Parallax.swift

//
//  Parallax.swift
//
//  Created by Nikolay Fiantsev on 21/02/2018.
//  Copyright © 2018 Nikolay Fiantsev. All rights reserved.
//

import UIKit

enum ParallaxMagnitude: Float {
  case background       =  10.0
  case foregroundSmall  = -10.0
  case foregroundMiddle = -20.0
  case foregroundMax    = -40.0
}

protocol Parallax {
  var foregroundViews: [UIView] { get }
  var backgroundView: UIView { get }
  func addParallax()
}

extension Parallax {
  
  func addParallax() {
    _addParallaxMotionEffect(toView: backgroundView, magnitude: .background)
    _addParallaxMotionEffect(toViews: foregroundViews, magnitude: .foregroundMiddle)
  }
  
  private func _addParallaxMotionEffect(toViews views: [UIView], magnitude: ParallaxMagnitude) {
    views.forEach {
      _addParallaxMotionEffect(toView: $0, magnitude: magnitude)
    }
  }
  
  private func _addParallaxMotionEffect(toView view: UIView, magnitude: ParallaxMagnitude) {
    let xMotion = UIInterpolatingMotionEffect(keyPath: "center.x", type: .tiltAlongHorizontalAxis)
    xMotion.minimumRelativeValue = -magnitude.rawValue
    xMotion.maximumRelativeValue = magnitude.rawValue
    
    let yMotion = UIInterpolatingMotionEffect(keyPath: "center.y", type: .tiltAlongVerticalAxis)
    yMotion.minimumRelativeValue = -magnitude.rawValue
    yMotion.maximumRelativeValue = magnitude.rawValue
    
    let group = UIMotionEffectGroup()
    group.motionEffects = [xMotion, yMotion]
    view.addMotionEffect(group)
  }
  
}