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How to add a shadow to a UIView

Paul Hudson       @twostraws

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:

yourView.layer.shadowColor = UIColor.black.cgColor
yourView.layer.shadowOpacity = 1
yourView.layer.shadowOffset = CGSize.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

Available from iOS 3.2

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