Auto Layout DSL

AutoLayoutDSL.swift

import UIKit

/// Represents a single `NSLayoutConstraint`
enum LayoutAnchor {
    case constant(attribute: NSLayoutConstraint.Attribute,
                  relation: NSLayoutConstraint.Relation,
                  constant: CGFloat)

    case relative(attribute: NSLayoutConstraint.Attribute,
                  relation: NSLayoutConstraint.Relation,
                  relatedTo: NSLayoutConstraint.Attribute,
                  multiplier: CGFloat,
                  constant: CGFloat)
}

// MARK: - Factory methods

extension LayoutAnchor {
    static let leading = relative(attribute: .leading, relation: .equal, relatedTo: .leading)
    static let trailing = relative(attribute: .trailing, relation: .equal, relatedTo: .trailing)
    static let top = relative(attribute: .top, relation: .equal, relatedTo: .top)
    static let bottom = relative(attribute: .bottom, relation: .equal, relatedTo: .bottom)

    static let centerX = relative(attribute: .centerX, relation: .equal, relatedTo: .centerX)
    static let centerY = relative(attribute: .centerY, relation: .equal, relatedTo: .centerY)

    static let width = constant(attribute: .width, relation: .equal)
    static let height = constant(attribute: .height, relation: .equal)

    static func constant(
        attribute: NSLayoutConstraint.Attribute,
        relation: NSLayoutConstraint.Relation
    ) -> (CGFloat) -> LayoutAnchor {
        return { constant in
            .constant(attribute: attribute, relation: relation, constant: constant)
        }
    }

    static func relative(
        attribute: NSLayoutConstraint.Attribute,
        relation: NSLayoutConstraint.Relation,
        relatedTo: NSLayoutConstraint.Attribute,
        multiplier: CGFloat = 1
    ) -> (CGFloat) -> LayoutAnchor {
        return { constant in
            .relative(attribute: attribute, relation: relation, relatedTo: relatedTo, multiplier: multiplier, constant: constant)
        }
    }
}

// MARK: - Conveniences

extension UIView {
    func addSubview(_ subview: UIView, anchors: [LayoutAnchor]) {
        subview.translatesAutoresizingMaskIntoConstraints = false
        addSubview(subview)
        subview.activate(anchors: anchors, relativeTo: self)
    }

    func activate(anchors: [LayoutAnchor], relativeTo item: UIView? = nil) {
        let constraints = anchors.map { NSLayoutConstraint(from: self, to: item, anchor: $0) }
        NSLayoutConstraint.activate(constraints)
    }
}

extension NSLayoutConstraint {
    convenience init(from: UIView, to item: UIView?, anchor: LayoutAnchor) {
        switch anchor {
        case let .constant(attribute: attr, relation: relation, constant: constant):
            self.init(
                item: from,
                attribute: attr,
                relatedBy: relation,
                toItem: nil,
                attribute: .notAnAttribute,
                multiplier: 1,
                constant: constant
            )
        case let .relative(attribute: attr,
                           relation: relation,
                           relatedTo: relatedTo,
                           multiplier: multiplier,
                           constant: constant):
            self.init(
                item: from,
                attribute: attr,
                relatedBy: relation,
                toItem: item,
                attribute: relatedTo,
                multiplier: multiplier,
                constant: constant
            )
        }
    }
}

ExampleViewController.swift

import UIKit

class ExampleViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()

        let yellow = UIView()
        yellow.backgroundColor = .yellow
        view.addSubview(yellow, anchors: [.leading(0), .trailing(0), .bottom(0), .top(0)])

        let redBox = UIView()
        redBox.backgroundColor = .red
        view.addSubview(redBox, anchors: [.centerX(0), .centerY(0), .width(100), .height(100)])
    }
}

LICENSE.md

This is free and unencumbered software released into the public domain.

Anyone is free to copy, modify, publish, use, compile, sell, or distribute this software, either in source code form or as a compiled binary, for any purpose, commercial or non-commercial, and by any means.

In jurisdictions that recognize copyright laws, the author or authors of this software dedicate any and all copyright interest in the software to the public domain. We make this dedication for the benefit of the public at large and to the detriment of our heirs and successors. We intend this dedication to be an overt act of relinquishment in perpetuity of all present and future rights to this software under copyright law.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

For more information, please refer to https://unlicense.org

What if we want to give aspect ratio to any view ?

• Suppose if we want to assign same width to any view as per the view height arranged in Stackview.