KeyboardTransition.swift

//
//  KeyboardTransition.swift
//  ViewControllerKeyboardAnimated
//
//  Created by Ryan Nystrom on 11/7/17.
//  Copyright © 2017 Ryan Nystrom. All rights reserved.
//

import UIKit

protocol Keyboardable {
    var focusResponder: UIResponder { get }
}

extension UIViewAnimationCurve {
    var toOptions: UIViewAnimationOptions {
        return UIViewAnimationOptions(rawValue: UInt(rawValue << 16))
    }
}

class KeyboardTransition: NSObject, UIViewControllerTransitioningDelegate, UIViewControllerAnimatedTransitioning {

    private struct TransitionDetails {
        let duration: TimeInterval
        let curve: UIViewAnimationCurve
    }

    private weak var controller: (UIViewController & Keyboardable)?
    private var transitionDetails: TransitionDetails?
    private var transitionContext: UIViewControllerContextTransitioning?

    init(controller: UIViewController & Keyboardable) {
        self.controller = controller

        super.init()

        NotificationCenter.default.addObserver(
            self,
            selector: #selector(onKeyboardWillShow(notification:)),
            name: .UIKeyboardWillShow,
            object: nil
        )
    }

    // MARK: UIViewControllerTransitioningDelegate

    func animationController(forPresented presented: UIViewController, presenting: UIViewController, source: UIViewController) -> UIViewControllerAnimatedTransitioning? {
        return self
    }

    func animationController(forDismissed dismissed: UIViewController) -> UIViewControllerAnimatedTransitioning? {
        return nil
    }

    // MARK: UIViewControllerAnimatedTransitioning

    func transitionDuration(using transitionContext: UIViewControllerContextTransitioning?) -> TimeInterval {
        return 0.3
    }

    func animateTransition(using transitionContext: UIViewControllerContextTransitioning) {
        guard let toVC = transitionContext.viewController(forKey: .to),
            let fromVC = transitionContext.viewController(forKey: .from) else {
                transitionContext.completeTransition(true)
                return
        }

        self.transitionContext = transitionContext

        transitionContext.containerView.addSubview(toVC.view)

        var frame = fromVC.view.frame
        frame.origin.y = fromVC.view.frame.maxY
        toVC.view.frame = frame

        controller?.focusResponder.becomeFirstResponder()

        didChange()
    }

    // MARK: Private API

    func didChange() {
        guard let details = transitionDetails, let context = transitionContext else { return }

        guard let toVC = context.viewController(forKey: .to),
            let fromVC = context.viewController(forKey: .from) else {
                context.completeTransition(true)
                return
        }

        transitionContext?.containerView.addSubview(toVC.view)

        var frame = fromVC.view.frame
        frame.origin.y = fromVC.view.frame.maxY
        toVC.view.frame = frame

        UIView.animateKeyframes(withDuration: details.duration, delay: 0, options: [.beginFromCurrentState, UIViewKeyframeAnimationOptions(rawValue: details.curve.toOptions.rawValue)], animations: {
            toVC.view.frame = fromVC.view.frame
        }) { (_) in
            context.completeTransition(true)
        }
    }

    // MARK: Notifications

    @objc func onKeyboardWillShow(notification: NSNotification) {
        transitionDetails = TransitionDetails(
            duration: notification.userInfo?[UIKeyboardAnimationDurationUserInfoKey] as? TimeInterval ?? 0.3,
            curve: notification.userInfo?[UIKeyboardAnimationCurveUserInfoKey] as? UIViewAnimationCurve ?? UIViewAnimationCurve.easeInOut
        )
        NotificationCenter.default.removeObserver(self)
        didChange()
    }
    
}