observer_cleanup.swift

import UIKit

typealias KeyboardHeightDuration = (height: CGFloat, duration: Double)

// New notification type
let RemoveKeyboardNotifications = "RemoveKeyboardNotifications"

protocol KeyboardAvoidable: class {
    var layoutConstraintsForKeyboard: [NSLayoutConstraint] { get }
    func addKeyboardObservers()
    func removeKeyboardObservers()
}

extension KeyboardAvoidable where Self: UIViewController {

    func addKeyboardObservers() {

        // Store returned observer one for willShow
        let observerOne = NSNotificationCenter
            .defaultCenter()
            .addObserverForName(UIKeyboardWillShowNotification,
                                object: nil,
                                queue: nil) { [weak self] notification in

                                    self?.keyboardWillShow(notification)
        }

        // Store returned observer two for willHide
        let observerTwo = NSNotificationCenter
            .defaultCenter()
            .addObserverForName(UIKeyboardWillHideNotification,
                                object: nil,
                                queue: nil) { [weak self] notification in

                                    self?.keyboardWillHide(notification)
        }
        
        // Create an object holder for a new listener event
        // that will be triggered in `removeKeyboardObservers`
        var observerThree: NSObjectProtocol?
        
        observerThree = NSNotificationCenter
            .defaultCenter()
            .addObserverForName(RemoveKeyboardNotifications,
                                object: nil,
                                queue: nil) { [weak observerOne, weak observerTwo] notification in
                                    
                guard let observerOne = observerOne else { return }
                
                // Remove observer for willShow from above
                NSNotificationCenter
                    .defaultCenter()
                    .removeObserver(observerOne,
                                    name: UIKeyboardWillShowNotification,
                                    object: nil)
                                    
                guard let observerTwo = observerTwo else { return }
                
                // Remove observer for willHide from above
                NSNotificationCenter
                    .defaultCenter()
                    .removeObserver(observerTwo,
                                    name: UIKeyboardWillHideNotification,
                                    object: nil)
                
                guard let observerThree = observerThree else { return }
                
                // Remove observer attached to this entire block that we previously setup space for
                NSNotificationCenter
                    .defaultCenter()
                    .removeObserver(observerThree,
                                    name: RemoveKeyboardNotifications,
                                    object: nil)
                                    
        }
    }

    func removeKeyboardObservers() {
        // Trigger new notification
        NSNotificationCenter
            .defaultCenter()
            .postNotificationName(RemoveKeyboardNotifications, object: nil)
    }

    private func keyboardWillShow(notification: NSNotification) {
        guard var info = getKeyboardInfo(notification) else { return }

        if let tabBarHeight = tabBarController?.tabBar.frame.height {
            info.height -= tabBarHeight
        }

        animateConstraints(info.height, duration: info.duration)
    }

    private func keyboardWillHide(notification: NSNotification) {
        guard let info = getKeyboardInfo(notification) else { return }

        animateConstraints(0, duration: info.duration)
    }

    private func getKeyboardInfo(notification: NSNotification) -> KeyboardHeightDuration? {
        guard let userInfo = notification.userInfo else { return nil }
        guard let rect = userInfo[UIKeyboardFrameEndUserInfoKey]?.CGRectValue() else { return nil }
        guard let duration = userInfo[UIKeyboardAnimationDurationUserInfoKey]?.doubleValue else { return nil }

        return (rect.height, duration)
    }

    private func animateConstraints(constant: CGFloat, duration: Double) {
        layoutConstraintsForKeyboard.forEach { c in
            c.constant = constant
        }

        UIView.animateWithDuration(duration) {
            self.view.layoutIfNeeded()
        }
    }
}

// Example Implementation

//    final class NiceViewController: UIViewController {
//
//        @IBOutlet weak var scrollViewBottomConstraint: NSLayoutConstraint!
//
//        override func viewWillAppear(animated: Bool) {
//            super.viewWillAppear(animated)
//
//            addKeyboardObservers()
//        }
//
//        override func viewWillDisappear(animated: Bool) {
//            super.viewWillDisappear(animated)
//
//            removeKeyboardObservers()
//        }
//    }
//
//    extension NiceViewController: KeyboardAvoidable {
//
//        var layoutConstraintsForKeyboard: [NSLayoutConstraint] {
//            return [scrollViewBottomConstraint]
//        }
//    }