NEW! Master Swift design patterns with my latest book! >>

How to render example content using prepareForInterfaceBuilder()

Paul Hudson       @twostraws

Whenever you create a custom UIView subclass using @IBDesignable, it’s usually a good idea to provide it with some sample content so it can render meaningfully at design time.

For example, here’s a simple ShapeView class that renders a UIBezierPath inside a view, using CAShapeLayer:

@IBDesignable class ShapeView: UIView {
    @IBInspectable var strokeColor: UIColor =
    @IBInspectable var fillColor: UIColor = UIColor.clear
    var path: UIBezierPath?

    override class var layerClass: AnyClass {
        return CAShapeLayer.self

    override func layoutSubviews() {
        guard let layer = layer as? CAShapeLayer else { return }
        layer.path = path?.cgPath
        layer.strokeColor = strokeColor.cgColor
        layer.fillColor = fillColor.cgColor

While that might work well enough at runtime, you won’t be able to see anything when used with Interface Builder because it relies on a bezier path being set. So, while you can adjust the stroke and fill colors all you want, you can’t see how those changes look.

To fix this, Xcode lets us add a special method in the view called prepareForInterfaceBuilder(). If present, this is called by Interface Builder when your custom view is being drawn, and it’s your chance to provide some example content so others can see how it looks.

In this instance, setting the path property to a default shape does the job neatly:

override func prepareForInterfaceBuilder() {
    let drawRect = CGRect(x: 0, y: 0, width: 128, height: 128)
    path = UIBezierPath(rect: drawRect)

This method is only called at design time, so you don’t have to worry about it being run in shipping code.

Available from iOS

Did this solution work for you? Please pass it on!

Other people are reading…

About the Swift Knowledge Base

This is part of the Swift Knowledge Base, a free, searchable collection of solutions for common iOS questions.

Upgrade to the premium experience

Get all 40 projects in PDF and ePub, plus exclusive content that will take your Swift learning to the next level – buy the Hacking with Swift book today!

Buy Practical iOS 12 Buy Pro Swift Buy Swift Design Patterns Buy Practical iOS 11 Buy Swift Coding Challenges Buy Server-Side Swift (Vapor Edition) Buy Server-Side Swift (Kitura Edition) Buy Hacking with macOS Buy Advanced iOS Volume One Buy Hacking with watchOS Buy Hacking with tvOS Buy Hacking with Swift Buy Dive Into SpriteKit Buy Swift in Sixty Seconds Buy Objective-C for Swift Developers Buy Beyond Code

Was this page useful? Let me know!

Click here to visit the Hacking with Swift store >>