Move view when keyboard appears, changes frame, disappears

KeyboardRelatedConstraintAnimator.swift

import UIKit

/// Animate and modify constraint's constant when keyboard appears, changes frames or disappears.
/// Use-case: move content above keyboard when it appears.
class KeyboardRelatedConstraintAnimator {
	unowned let constraint: NSLayoutConstraint
	unowned let view: UIView
	
	private var defaultValue: CGFloat
	
	/// - note: Keyboard notifications observers are added on init and removed on deinit
	///
	/// - Parameters:
	///   - constraint: *(unowned)* constraint wich constant will be modified on keyboard animations, class respect's constraint's default constant, it is saved on initialization
	///   - view: *(unowned)* view owning constraint, animations will be performed in that view
	init(constraint: NSLayoutConstraint, in view: UIView) {
		self.constraint = constraint
		self.defaultValue = constraint.constant
		self.view = view
	}
	
	func addObservers() {
		NotificationCenter.default.addObserver(forName: .UIKeyboardWillChangeFrame, object: nil, queue: .main) { [weak self] (notification) in
			self?.animate(notification: notification, willHide: false)
		}
		
		NotificationCenter.default.addObserver(forName: .UIKeyboardWillHide, object: nil, queue: .main) { [weak self] (notification) in
			self?.animate(notification: notification, willHide: true)
		}
	}
	
	func removeObservers() {
		NotificationCenter.default.removeObserver(self, name: .UIKeyboardWillChangeFrame, object: nil)
		NotificationCenter.default.removeObserver(self, name: .UIKeyboardWillHide, object: nil)
	}
	
	private func animate(notification: Notification, willHide: Bool) {
		guard let userInfo = notification.userInfo,
			let animationDuration = userInfo[UIKeyboardAnimationDurationUserInfoKey] as? Double,
			let keyboardEndFrame = (userInfo[UIKeyboardFrameEndUserInfoKey] as? NSValue)?.cgRectValue,
			let rawAnimationCurveNumber = userInfo[UIKeyboardAnimationCurveUserInfoKey] as? NSNumber else { return }
		
		let rawAnimationCurve = rawAnimationCurveNumber.uint32Value << 16
		let animationCurve = UIViewAnimationOptions(rawValue: UInt(rawAnimationCurve))
		
		self.constraint.constant = willHide ? self.defaultValue : keyboardEndFrame.height + self.defaultValue
		
		UIView.animate(withDuration: animationDuration, delay: 0, options: [UIViewAnimationOptions.beginFromCurrentState, animationCurve], animations: {
			self.view.layoutIfNeeded()
		}, completion: nil)
	}
}