參照此處所描述的陰影定義方式 https://wcc723.gitbooks.io/google_design_translate/content/layout-principles.html

ExampleView.swift

//
//  ShadowViewB.swift
//  SwiftShadow
//
//  Created by 默司 on 2016/8/4.
//  Copyright © 2016年 默司. All rights reserved.
//

import UIKit

@IBDesignable
class ExampleView: UIView {
    
    let innerView = UIView()
    
    var cornerRadius: CGFloat = 10 {
        didSet {
            self.layer.cornerRadius = cornerRadius
            self.drawMaterialShadow()
        }
    }
    
    required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
        
        setup()
    }
    
    override init(frame: CGRect) {
        super.init(frame: frame)
        
        setup()
    }
    
    func setup() {
        self.layer.cornerRadius = cornerRadius
        self.drawMaterialShadow()
        
        self.innerView.frame = CGRectMake(0, 0, self.bounds.width - 32, self.bounds.height - 32)
        self.innerView.backgroundColor = UIColor.whiteColor()
        
        self.addSubview(innerView)
        
        self.innerView.center = CGPointMake(self.bounds.width/2, self.bounds.height/2)
        
        self.innerView.drawMaterialShadow()
    }
    
}

UIView+MaterialShadow.swift

//
//  UIView+MaterialShadow.swift
//  SwiftShadow
//
//  Created by 默司 on 2016/8/5.
//  Copyright © 2016年 默司. All rights reserved.
//

import UIKit

extension UIView {
    func drawMaterialShadow() {
        drawMaterialShadow(1)
    }
    
    func drawMaterialShadow(depth: UInt) {
        var depth = depth
        
        if depth < 1 {
            depth = 1
        }
        
        if depth > 5 {
            depth = 5
        }
        
        let shadowLayer = CAShapeLayer()
        
        let path = UIBezierPath(roundedRect: self.bounds, cornerRadius: self.layer.cornerRadius)
        let topShadowLayer = CAShapeLayer()
        topShadowLayer.shadowPath = path.CGPath
        topShadowLayer.shadowOpacity = 1.0
        
        let bottomShadowLayer = CAShapeLayer()
        bottomShadowLayer.shadowPath = path.CGPath
        bottomShadowLayer.shadowOpacity = 1.0
        
        switch depth {
        case 1:
            topShadowLayer.shadowColor = UIColor ( red: 0.0, green: 0.0, blue: 0.0, alpha: 0.12 ).CGColor
            topShadowLayer.shadowRadius = 1.5
            topShadowLayer.shadowOffset =  CGSizeMake(0, 1.0)
            
            bottomShadowLayer.shadowColor = UIColor ( red: 0.0, green: 0.0, blue: 0.0, alpha: 0.24 ).CGColor
            bottomShadowLayer.shadowRadius = 1.0
            bottomShadowLayer.shadowOffset =  CGSizeMake(0, 1.0)
            break
        case 2:
            topShadowLayer.shadowColor = UIColor ( red: 0.0, green: 0.0, blue: 0.0, alpha: 0.16 ).CGColor
            topShadowLayer.shadowRadius = 3.0
            topShadowLayer.shadowOffset =  CGSizeMake(0, 3.0)
            
            bottomShadowLayer.shadowColor = UIColor ( red: 0.0, green: 0.0, blue: 0.0, alpha: 0.23 ).CGColor
            bottomShadowLayer.shadowRadius = 3.0
            bottomShadowLayer.shadowOffset =  CGSizeMake(0, 3.0)
            break
        case 3:
            topShadowLayer.shadowColor = UIColor ( red: 0.0, green: 0.0, blue: 0.0, alpha: 0.19 ).CGColor
            topShadowLayer.shadowRadius = 10.0
            topShadowLayer.shadowOffset =  CGSizeMake(0, 10.0)
            
            bottomShadowLayer.shadowColor = UIColor ( red: 0.0, green: 0.0, blue: 0.0, alpha: 0.23 ).CGColor
            bottomShadowLayer.shadowRadius = 6.0
            bottomShadowLayer.shadowOffset =  CGSizeMake(0, 3.0)
            break
        case 4:
            topShadowLayer.shadowColor = UIColor ( red: 0.0, green: 0.0, blue: 0.0, alpha: 0.25 ).CGColor
            topShadowLayer.shadowRadius = 14.0
            topShadowLayer.shadowOffset =  CGSizeMake(0, 14.0)
            
            bottomShadowLayer.shadowColor = UIColor ( red: 0.0, green: 0.0, blue: 0.0, alpha: 0.22 ).CGColor
            bottomShadowLayer.shadowRadius = 10.0
            bottomShadowLayer.shadowOffset =  CGSizeMake(0, 5.0)
            break
        case 5:
            topShadowLayer.shadowColor = UIColor ( red: 0.0, green: 0.0, blue: 0.0, alpha: 0.30 ).CGColor
            topShadowLayer.shadowRadius = 19.0
            topShadowLayer.shadowOffset =  CGSizeMake(0, 19.0)
            
            bottomShadowLayer.shadowColor = UIColor ( red: 0.0, green: 0.0, blue: 0.0, alpha: 0.22 ).CGColor
            bottomShadowLayer.shadowRadius = 15.0
            bottomShadowLayer.shadowOffset =  CGSizeMake(0, 6.0)
            break
        default:
            break
        }
        
        shadowLayer.addSublayer(bottomShadowLayer)
        shadowLayer.addSublayer(topShadowLayer)
        
        let layer = CALayer()
        layer.frame = self.bounds
        layer.cornerRadius = self.layer.cornerRadius
        layer.backgroundColor = self.backgroundColor?.CGColor
        
        self.layer.insertSublayer(layer, atIndex: 0)
        self.layer.insertSublayer(shadowLayer, atIndex: 0)
    }
}