displayModeButton.swift

    
    /// 1
    lazy var displayModeCustomButton: Button = {
        let button = Button(type: .system, image: SplitViewControllerViewModel.displayModeButtonImageFor(self.displayMode), target: self, selector: #selector(togglePrefferDisplayModeExecutingCompletion))
        button.constrainWidth(constant: 44.0)
        button.constrainHeight(constant: 44.0)
        button.imageEdgeInsets = .init(top: 8.0, left: 8.0, bottom: 8.0, right: 8.0)
        return button
    }()

/// 2
    @objc func togglePrefferDisplayModeExecutingCompletion(_ executing: Bool = true) {
        UIView.animate(withDuration: 0.3, animations: {
          /// 3
            self.preferredDisplayMode = self.displayMode == .allVisible ? .primaryHidden : .allVisible
            self.displayModeCustomButton.setImage(SplitViewControllerViewModel.displayModeButtonImageFor(self.preferredDisplayMode), for: .normal)
            
        }) { _ in
            /// perform some action if needed after animation completion.
        }
    }

extension SplitViewController: UISplitViewControllerDelegate {

  /// 4
    func splitViewController(_ svc: UISplitViewController, willChangeTo displayMode: UISplitViewController.DisplayMode) {
      /// delegate method that is triggered as soon `prefferDisplayMode` is set.
    }
}