aspectFitInRect() - returns biggest rect of current aspect ratio that will fit into target

gistfile1.txt

extension CGRect
{
    func aspectFitInRect(target target: CGRect) -> CGRect
    {
        let scale: CGFloat =
        {
            let scale = target.width / self.width
            
            return self.height * scale <= target.height ?
                scale :
                target.height / self.height
        }()
        
        let width = self.width * scale
        let height = self.height * scale
        let x = target.midX - width / 2
        let y = target.midY - height / 2
        
        return CGRect(x: x,
            y: y,
            width: width,
            height: height)
    }
}

I think you should be able to simplify the calculation of scale:

let scale = min(target.width / self.width, target.height / self.height)

The rationale is that you have two scaling values, each of them is going to scale the target to one of the dimensions of the rect. The smaller one guarantees that the other dimension does not overflow.