Bind closure to Storyboard Segues (Master-Detail example)

SegueInteractor.swift

// SegueInteractor.swift

// SegueIdentifier enumerates just the segue ID strings in the storyboard. VC's don't switch on this...
private enum SegueIdentifier: String {
    case ShowDetail
}

// SegueInteractor binds closures to segues. VC's switch on this instead!
enum SegueInteractor {
    case PrepareShowDetail((EventEntity) -> Void)

    init(segue: UIStoryboardSegue) {
        switch SegueIdentifier(rawValue: segue.identifier!)! {
        case .ShowDetail: self = PrepareShowDetail() { segue.detailController.prepare($0) }
        }
    }
}

// This is to keep the segue init above nice and compact on one line.
// Add as many of these as needed to "SegueInteractor". This file might grow a bit, 
// but should be limited to represent the containment structures in the associated storyboard.
extension UIStoryboardSegue {
    var detailController: DetailViewController {
        let nav = destinationViewController as! UINavigationController
        return nav.topViewController as! DetailViewController
    }
}

// DetailViewController.swift

class DetailViewController: UIViewController {
    var event: EventEntity {
        didSet {
            // build view model / update view in viewWillAppear
        }
    }
}

extension DetailViewController {
    
    // Destination VC defines a method to be bound to a SegueInteractor as a closure.
    // Dependency injection happens here. 
    func prepare(event: EventEntity) {
        self.event = event
        // other vc (not view) setup can happen here if needed
    }
}

// MasterViewController.swift

class MasterViewController: UITableViewController {
    lazy var tableViewModel: MasterTableViewModel: { /* etc... */ }()

    // Source VC avoids Destination VC dependency!
    override func prepareForSegue(segue: UIStoryboardSegue, sender: AnyObject?) {
        switch SegueInteractor(segue: segue) {
        case .PrepareShowDetail(let prepare):
            guard let selectedIndexPath = tableView.indexPathForSelectedRow else { fatalError("no selection") }
            guard let eventEntity = tableViewModel.entityAtIndexPath(selectedIndexPath) else { fatalError("event entity not found") }
            prepare(eventEntity)
        }
    }
}