Custom view which can be rendered by Interface Builder and a running iOS app alike. It uses autolayout for the subviews.

MYCustomView.swift

//
//  MYCustomView.swift
//  StatWidget

import UIKit


/**
 Custom view which can be rendered by Interface Builder
 and a running iOS app alike. It uses autolayout for the
 subviews.
 */
@IBDesignable class MYCustomView: UIView {
    
    // MARK: View initialization and update
    
    // Do not change the value of this property
    // manually. If the constraints have already
    // been created it will be set to false.
    private var shouldCreateConstraints = true
    
    // Used by running iOS app
    required init?(coder: NSCoder) {
        super.init(coder: coder)
        createSubviews()
    }
    
    // Used by Inteface Builder
    override init(frame: CGRect) {
        super.init(frame: frame)
        createSubviews()
    }
    
    // Updates the layout of the subviews whenever
    // needed.
    override func layoutSubviews() {
        if(shouldCreateConstraints) {
            createConstraints()
            shouldCreateConstraints = false
        }
        // else { updateConstraints() }
    }
    
    // MARK: "Outlets" for subviews
    
    private var label: UILabel!
    
    // MARK: Properties inspectable by Interface Builder
    
    // They get set immediately after init(CGRect) is called
    // so make sure the views they are referring to have
    // already been initialized. Else you will get a nil
    // reference error.
    
    @IBInspectable var bgColor: UIColor = UIColor.whiteColor() {
        didSet {
            self.backgroundColor = bgColor
        }
    }
    
    @IBInspectable var textColor: UIColor = UIColor.blackColor() {
        didSet {
            label.textColor = textColor
        }
    }
    
    // MARK: Subviews creation and setup
    
    // Never call this method manually. It will be called by layoutSubviews
    // whenever needed.
    private func createSubviews() {
        label = UILabel()
        label.font = UIFont.boldSystemFontOfSize(15)
        label.text = "Ehi, I'm centered!"
        
        // Set to false if you are going to use constraints to position it,
        // else it won't show up correctly
        label.translatesAutoresizingMaskIntoConstraints = false
        self.addSubview(label)
    }
    
    // MARK: Constraints creation and setup
    
    // Never call this method manually. It will be called by layoutSubviews
    // whenever needed.
    private func createConstraints() {
        label.centerXAnchor.constraintEqualToAnchor(self.centerXAnchor).active = true
        label.centerYAnchor.constraintEqualToAnchor(self.centerYAnchor).active = true
    }
    
    // MARK: Rest of the class
    
    // Add additional code here if needed
    
}