AutoLayout by code

Basic Setup

let firstView = UIView()
let secondView = UIView()

override func viewDidLoad() {
    super.viewDidLoad()

    firstView.backgroundColor = .darkGray
    secondView.backgroundColor = .blue

    view.addSubview(firstView)
    firstView.addSubview(secondView)

    setupAutoresizingMask()
    activeNSLayoutConstraintConstaints()
    activateVisualFormat()
    activeLayoutAnchors()
}

Using setupAutoresizingMask

private func setupAutoresizingMask() {
    firstView.frame = CGRect(x: 50, y: 50, width: view.bounds.width - 100, height: view.bounds.height - 100)
    secondView.frame = CGRect(x: 25, y: 25, width: firstView.bounds.width - 50, height: firstView.bounds.height - 50)

    firstView.autoresizingMask = [.flexibleWidth, .flexibleHeight]
    secondView.autoresizingMask = [.flexibleWidth, .flexibleHeight]
    }

Using NSLayoutConstraint (iOS 6.0 부터 사용됨)

private func activeNSLayoutConstraintConstaints() {
    firstView.translatesAutoresizingMaskIntoConstraints = false  // 반드시 false로 시작
    secondView.translatesAutoresizingMaskIntoConstraints = false

    // firstView <-> View
    NSLayoutConstraint(item: firstView, attribute: .top, relatedBy: .equal, toItem: view, attribute: .top, multiplier: 1, constant: 50).isActive = true
    NSLayoutConstraint(item: firstView, attribute: .leading, relatedBy: .equal, toItem: view, attribute: .leading, multiplier: 1, constant: 50).isActive = true
    NSLayoutConstraint(item: firstView, attribute: .trailing, relatedBy: .equal, toItem: view, attribute: .trailing, multiplier: 1, constant: -50).isActive = true
    NSLayoutConstraint(item: firstView, attribute: .bottom, relatedBy: .equal, toItem: view, attribute: .bottom, multiplier: 1, constant: -50).isActive = true

    // secondView <-> firstView
    NSLayoutConstraint(item: secondView, attribute: .top, relatedBy: .equal, toItem: firstView, attribute: .top, multiplier: 1, constant: 50).isActive = true
    NSLayoutConstraint(item: secondView, attribute: .leading, relatedBy: .equal, toItem: firstView, attribute: .leading, multiplier: 1, constant: 50).isActive = true
    NSLayoutConstraint(item: secondView, attribute: .trailing, relatedBy: .equal, toItem: firstView, attribute: .trailing, multiplier: 1, constant: -50).isActive = true
    NSLayoutConstraint(item: secondView, attribute: .bottom, relatedBy: .equal, toItem: firstView, attribute: .bottom, multiplier: 1, constant: -50).isActive = true
}

Using activateVisualFormat (iOS 6.0 부터 사용됨)

private func activateVisualFormat() {
    firstView.translatesAutoresizingMaskIntoConstraints = false
    secondView.translatesAutoresizingMaskIntoConstraints = false

    let views: [String: Any] = ["firstView": firstView, "secondView":secondView]
    let metrics: [String: Any] = ["margin": 50]

    var allConstraints: [NSLayoutConstraint] = []
    allConstraints += NSLayoutConstraint.constraints(withVisualFormat: "H:|-margin-[firstView]-margin-|", metrics: metrics, views: views)
    allConstraints += NSLayoutConstraint.constraints(withVisualFormat: "V:|-margin-[firstView]-margin-|", metrics: metrics, views: views)
    allConstraints += NSLayoutConstraint.constraints(withVisualFormat: "H:|-30-[secondView]-30-|", metrics: nil, views: views)
    allConstraints += NSLayoutConstraint.constraints(withVisualFormat: "V:|-30-[secondView]-30-|", metrics: nil, views: views)

    NSLayoutConstraint.activate(allConstraints)
}

Visual Format Syntax(이유는 모르겠으나 링크가 이상함. 필요한분은 하단링크 복/붙으로 이동하시길) https://developer.apple.com/library/archive/documentation/UserExperience/Conceptual/AutolayoutPG/VisualFormatLanguage.html

Using activateLayoutAnchors (iOS 9.0)

private func activelayoutAnchors() {

    firstView.translatesAutoresizingMaskIntoConstraints = false
    firstView.topAnchor.constraint(equalTo: view.topAnchor, constant: 50).isActive = true
    firstView.leadingAnchor.constraint(equalTo: view.leadingAnchor, constant: 50).isActive = true
    firstView.trailingAnchor.constraint(equalTo: view.trailingAnchor, constant: -50).isActive = true
    firstView.bottomAnchor.constraint(equalTo: view.bottomAnchor, constant: -50).isActive = true

    secondView.translatesAutoresizingMaskIntoConstraints = false
    secondView.topAnchor.constraint(equalTo: firstView.topAnchor, constant: 50).isActive = true
    secondView.leadingAnchor.constraint(equalTo: firstView.leadingAnchor, constant: 50).isActive = true
    secondView.trailingAnchor.constraint(equalTo: firstView.trailingAnchor, constant: -50).isActive = true
    secondView.bottomAnchor.constraint(equalTo: firstView.bottomAnchor, constant: -50).isActive = true
}