How to render example content using prepareForInterfaceBuilder()

Swift version: 5.2

Paul Hudson    @twostraws    May 28th 2019

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 = UIColor.black
    @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.

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