Xib-backed UIView subclass. Set the class name and outlets on the view's owner, not on the view itself!

XibView.swift

//
//  XibView.swift
//  
//
//  Created by Sam Rayner on 29/05/2016.
//  Copyright © 2016 Sam Rayner. All rights reserved.
//

import UIKit

@IBDesignable
class XibView: UIView {
    override init(frame: CGRect) {
        super.init(frame: frame)
        addXibView()
    }

    required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
        addXibView()
    }

    func addXibView() {
        guard let view = loadViewFromNib() else { return }
        backgroundColor = .clearColor()
        addSubview(view)
        fillWithView(view)
    }

    func loadViewFromNib() -> UIView? {
        let klass = self.dynamicType
        return UINib(nibName: "\(klass)", bundle: NSBundle(forClass: klass)).instantiateWithOwner(self, options: nil).first as? UIView
    }
    
    func fillWithView(view: UIView) {
        view.frame = bounds
        view.translatesAutoresizingMaskIntoConstraints = false
        addConstraint(NSLayoutConstraint(item: view, attribute: NSLayoutAttribute.Top, relatedBy: NSLayoutRelation.Equal, toItem: self, attribute: NSLayoutAttribute.Top, multiplier: 1, constant: 0))
        addConstraint(NSLayoutConstraint(item: view, attribute: NSLayoutAttribute.Bottom, relatedBy: NSLayoutRelation.Equal, toItem: self, attribute: NSLayoutAttribute.Bottom, multiplier: 1, constant: 0))
        addConstraint(NSLayoutConstraint(item: view, attribute: NSLayoutAttribute.Leading, relatedBy: NSLayoutRelation.Equal, toItem: self, attribute: NSLayoutAttribute.Leading, multiplier: 1, constant: 0))
        addConstraint(NSLayoutConstraint(item: view, attribute: NSLayoutAttribute.Trailing, relatedBy: NSLayoutRelation.Equal, toItem: self, attribute: NSLayoutAttribute.Trailing, multiplier: 1, constant: 0))
    }
}