Add Drop Shadow To UIVisualEffectView

May 15, 2017

The iOS 10 Maps app displays a UIVisualEffectView with a drop shadow effect. The Maps app table view and map options view (vertical "toolbar") appear to hover above the map content.

iOS 10 Maps app

Let's implement the same visual style

A sample iOS project and Playground is available on GitHub.

Core ingredients

  • one MKMapView
  • one reusable "helper" view we will name PassThroughView
  • one UIVisualEffectView with a UIBlurEffect (.extraLight is nice)
  • one UIImageView with a 9-part UIImage containing the shadow and a transparent center

A key to the solution is using a 9-part UIImage with a transparent "middle".


Dynamically creating 9-part shadow image

First let's create a simple representation of a shadow.

struct Shadow {
    let offset: CGSize
    let blur: CGFloat
    let color: UIColor

Next let's add a custom method to UIImage that generates a 9-part shadow image.

Note: there are several ways to generate this type of image. Feel free to adjust the implementation to suite your needs. For example you may want to add a thin inner border.

extension UIImage {

    static func resizableShadowImage(withSideLength sideLength: CGFloat, cornerRadius: CGFloat, shadow: Shadow) -> UIImage {
        // The image is a square, which makes it easier to set up the cap insets.
        // Note: this implementation assumes an offset of CGSize(0, 0)

        let lengthAdjustment = sideLength + (shadow.blur * 2.0)
        let graphicContextSize = CGSize(width: lengthAdjustment, height: lengthAdjustment)

        // Note: the image is transparent
        UIGraphicsBeginImageContextWithOptions(graphicContextSize, false, UIScreen.main.scale)
        let context = UIGraphicsGetCurrentContext()!
        defer {

        let roundedRect = CGRect(x: shadow.blur, y: shadow.blur, width: sideLength, height: sideLength)
        let shadowPath = UIBezierPath(roundedRect: roundedRect, cornerRadius: cornerRadius)
        let color = shadow.color.cgColor

        // Cut out the middle
        context.clip(using: .evenOdd)

        context.setShadow(offset: shadow.offset, blur: shadow.blur, color: color)

        let capInset = cornerRadius + shadow.blur
        let edgeInsets = UIEdgeInsets(top: capInset, left: capInset, bottom: capInset, right: capInset)
        let image = UIGraphicsGetImageFromCurrentImageContext()!

        return image.resizableImage(withCapInsets: edgeInsets, resizingMode: .tile)

Building A Sample App

Sample app view hierarchy

+ map view (fills the entire screen)
  + parent view (e.g. `PassThroughView`)
    + visual effect view
      + content view
        + your views (with clear backgrounds)
    + image view with 9-part shadow image with "hole" cut in the middle
Sample appSample app view hierarchy

Creating A Reusable "PassThroughView"

Setting up some basic view properties

The Properties are used when laying out the views and generating the shadow image.

final class PassThroughView: UIView {

    fileprivate enum Properties {
        static let cornerRadius: CGFloat = 10.0
        static let shadow: Shadow = Shadow(offset: CGSize(), blur: 6.0, color: .lightGray)


Exposing an API for adding subviews

Developers add subviews to the PassThroughView's contentView. The contentView is a simple proxy to the visual effect view's contentView.

     var contentView: UIView {
        return visualEffectView.contentView


Setting up some basic view properties

The initializers call a private initializer named selfInit to set up the view hierarchy and apply the layout constraints.

     private lazy var visualEffectView = self.lazyVisualEffectView()
    private lazy var shadowView = self.lazyShadowView()

    convenience init() {
        self.init(frame: CGRect())

    override init(frame: CGRect) {
        super.init(frame: frame)


    required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)



Add the subviews (visual effect view and shadow image view)

The shadowView sits below the visualEffectView. This is because the generated shadow image is not pixel-perfect aligned with the visual effect view's rounded corners. I am sure there are some optimizations that can be made.

     private func selfInit() {
        backgroundColor = .clear



Apply the constraints

You'll notice the shadow view constraints include the blurRadius. The shadow view hugs or wraps the visual effect view.

         let blurRadius = Properties.shadow.blur
            visualEffectView.topAnchor.constraint(equalTo: topAnchor),
            visualEffectView.trailingAnchor.constraint(equalTo: trailingAnchor),
            visualEffectView.bottomAnchor.constraint(equalTo: bottomAnchor),
            visualEffectView.leadingAnchor.constraint(equalTo: leadingAnchor),

            shadowView.topAnchor.constraint(equalTo: topAnchor, constant: -blurRadius),
            shadowView.trailingAnchor.constraint(equalTo: trailingAnchor, constant: blurRadius),
            shadowView.bottomAnchor.constraint(equalTo: bottomAnchor, constant: blurRadius),
            shadowView.leadingAnchor.constraint(equalTo: leadingAnchor, constant: -blurRadius),

Creating the visual effect view

extension PassThroughView {

    private func lazyVisualEffectView() -> UIVisualEffectView {
        let view = UIVisualEffectView(effect: UIBlurEffect(style: .extraLight))
        view.translatesAutoresizingMaskIntoConstraints = false
        view.layer.cornerRadius = Properties.cornerRadius
        view.layer.masksToBounds = true

        return view


Setting up the image view with a 9-part image view

The Properties we declared above are used to create the shadow image.

     private func lazyShadowView() -> UIImageView {

        let image = resizeableShadowImage(
            withCornerRadius: Properties.cornerRadius,
            shadow: Properties.shadow,
            shouldDrawCapInsets: showCapInsetLines

        let view = UIImageView(image: image)
        view.translatesAutoresizingMaskIntoConstraints = false

        return view

    private func resizeableShadowImage(
        withCornerRadius cornerRadius: CGFloat,
        shadow: Shadow,
        shouldDrawCapInsets: Bool
    ) -> UIImage {

        let sideLength = cornerRadius * 5 // trial and error: a multiple of 5 seems to create a decent shadow image
        return UIImage.resizableShadowImage(
            withSideLength: sideLength,
            cornerRadius: cornerRadius,
            shadow: shadow,
            shouldDrawCapInsets: showCapInsetLines

Get the code

Be sure to check out the sample iOS project and Playground on GitHub.

Got a better way to generate the shadow image? Let me know via a pull request.

Also, take a look at my post named Visualize iOS 10 Maps App View Hierarchy In Xcode. It shows how to visualize any iOS simulator app's view hierarchy.