MyViewController.swift

public class MyViewController: UIViewController {
    var viewModel: MyViewModel? {
        willSet(maybeViewModel) {
            if let _ = self.viewModel {
                unbindViewModel()
            }
        }
        
        didSet {
            if isViewLoaded(), let viewModel = self.viewModel {
                bindViewModel(viewModel)
            }
        }
    }
}