don't try this at home, kids

LayoutConstraint.swift

//
//  LayoutConstraint.swift
//  LayoutConstraints
//
//  Created by PATRICK PERINI on 12/10/15.
//  Copyright © 2015 atomic. All rights reserved.
//

import UIKit

public struct LayoutConstraint {
    // MARK: Properties
    let relation: NSLayoutRelation?
    let multiplier: CGFloat?
    let constant: CGFloat?
    
    let firstView: UIView?
    let firstAttribute: NSLayoutAttribute?
    
    let secondView: UIView?
    let secondAttribute: NSLayoutAttribute?
    
    var layoutConstraint: NSLayoutConstraint? {
        guard let firstView = self.firstView else { return nil }
        return NSLayoutConstraint(item: firstView,
            attribute: self.firstAttribute ?? .NotAnAttribute,
            relatedBy: self.relation ?? .Equal,
            toItem: self.secondView,
            attribute: self.secondAttribute ?? .NotAnAttribute,
            multiplier: self.multiplier ?? 1.0,
            constant: self.constant ?? 0.0)
    }
}

extension NSLayoutConstraint {
    // MARK: Properties
    var constraint: LayoutConstraint {
        return LayoutConstraint(relation: self.relation,
            multiplier: self.multiplier,
            constant: self.constant,
            firstView: self.firstItem as? UIView,
            firstAttribute: self.firstAttribute,
            secondView: self.secondItem as? UIView,
            secondAttribute: self.secondAttribute)
    }
}

// MARK: Conversion
extension LayoutConstraint: FloatLiteralConvertible {
    // MARK: Initializers
    public init(floatLiteral value: FloatLiteralType) {
        self.relation = nil
        self.multiplier = nil
        self.constant = CGFloat(value)
        
        self.firstView = nil
        self.firstAttribute = nil
        self.secondView = nil
        self.secondAttribute = .NotAnAttribute
    }
}

// MARK: Operators
func *(lhs: CGFloat, rhs: LayoutConstraint) -> LayoutConstraint {
    return LayoutConstraint(relation: rhs.relation,
        multiplier: lhs,
        constant: rhs.constant,
        firstView: rhs.firstView,
        firstAttribute: rhs.firstAttribute,
        secondView: rhs.secondView,
        secondAttribute: rhs.secondAttribute)
}

func +(lhs: LayoutConstraint, rhs: CGFloat) -> LayoutConstraint {
    return LayoutConstraint(relation: lhs.relation,
        multiplier: lhs.multiplier,
        constant: rhs,
        firstView: lhs.firstView,
        firstAttribute: lhs.firstAttribute,
        secondView: lhs.secondView,
        secondAttribute: lhs.secondAttribute)
}

func -(lhs: LayoutConstraint, rhs: CGFloat) -> LayoutConstraint {
    return LayoutConstraint(relation: lhs.relation,
        multiplier: lhs.multiplier,
        constant: -rhs,
        firstView: lhs.firstView,
        firstAttribute: lhs.firstAttribute,
        secondView: lhs.secondView,
        secondAttribute: lhs.secondAttribute)
}

prefix operator >= {}
prefix func >=(rhs: LayoutConstraint) -> LayoutConstraint {
    return LayoutConstraint(relation: .GreaterThanOrEqual,
        multiplier: rhs.multiplier,
        constant: rhs.constant,
        firstView: rhs.firstView,
        firstAttribute: rhs.firstAttribute,
        secondView: rhs.secondView,
        secondAttribute: rhs.secondAttribute)
}

prefix operator <= {}
prefix func <=(rhs: LayoutConstraint) -> LayoutConstraint {
    return LayoutConstraint(relation: .LessThanOrEqual,
        multiplier: rhs.multiplier,
        constant: rhs.constant,
        firstView: rhs.firstView,
        firstAttribute: rhs.firstAttribute,
        secondView: rhs.secondView,
        secondAttribute: rhs.secondAttribute)
}

UIView+LayoutConstraint.swift

//
//  UIView+LayoutConstraint.swift
//  LayoutConstraints
//
//  Created by PATRICK PERINI on 12/4/15.
//  Copyright © 2015 atomic. All rights reserved.
//

import UIKit

extension UIView {
    // MARK: Properties
    // ... Center / Size
    var centerX: LayoutConstraint {
        get { return self.constraint(attribute: .CenterX) }
        set { self.addConstraint(constraint: newValue, attribute: .CenterX) }
    }
    
    var centerY: LayoutConstraint {
        get { return self.constraint(attribute: .CenterY) }
        set { self.addConstraint(constraint: newValue, attribute: .CenterY) }
    }
    
    var width: LayoutConstraint {
        get { return self.constraint(attribute: .Width) }
        set { self.addConstraint(constraint: newValue, attribute: .Width) }
    }
    
    var height: LayoutConstraint {
        get { return self.constraint(attribute: .Height) }
        set { self.addConstraint(constraint: newValue, attribute: .Height) }
    }
    
