问如何获得类似于支持iOS 6的iOS 7主屏幕的视差运动偏移？

EN

import UIKit

class CustomMotionEffect: UIMotionEffect {

    override func keyPathsAndRelativeValues(forViewerOffset viewerOffset: UIOffset) -> [String : Any]? {
        var motionEffectDictionary = [String : Any]()

        //Horizontal
        if viewerOffset.horizontal > 0 { //Device tilted right

        } else if viewerOffset.horizontal < 0 { //Device tilted leftß

        } else {//Device horizontally perpendicular

        }

        //Vertical
        if viewerOffset.vertical > 0 { //Device tilted down

        } else if viewerOffset.vertical < 0 { //Device tilted up

        } else { //Device vertically perpendicular

        }
        return motionEffectDictionary
    }

}