KeyboardAvoidance.swift

import UIKit

public protocol KeyboardAvoidance: class {
    var scrollView: UIScrollView { get }
}

private var willShowTokenKey = "willShowTokenKey"
private var willHideTokenKey = "willHideTokenKey"

public extension KeyboardAvoidance where Self: UIViewController {
    
    var willShowToken: NSObjectProtocol? {
        get {
            return objc_getAssociatedObject(self, &willShowTokenKey) as? NSObjectProtocol
        }
        set(newValue) {
            objc_setAssociatedObject(self, &willShowTokenKey, newValue, objc_AssociationPolicy.OBJC_ASSOCIATION_RETAIN)
        }
    }
    
    var willHideToken: NSObjectProtocol? {
        get {
            return objc_getAssociatedObject(self, &willHideTokenKey) as? NSObjectProtocol
        }
        set(newValue) {
            objc_setAssociatedObject(self, &willHideTokenKey, newValue, objc_AssociationPolicy.OBJC_ASSOCIATION_RETAIN)
        }
    }
    
    func startKeyboardAvoidance() {
        
        willShowToken = NSNotificationCenter.defaultCenter().addObserverForName(UIKeyboardWillShowNotification, object: nil, queue: NSOperationQueue.mainQueue()) { [weak self] notif in
           self?.scrollView.keyboardWillShow(notif)
        }
        
        willHideToken = NSNotificationCenter.defaultCenter().addObserverForName(UIKeyboardWillHideNotification, object: nil, queue: NSOperationQueue.mainQueue()) { [weak self] notif in
            self?.scrollView.keyboardWillHide(notif)
        }
        
    }
    
    func stopKeyboardAvoidance() {
        _ = willShowToken.map(NSNotificationCenter.defaultCenter().removeObserver)
        _ = willHideToken.map(NSNotificationCenter.defaultCenter().removeObserver)
    }
    
}