    // ... Edges
    var left: LayoutConstraint {
        get { return self.constraint(attribute: .Left) }
        set { self.addConstraint(constraint: newValue, attribute: .Left) }
    }
    
    var right: LayoutConstraint {
        get { return self.constraint(attribute: .Right) }
        set { self.addConstraint(constraint: newValue, attribute: .Right) }
    }
    
    var top: LayoutConstraint {
        get { return self.constraint(attribute: .Top) }
        set { self.addConstraint(constraint: newValue, attribute: .Top) }
    }
    
    var bottom: LayoutConstraint {
        get { return self.constraint(attribute: .Bottom) }
        set { self.addConstraint(constraint: newValue, attribute: .Bottom) }
    }
}

extension UIView {
    // MARK: Accessors
    private func constraint(attribute attribute: NSLayoutAttribute) -> LayoutConstraint {
        return LayoutConstraint(relation: nil,
            multiplier: nil,
            constant: nil,
            firstView: self,
            firstAttribute: attribute,
            secondView: nil,
            secondAttribute: nil)
    }
    
    private func layoutConstraintForAttribute(attribute: NSLayoutAttribute) -> NSLayoutConstraint? {
        return (self.constraints + (self.superview?.constraints ?? [])).filter { (possibleConstraint: NSLayoutConstraint) in
            return ((possibleConstraint.firstItem === self && possibleConstraint.firstAttribute == attribute) ||
                (possibleConstraint.secondItem === self && possibleConstraint.secondAttribute == attribute))
        }.first
    }
    
    // MARK: Mutators
    private func addConstraint(constraint constraint: LayoutConstraint, attribute: NSLayoutAttribute) {
        let targetView = constraint.firstView ?? self
        targetView.removeConstraint(constraint: constraint, toView: self, attribute: attribute)
        
        guard let layoutConstraint = LayoutConstraint(relation: constraint.relation,
            multiplier: constraint.multiplier,
            constant: constraint.constant,
            firstView: self,
            firstAttribute: attribute,
            secondView: constraint.firstView,
            secondAttribute: constraint.firstAttribute).layoutConstraint else { return }
        targetView.addConstraint(layoutConstraint)
    }
    
    private func removeConstraint(constraint constraint: LayoutConstraint, toView view: UIView, attribute: NSLayoutAttribute) {
        guard let layoutConstraint = self.constraints.filter({ (possibleConstraint: NSLayoutConstraint) in
            return ((possibleConstraint.firstItem === view || possibleConstraint.secondItem === view) &&
                (possibleConstraint.firstAttribute == attribute) &&
                (possibleConstraint.secondAttribute == constraint.firstAttribute ?? NSLayoutAttribute(rawValue: 0)))
        }).first else { return }
        
        self.removeConstraint(layoutConstraint)
    }
}

// MARK: Operators
func +=(lhs: LayoutConstraint, rhs: CGFloat) {
    guard let layoutConstraint = lhs.firstView?.layoutConstraintForAttribute(lhs.firstAttribute ?? .NotAnAttribute) else { return }
    layoutConstraint.constant += rhs
}

func -=(lhs: LayoutConstraint, rhs: CGFloat) {
    guard let layoutConstraint = lhs.firstView?.layoutConstraintForAttribute(lhs.firstAttribute ?? .NotAnAttribute) else { return }
    layoutConstraint.constant += rhs
}

ViewController.swift

//
//  ViewController.swift
//  LayoutConstraints
//
//  Created by PATRICK PERINI on 12/4/15.
//  Copyright © 2015 atomic. All rights reserved.
//

import UIKit

class ViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()
        
        
        let childView = UIView()
        self.view.addSubview(childView)
        childView.backgroundColor = UIColor.redColor()
        
        self.view.translatesAutoresizingMaskIntoConstraints = false
        childView.translatesAutoresizingMaskIntoConstraints = false
        
        childView.height = 0.5 * self.view.height - 5.0
        childView.width = 100.5
        
        childView.left = self.view.left + 50.0
        self.view.layoutIfNeeded()
        
        dispatch_after(dispatch_time(DISPATCH_TIME_NOW, Int64(2) * Int64(NSEC_PER_SEC)), dispatch_get_main_queue()) {
            UIView.animateWithDuration(1.0, delay: 0.0, options: UIViewAnimationOptions(), animations: {
                childView.left += 100.0
                childView.height = self.view.height - 50.0
                self.view.layoutIfNeeded()
            }, completion: nil)
        }
        
        // Do any additional setup after loading the view, typically from a nib.
    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }
}