KeyboardObserver.swift

//
//  KeyboardObserver.swift
//  Product
//
//  Created by muukii on 3/17/16.
//  Copyright © 2016 eure. All rights reserved.
//

import Foundation
import RxSwift
import RxCocoa

public final class KeyboardObserver {
    
    public struct KeyboardInfo {

        public let frameBegin: CGRect
        public let frameEnd: CGRect
        
        init(notification: NSNotification) {
            let frameEnd = notification.userInfo?[UIKeyboardFrameEndUserInfoKey]?.CGRectValue
            let frameBegin = notification.userInfo?[UIKeyboardFrameBeginUserInfoKey]?.CGRectValue
            self.frameBegin = frameBegin!
            self.frameEnd = frameEnd!
        }
    }
    
    public let willChangeFrame = PublishSubject<KeyboardInfo>()
    public let didChangeFrame = PublishSubject<KeyboardInfo>()
    
    public let willShow = PublishSubject<KeyboardInfo>()
    public let didShow = PublishSubject<KeyboardInfo>()
    public let willHide = PublishSubject<KeyboardInfo>()
    public let didHide = PublishSubject<KeyboardInfo>()
    
    public init() {
        
        NSNotificationCenter.defaultCenter()
            .rx_notification(UIKeyboardWillChangeFrameNotification)
            .map { KeyboardInfo(notification: $0) }
            .bindTo(self.willChangeFrame)
            .addDisposableTo(self.disposeBag)
        
        NSNotificationCenter.defaultCenter()
            .rx_notification(UIKeyboardDidChangeFrameNotification)
            .map { KeyboardInfo(notification: $0) }
            .bindTo(self.didChangeFrame)
            .addDisposableTo(self.disposeBag)
        
        NSNotificationCenter.defaultCenter()
            .rx_notification(UIKeyboardWillShowNotification)
            .map { KeyboardInfo(notification: $0) }
            .bindTo(self.willShow)
            .addDisposableTo(self.disposeBag)
        
        NSNotificationCenter.defaultCenter()
            .rx_notification(UIKeyboardDidShowNotification)
            .map { KeyboardInfo(notification: $0) }
            .bindTo(self.didShow)
            .addDisposableTo(self.disposeBag)
        
        NSNotificationCenter.defaultCenter()
            .rx_notification(UIKeyboardWillHideNotification)
            .map { KeyboardInfo(notification: $0) }
            .bindTo(self.willHide)
            .addDisposableTo(self.disposeBag)
        
        NSNotificationCenter.defaultCenter()
            .rx_notification(UIKeyboardDidHideNotification)
            .map { KeyboardInfo(notification: $0) }
            .bindTo(self.didHide)
            .addDisposableTo(self.disposeBag)
    }
    
    private let disposeBag = DisposeBag()
}

In case anyone needs a Swift 5 version:

import Foundation
import RxSwift
import RxCocoa

struct KeyboardObserver {
    struct KeyboardInfo {
        
        let frameBegin: CGRect
        let frameEnd: CGRect
        
        init(notification: Notification) {
            self.frameBegin = (notification.userInfo?[UIResponder.keyboardFrameBeginUserInfoKey] as? NSValue)?.cgRectValue ?? CGRect.zero
            self.frameEnd = (notification.userInfo?[UIResponder.keyboardFrameEndUserInfoKey] as? NSValue)?.cgRectValue ?? CGRect.zero
        }
    }
    
    private let disposeBag = DisposeBag()
    
    let willChangeFrame = PublishSubject<KeyboardInfo>()
    let didChangeFrame = PublishSubject<KeyboardInfo>()
    
    let willShow = PublishSubject<KeyboardInfo>()
    let didShow = PublishSubject<KeyboardInfo>()
    let willHide = PublishSubject<KeyboardInfo>()
    let didHide = PublishSubject<KeyboardInfo>()
    
    init() {
        
        NotificationCenter.default.rx.notification(UIResponder.keyboardWillChangeFrameNotification)
            .observeOn(MainScheduler.instance)
            .map { KeyboardInfo(notification: $0) }
            .bind(to: self.willChangeFrame)
            .disposed(by: self.disposeBag)
        
        NotificationCenter.default.rx.notification(UIResponder.keyboardDidChangeFrameNotification)
            .observeOn(MainScheduler.instance)
            .map { KeyboardInfo(notification: $0) }
            .bind(to: self.didChangeFrame)
            .disposed(by: self.disposeBag)
        
        NotificationCenter.default.rx.notification(UIResponder.keyboardWillShowNotification)
            .observeOn(MainScheduler.instance)
            .map { KeyboardInfo(notification: $0) }
            .bind(to: self.willShow)
            .disposed(by: self.disposeBag)
        
        NotificationCenter.default.rx.notification(UIResponder.keyboardDidShowNotification)
            .observeOn(MainScheduler.instance)
            .map { KeyboardInfo(notification: $0) }
            .bind(to: self.didShow)
            .disposed(by: self.disposeBag)
        
        NotificationCenter.default.rx.notification(UIResponder.keyboardWillHideNotification)
            .observeOn(MainScheduler.instance)
            .map { KeyboardInfo(notification: $0) }
            .bind(to: self.willHide)
            .disposed(by: self.disposeBag)
        
        NotificationCenter.default.rx.notification(UIResponder.keyboardDidHideNotification)
            .observeOn(MainScheduler.instance)
            .map { KeyboardInfo(notification: $0) }
            .bind(to: self.didHide)
            .disposed(by: self.disposeBag)
    }
}