protocol UI

protocol UIOwner

final class SomeViewController: UIViewController, UIOwner {
  
  var viewUI: SomeUI!
  
  override func loadView() {
    // Instantiates `viewUI` at `loadView()`.
    viewUI = .init(owner: self)
  }
  
  override func viewDidLoad() {
    super.viewDidLoad()
    // Do something with `viewUI.label`...
  }
}

final class SomeUI: UI {
  
  // It is recommended to declare it as `unowned`.
  unowned var owner: SomeViewController
  
  // It is recommended to declare any UI component variables using `lazy` keyword.
  lazy var label = UILabel()
  
  init(owner: SomeViewController) {
    self.owner = owner
    
    // **Important** 
    // You must write the following line because `owner.view` isn't made yet.
    owner.view = .init()
    // You should write the following line 
    // because the default value of `UIView.backgroundColor` is `nil`, 
    // which results in a transparent background color.
    view.backgroundColor = .white
    
    // By using `do` method of `Then`, we can configure the label like this...
    label.do {
      $0.text = "Hello World!"
      $0.textColor = .blue
      $0.font = .systemFont(ofSize: 15)
      $0.numberOfLines = 1
      $0.lineBreakMode = .byWordWrapping
    }
  }
}

final class SomeView: UIView, UIOwner {
  
  var viewUI: SomeUI!
  
  override init(frame: CGRect) {
    super.init(frame: frame)
    // Instantiates `viewUI` at any initializer.
    viewUI = .init(owner: self)
  }
  
  required init?(coder aDecoder: NSCoder) {
    fatalError("init(coder:) has not been implemented")
  }
}

final class SomeUI: UI {
  
  unowned var owner: SomeView
  
  lazy var label = UILabel()
  
  init(owner: SomeView) {
    self.owner = owner
    
    label.do {
      $0.text = "Hello World!"
      $0.textColor = .blue
      $0.font = .systemFont(ofSize: 15)
      $0.numberOfLines = 1
      $0.lineBreakMode = .byWordWrapping
    }
  }
}

// Same as `addTarget(self, action: #selector(buttonDidTap(_:), for: .touchUpInside)`
button.tapped(onTarget: self, action: #selector(buttonDidTap(_:)))

// Same as `UITapGestureRecognizer(target: self, action: #selector(viewDidTap(_:))`
view.addGestureRecognizer(.tap, onTarget: self, action: #selector(viewDidTap(_:)))

// Same as `addTarget(self, action: #selector(sliderValueChanged(_:), for: .valueChanged)`
slider.valueChanged(onTarget: self, action: #selector(sliderValueChanged(_:)))

// Same as `addtarget(self, action: #selector(textFieldTextDidChange(_:), for: .editingChanged)`
textField.textChanged(onTarget: self, action: #selector(textFieldTextDidChange(_:)))

// Same as `UIEdgeInsets(top: 4, left: 4, bottom: 4, right: 4)`
textView.textContainerInset = .all(4)

// Same as `UIEdgeInsets(top: 4, left: 8, bottom: 4, right: 4)`
someButton.titleEdgeInsets = .symmetric(horizontal: 8, vertical: 4)

// Same as `UIEdgeInsets(top: 0, left: 4, bottom: 0, right: 0)`
tableView.separatorInset = .left(4)

// Same as `CGRect(x: 100, y: 100, width: 100, height: 100)`
label.frame = .all(100)

// Same as `CGSize(width: 100, height: 100)`
label.frame.size = .all(100)

// Same as `CGPoint(width: 100, height: 100)`
label.frame.origin = .all(100)