A basic, thin wrapper on keyboard appearance handling. This is mostly just a starting point: this only updates its `var currentMetrics`, which leaves to the user the exercise of adding whatever update hooks make sense for their codebase (likely through subclassing).

KeyboardMetricsService.swift

//
//  KeyboardMetricsService.swift
//  CrowdRise
//
//  Created by Josh Avant on 3/10/16.
//  Copyright © 2016 CrowdRise. All rights reserved.
//

import UIKit

class KeyboardMetricsService: NSObject {
    struct Metrics {
        let beginFrame: CGRect
        let endFrame: CGRect
        let animationDuration: Double
        let animationCurve: UInt // rawValue for UIViewAnimationCurve
        
        init(userInfo: [NSObject : AnyObject]) {
            let beginFrameValue = userInfo[UIKeyboardFrameBeginUserInfoKey] as? NSValue ?? NSValue(CGRect: CGRect.zero)
            beginFrame = beginFrameValue.CGRectValue()
            
            let endFrameValue = userInfo[UIKeyboardFrameEndUserInfoKey] as? NSValue ?? NSValue(CGRect: CGRect.zero)
            endFrame = endFrameValue.CGRectValue()
            
            let animationDurationNumber = userInfo[UIKeyboardAnimationDurationUserInfoKey] as? NSNumber ?? NSNumber(double: 0)
            animationDuration = animationDurationNumber.doubleValue
            
            let animationCurveNumber = userInfo[UIKeyboardAnimationDurationUserInfoKey] as? NSNumber ?? NSNumber(unsignedInteger: 0)
            animationCurve = animationCurveNumber.unsignedIntegerValue
        }
    }
    
    enum ChangeType {
        case WillChange
        case DidChange
    }

    private(set) var currentMetrics: Metrics?
    
    // Injected Dependencies
    var notificationCenter: NSNotificationCenter? {
        willSet {
            if notificationCenter != nil {
                removeObservers()
            }
        }
        didSet {
            if notificationCenter != nil {
                addObservers()
            }
        }
    }
    
    func handleKeyboardMetricsChange(changeType: ChangeType, metrics: Metrics) {
        currentMetrics = metrics
    }
    
    private func addObservers() {
        self.notificationCenter?.addObserver(self,
            selector: "keyboardWillChangeFrameNotification:",
            name: UIKeyboardWillChangeFrameNotification,
            object: nil)
        
        self.notificationCenter?.addObserver(self,
            selector: "keyboardDidChangeFrameNotification:",
            name: UIKeyboardDidChangeFrameNotification,
            object: nil)
        
    }
    
    private func removeObservers() {
        self.notificationCenter?.removeObserver(self,
            name: UIKeyboardWillChangeFrameNotification,
            object: nil)
        
        self.notificationCenter?.removeObserver(self,
            name: UIKeyboardDidChangeFrameNotification,
            object: nil)
    }
    
    private dynamic func keyboardWillChangeFrameNotification(notification: NSNotification) {
        if let userInfo = notification.userInfo {
            let metrics = Metrics(userInfo: userInfo)
            self.handleKeyboardMetricsChange(.WillChange, metrics: metrics)
        }
    }
    
    private dynamic func keyboardDidChangeFrameNotification(notification: NSNotification) {
        if let userInfo = notification.userInfo {
            let metrics = Metrics(userInfo: userInfo)
            self.handleKeyboardMetricsChange(.DidChange, metrics: metrics)
        }
    }
    
    deinit {
        removeObservers()
    }
}