UIViewController+ScrollWhenKeyboardAppear.swift

//
//  UIViewController+ScrollWhenKeyboardAppear.swift
//  LIFE
//
//  Created by Ryoga Kitagawa on 8/8/16.
//  Copyright © 2016 Givery Inc. All rights reserved.
//

import Foundation

extension UIViewController {
    func startKeyboardAvoiding() {
        NSNotificationCenter.defaultCenter().addObserver(self, selector: #selector(UIViewController.scrollTextFieldWhehKeybordShown(_:)), name: UIKeyboardWillShowNotification, object: nil)
        NSNotificationCenter.defaultCenter().addObserver(self, selector: #selector(UIViewController.restoreScrollTextField(_:)), name: UIKeyboardWillHideNotification, object: nil)
    }

    func endKeyboardAvoidance() {
        NSNotificationCenter.defaultCenter().removeObserver(self)
    }

    @objc func scrollTextFieldWhehKeybordShown(notification: NSNotification) {
        guard let textField = self.view.currentFirstResponder() as? UIView,
            scrollView = textField.findSuperView(UIScrollView.self),
            userInfo = notification.userInfo,
            keyboardFrame = userInfo[UIKeyboardFrameEndUserInfoKey]?.CGRectValue,
            animationDuration = userInfo[UIKeyboardAnimationDurationUserInfoKey]?.doubleValue
            else { return }

        let margin = keyboardMarginForResponderView(textField)
        let maxOffset = scrollView.contentSize.height - scrollView.bounds.height + keyboardFrame.height
        let origOffset = scrollView.contentOffset

        scrollView.contentInset = UIEdgeInsetsZero
        scrollView.scrollIndicatorInsets = UIEdgeInsetsZero

        let convertedKeyboardFrame = scrollView.convertRect(keyboardFrame, fromView: nil)
        let convertedTextFieldFrame = textField.convertRect(textField.bounds, toView: scrollView)

        let diffOfOffsetY = convertedTextFieldFrame.maxY - convertedKeyboardFrame.minY + margin
        if diffOfOffsetY > 0 {
            UIView.animateWithDuration(animationDuration) {
                let offsetY = min(diffOfOffsetY + origOffset.y, maxOffset)
                let insetsY = max(diffOfOffsetY + origOffset.y - maxOffset, 0)
                let contentInsets = UIEdgeInsetsMake(0, 0, insetsY, 0)
                scrollView.contentInset = contentInsets
                scrollView.scrollIndicatorInsets = contentInsets
                scrollView.contentOffset = CGPointMake(0, offsetY)
            }
        }
    }

    @objc func restoreScrollTextField(notification: NSNotification) {
        guard let textField = self.view.currentFirstResponder() as? UIView,
            let scrollView = textField.findSuperView(UIScrollView.self)
            else { return }

        scrollView.contentInset = UIEdgeInsetsZero
        scrollView.scrollIndicatorInsets = UIEdgeInsetsZero
    }

    func keyboardMarginForResponderView(view: UIView) -> CGFloat {
        return 8
    }
}

private extension UIView {
    func currentFirstResponder() -> UIResponder? {
        if self.isFirstResponder() {
            return self
        }

        for view in self.subviews {
            if let responder = view.currentFirstResponder() {
                return responder
            }
        }

        return nil
    }

    func insertTextToCurrentField(text: String) {
        guard let field = self.currentFirstResponder() as? UIKeyInput else { return }
        field.insertText(text)
    }

    func findSuperView<T>(ofType: T.Type) -> T? {
        if let s = self.superview {
            switch s {
            case let s as T:
                return s
            default:
                return s.findSuperView(ofType)
            }
        }
        return nil
    }
}