UIView+AnchorConstraints.swift

import UIKit
import PlaygroundSupport

extension UIView {

    /// Anchor self to `view` center x and y axis.
    ///
    /// - parameters:
    ///     - centerX: Center align to the view along the x axis.
    ///     - centerY: Center align to the view along the y axis.
    ///     - view: View to apply the relevant constraints.
    ///     - shouldRespectSafeAreaLayoutGuide: If true the constraints will
    /// respect the safe area layout guides.
    public func anchor(
        centerX: Bool = false,
        centerY: Bool = false,
        to view: UIView,
        shouldRespectSafeAreaLayoutGuide: Bool = false) {

        // Disabling the `translatesAutoresizingMaskIntoConstraints` property
        // to use Auto Layout constraints
        translatesAutoresizingMaskIntoConstraints = false

        // Center constraints
        if #available(iOS 11, *), shouldRespectSafeAreaLayoutGuide {
            centerXAnchor.constraint(equalTo: view.safeAreaLayoutGuide.centerXAnchor).isActive = centerX
            centerYAnchor.constraint(equalTo: view.safeAreaLayoutGuide.centerYAnchor).isActive = centerY
        } else {
            centerXAnchor.constraint(equalTo: view.centerXAnchor).isActive = centerX
            centerYAnchor.constraint(equalTo: view.centerYAnchor).isActive = centerY
        }
    }

    /// Constrain width and height to another view.
    ///
    /// - parameters:
    ///     - view: View to apply the relevant constraints.
    ///     - shouldRespectSafeAreaLayoutGuide: If true the constraints will
    /// respect the safe area layout guides.
    public func anchorSize(
        to view: UIView,
        shouldRespectSafeAreaLayoutGuide: Bool = false) {

        // Disabling the `translatesAutoresizingMaskIntoConstraints` property
        // to use Auto Layout constraints
        translatesAutoresizingMaskIntoConstraints = false

        // Size constraints
        if #available(iOS 11, *), shouldRespectSafeAreaLayoutGuide {
            widthAnchor.constraint(equalTo: view.safeAreaLayoutGuide.widthAnchor).isActive = true
            heightAnchor.constraint(equalTo: view.safeAreaLayoutGuide.heightAnchor).isActive = true
        } else {
            widthAnchor.constraint(equalTo: view.widthAnchor).isActive = true
            heightAnchor.constraint(equalTo: view.heightAnchor).isActive = true
        }
    }

    /// Anchor the view to various constraints.
    ///
    /// - parameters:
    ///     - top: Align the views top to `NSLayoutYAxisAnchor`
    ///     - bottom: Align the views bottom to `NSLayoutYAxisAnchor`
    ///     - leading: Align the views leading to `NSLayoutXAxisAnchor`
    ///     - trailing: Align the views trailing to `NSLayoutXAxisAnchor`
    ///     - padding: Padding constraints
    ///     - size: Size constraints
    public func anchor(
        top: NSLayoutYAxisAnchor? = nil,
        bottom: NSLayoutYAxisAnchor? = nil,
        leading: NSLayoutXAxisAnchor? = nil,
        trailing: NSLayoutXAxisAnchor? = nil,
        padding: UIEdgeInsets,
        size: CGSize = .zero) {

        // Disabling the `translatesAutoresizingMaskIntoConstraints` property
        // to use Auto Layout constraints
        translatesAutoresizingMaskIntoConstraints = false

        // Top constraint
        if let top = top {
            topAnchor.constraint(
                equalTo: top,
                constant: padding.top).isActive = true
        }

        // Bottom constraint
        if let bottom = bottom {
            bottomAnchor.constraint(
                equalTo: bottom,
                constant: -padding.bottom).isActive = true
        }

        // Leading constraint
        if let leading = leading {
            leadingAnchor.constraint(
                equalTo: leading,
                constant: padding.left).isActive = true
        }

        // Trailing constraint
        if let trailing = trailing {
            trailingAnchor.constraint(
                equalTo: trailing,
                constant: -padding.right).isActive = true
        }

        // Each size constraint should only be set if NOT zero
        if size.width != 0 {
            widthAnchor.constraint(equalToConstant: size.width).isActive = true
        }
        if size.height != 0 {
            heightAnchor.constraint(equalToConstant: size.height).isActive = true
        }
    }
}

class MyViewController: UIViewController {

    override func loadView() {
        let view = UIView()
        view.backgroundColor = .white

        let greenBox = UIView()
        greenBox.backgroundColor = .green

        let redBox = UIView()
        redBox.backgroundColor = .red

        // Add all the views
        [greenBox, redBox].forEach { view.addSubview($0) }

        // Set constraints
        greenBox.anchor(
            top: view.safeAreaLayoutGuide.topAnchor,
            bottom: view.safeAreaLayoutGuide.bottomAnchor,
            leading: view.leadingAnchor,
            trailing: view.trailingAnchor,
            padding: .init(top: 8, left: 8, bottom: 8, right: 8))
        redBox.anchor(
            top: greenBox.topAnchor,
            leading: greenBox.leadingAnchor,
            trailing: greenBox.trailingAnchor,
            padding: .init(top: 8, left: 8, bottom: 8, right: 8),
            size: .init(width: 0, height: 100))

        self.view = view
    }
}

// Present the view controller in the Live View window
PlaygroundPage.current.liveView = MyViewController()