DisplayModeUpdatable.swift

/// 1
protocol DisplayModeUpdatable {
    func displayModeWillChangeTo(_ displayMode: UISplitViewController.DisplayMode)
    func displayModeDidChangeTo(_ displayMode: UISplitViewController.DisplayMode)
}

////
@objc func togglePrefferDisplayModeExecutingCompletion(_ executing: Bool = true) {
        UIView.animate(withDuration: 0.3, animations: {
            self.preferredDisplayMode = self.displayMode == .allVisible ? .primaryHidden : .allVisible
            self.displayModeCustomButton.setImage(SplitViewControllerViewModel.displayModeButtonImageFor(self.preferredDisplayMode), for: .normal)
        }) { _ in
            /// 2
             guard let detailViewOnDisplayModeChange = self.secondaryViewController as? UINavigationController,
              let displayModeUpdatable = detailViewOnDisplayModeChange.topViewController as? DisplayModeUpdatable 
            else { return }
            displayModeUpdatable.displayModeDidChangeTo(self.displayMode)
        }
    }

/// 3
    func splitViewController(_ svc: UISplitViewController, willChangeTo displayMode: UISplitViewController.DisplayMode) {
        guard let detailViewOnDisplayModeChange = svc.secondaryViewController as? UINavigationController, 
        let displayModeUpdatable = detailViewOnDisplayModeChange.topViewController as? DisplayModeUpdatable 
        else { return }
        displayModeUpdatable.displayModeWillChangeTo(displayMode)
    }

/// 4 DisplayModeUpdatable Protocol conformance
extension ContentDetailViewcontroller: DisplayModeUpdatable {
    
    func displayModeWillChangeTo(_ displayMode: UISplitViewController.DisplayMode) {
        /// 5
        verticalFeedTableView?.displayMode = displayMode
        let constant: CGFloat = displayMode
            != .allVisible ? 100 : 0
        tableViewTrailingConstraint?.constant = -constant
        tableViewLeadingConstraint?.constant = constant
        UIView.animate(withDuration: 0.3) {
            self.view.layoutIfNeeded()
        }
        verticalFeedTableView?.reload(animated: true) /// reloading to apply a nice animation effect when expand collapse.
    }
    
    func displayModeDidChangeTo(_ displayMode: UISplitViewController.DisplayMode) {
        // Perform an action if needed
    }
    
/// In VerticalFeedTableView.swift file.
func tableView(_ tableView: UITableView, heightForHeaderInSection section: Int) -> CGFloat {
    /// 6
    guard let dataSource = dataSource,
    case VerticalFeed.userPostsFeed(_) = dataSource.getModelAt(IndexPath(item: 0, section: section)) else { return 0 }
    return displayMode != .allVisible ? 140.0 : 0
    }
}