How to add a shadow to a UIView

Swift version: 5.6

Paul Hudson    @twostraws    June 1st 2019

iOS can dynamically generate shadows for any UIView, and these shadows automatically adjust to fit the shape of the item in question – even following the curves of text inside a UILabel. This functionality is built right in, so all you need to do is configure its properties, and there are four you need to care about:

• shadowColor sets the color of the shadow, and needs to be a CGColor.
• shadowOpacity sets how transparent the shadow is, where 0 is invisible and 1 is as strong as possible.
• shadowOffset sets how far away from the view the shadow should be, to give a 3D offset effect.
• shadowRadius sets how wide the shadow should be.

Here's a simple example to get you started:

let yourView = UIView()
yourView.layer.shadowColor = UIColor.black.cgColor
yourView.layer.shadowOpacity = 1
yourView.layer.shadowOffset = .zero
yourView.layer.shadowRadius = 10

Be warned: generating shadows dynamically is expensive, because iOS has to draw the shadow around the exact shape of your view's contents. If you can, set the shadowPath property to a specific value so that iOS doesn't need to calculate transparency dynamically. For example, this creates a shadow path equivalent to the frame of the view:

yourView.layer.shadowPath = UIBezierPath(rect: yourView.bounds).cgPath

Alternatively, ask iOS to cache the rendered shadow so that it doesn't need to be redrawn:

yourView.layer.shouldRasterize = true

If you want to go down the rasterization route, you should make sure iOS caches the shadow at the same drawing scale as the main screen, otherwise it will look pixelated:

yourView.layer.rasterizationScale = UIScreen.main.scale

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Available from iOS 3.2

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