let top = NSLayoutConstraint(item: view, attribute: .top, relatedBy: .equal, toItem: self, attribute: .top, multiplier: 1.0, constant: -10)
let trailing = NSLayoutConstraint(item: view, attribute: .trailing, relatedBy: .equal, toItem: self, attribute: .trailing, multiplier: 1.0, constant: 10)
let width = NSLayoutConstraint(item: view, attribute: .width, relatedBy: .equal, toItem: nil, attribute: .notAnAttribute, multiplier: 1.0, constant: 30)
let height = NSLayoutConstraint(item: view, attribute: .height, relatedBy: .equal, toItem: nil, attribute: .notAnAttribute, multiplier: 1.0, constant: 30)

view.addConstraints([width, height])
view.layoutIfNeeded()
self.addConstraints([top, trailing])
self.layoutIfNeeded()

view.layout
    .top(-10)
    .trailing(10)
    .width(fixed: 30)
    .height(fixed: 30)

view.layout
    .top(by: view._safeAreaLayoutGuide)
    .leading()
    .trailing()
    .bottom(by: view._safeAreaLayoutGuide)

import UIKit
import SimpleLayout_Swift

class FillExampleViewController: UIViewController {

    private lazy var subview: UIView = {
        let label = UILabel()
        label.text = "subview"
        label.textColor = .white

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

        subview.addSubview(label)
        label.layout
            .leading()
            .top()
            .width(fixed: 0, relation: .greaterThanOrEqual)
            .height(fixed: 0, relation: .greaterThanOrEqual)
        return subview
    }()

    override func viewDidLoad() {
        super.viewDidLoad()

        edgesForExtendedLayout = .bottom

        let label = UILabel()
        label.text = "view"

        view.addSubview(label)
        view.addSubview(subview)
        label.layout
            .leading()
            .top()
            .width(fixed: 0, relation: .greaterThanOrEqual)
            .height(fixed: 0, relation: .greaterThanOrEqual)
        subview.layout.fill(leading: 30, top: 30, trailing: -30, bottom: -30)
    }
}

import UIKit
import SimpleLayout_Swift

class ChainExampleViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()

        edgesForExtendedLayout = .bottom

        let subview1 = label(backgroundColor: .red, text: "subview1")
        let subview2 = label(backgroundColor: .purple, text: "subview2")
        let subview3 = label(backgroundColor: .blue, text: "subview3")

        view.addSubview(subview1)
        view.addSubview(subview2)
        view.addSubview(subview3)

        subview1.layout
            .leading()
            .top()
            .width()
            .height(fixed: 150)
        subview2.layout
            .leading(by: subview1)
            .top(by: subview1, attribute: .bottom)
            .width(fixed: view.width/2)
            .bottom()
        subview3.layout
            .leading(by: subview2, attribute: .trailing)
            .top(by: subview2)
            .trailing()
            .bottom(by: view.layout.safeAreaLayoutGuide)
    }

    private func label(backgroundColor: UIColor, text: String) -> UILabel {
        let label = UILabel()
        label.backgroundColor = backgroundColor
        label.text = text
        label.textAlignment = .center
        label.textColor = .white
        return label
    }
}