIResponder.keyboardWillChangeFrameNotification else { return nil }
        let u = notification.userInfo!

        animationCurve = UIView.AnimationCurve(rawValue: u[UIWindow.keyboardAnimationCurveUserInfoKey] as! Int)!
        animationDuration = u[UIWindow.keyboardAnimationDurationUserInfoKey] as! Double
        isLocal = u[UIWindow.keyboardIsLocalUserInfoKey] as! Bool
        frameBegin = u[UIWindow.keyboardFrameBeginUserInfoKey] as! CGRect
        frameEnd = u[UIWindow.keyboardFrameEndUserInfoKey] as! CGRect
    }
}