Programmatic vertical, aspect ratio, and horizontally centered constraints

gistfile1.txt

imageView.frame = self.frame
        imageView.image = defaultImage
        imageView.translatesAutoresizingMaskIntoConstraints = false
        addSubview(imageView)
        let views: [String: AnyObject] = ["imageView": imageView]
        let verticalConstraints = NSLayoutConstraint.constraintsWithVisualFormat("V:|[imageView]|",
            options: .AlignAllCenterX,
            metrics: nil,
            views: views)
        addConstraints(verticalConstraints)
        let centerXConstraint = NSLayoutConstraint(item: imageView,
            attribute: .CenterX,
            relatedBy: .Equal,
            toItem: self,
            attribute: .CenterX,
            multiplier: 1.0,
            constant: 0.0)
        addConstraint(centerXConstraint)
        let aspectRatio = NSLayoutConstraint(item: imageView,
            attribute: .Width,
            relatedBy: .Equal,
            toItem: imageView,
            attribute: .Height,
            multiplier: 2.0/3.0,
            constant: 0.0)
        imageView.addConstraint(aspectRatio)