Swift convenience wrapper for the userInfo values associated with a UIKeyboard notification

KeyboardNotification.swift

import UIKit

/// Wrapper for the NSNotification userInfo values associated with a keyboard notification.
///
/// It provides properties that retrieve userInfo dictionary values with these keys:
///
/// - UIKeyboardFrameBeginUserInfoKey
/// - UIKeyboardFrameEndUserInfoKey
/// - UIKeyboardAnimationDurationUserInfoKey
/// - UIKeyboardAnimationCurveUserInfoKey
public struct KeyboardNotification {
    
    let notification: Notification
    let userInfo: [AnyHashable : Any]
    
    /// Initializer
    ///
    /// :param: notification Keyboard-related notification
    public init(_ notification: Notification) {
        self.notification = notification
        if let userInfo = notification.userInfo {
            self.userInfo = userInfo
        } else {
            self.userInfo = [:]
        }
    }
    
    /// Start frame of the keyboard in screen coordinates
    public var screenFrameBegin: CGRect {
        if let value = userInfo[UIKeyboardFrameBeginUserInfoKey] as? NSValue {
            return value.cgRectValue
        }
        else {
            return .zero
        }
    }
    
    /// End frame of the keyboard in screen coordinates
    public var screenFrameEnd: CGRect {
        if let value = userInfo[UIKeyboardFrameEndUserInfoKey] as? NSValue {
            return value.cgRectValue
        }
        else {
            return .zero
        }
    }
    
    /// Keyboard animation duration
    public var animationDuration: Double {
        if let number = userInfo[UIKeyboardAnimationDurationUserInfoKey] as? NSNumber {
            return number.doubleValue
        }
        else {
            return 0.25
        }
    }
    
    /// Keyboard animation curve
    ///
    /// Note that the value returned by this method may not correspond to a
    /// UIViewAnimationCurve enum value.  For example, in iOS 7 and iOS 8,
    /// this returns the value 7.
    public var animationCurve: UInt {
        if let number = userInfo[UIKeyboardAnimationCurveUserInfoKey] as? NSNumber {
            return UInt(number.intValue)
        }
        return UInt(UIViewAnimationCurve.easeInOut.rawValue)
    }
    
    /// Start frame of the keyboard in coordinates of specified view
    ///
    /// :param: view UIView to whose coordinate system the frame will be converted
    /// :returns: frame rectangle in view's coordinate system
    public func frameBeginForView(view: UIView) -> CGRect {
        return view.convert(screenFrameBegin, from: view.window)
    }
    
    /// End frame of the keyboard in coordinates of specified view
    ///
    /// :param: view UIView to whose coordinate system the frame will be converted
    /// :returns: frame rectangle in view's coordinate system
    public func frameEndForView(view: UIView) -> CGRect {
        return view.convert(screenFrameEnd, from: view.window)
    }
}

ViewController.swift

// Example of using KeyboardNotification to update an NSLayoutConstraint

import UIKit

final class ViewController: UIViewController {

    // Outlet for a layout constraint that specifies distance from bottom of
    // a subview to the bottom of the view.
    // 
    // This constraint will be updated when the keyboard appears, disappears,
    // or changes size.
    @IBOutlet weak var bottomLayoutConstraint: NSLayoutConstraint!
    
    override func viewWillAppear(animated: Bool) {
        super.viewWillAppear(animated)

        NSNotificationCenter.defaultCenter().addObserver(self,
            selector: "keyboardWillChangeFrameNotification:",
            name: UIKeyboardWillChangeFrameNotification,
            object: nil)
    }

    override func viewDidDisappear(animated: Bool) {
        NSNotificationCenter.defaultCenter().removeObserver(self)
        super.viewDidDisappear(animated)
    }

    func keyboardWillChangeFrameNotification(notification: NSNotification) {
        let n = KeyboardNotification(notification)
        let keyboardFrame = n.frameEndForView(self.view)
        let animationDuration = n.animationDuration
        let animationCurve = n.animationCurve

        let viewFrame = self.view.frame
        let newBottomOffset = viewFrame.maxY - keyboardFrame.minY

        self.view.layoutIfNeeded()
        UIView.animateWithDuration(animationDuration,
            delay: 0,
            options: UIViewAnimationOptions(rawValue: animationCurve),
            animations: {
                self.bottomLayoutConstraint.constant = newBottomOffset
                self.view.layoutIfNeeded()
            },
            completion: nil
        )
    }
}

Updated for Swift 4