SegueHandlerType for Swift 3

SegueHandlerType.swift

/* The SegueHandlerType pattern, as seen on [1, 2], adapted for the changed Swift 3 syntax.

 [1] https://developer.apple.com/library/content/samplecode/Lister/Listings/Lister_SegueHandlerType_swift.html
 [2] https://www.natashatherobot.com/protocol-oriented-segue-identifiers-swift/
 */

protocol SegueHandlerType {

    // `typealias` has been changed to `associatedtype` for Protocols in Swift 3.
    associatedtype SegueIdentifier: RawRepresentable
}

extension SegueHandlerType where Self: UIViewController, SegueIdentifier.RawValue == String {

    // This used to be `performSegueWithIdentifier(...)`.
    func performSegue(withIdentifier identifier: SegueIdentifier, sender: Any?) {
        performSegue(withIdentifier: identifier.rawValue, sender: sender)
    }

    func segueIdentifier(for segue: UIStoryboardSegue) -> SegueIdentifier {
        guard let identifier = segue.identifier, let segueIdentifier = SegueIdentifier(rawValue: identifier) else {
            fatalError("Couldn't handle segue identifier \(String(describing: segue.identifier)) for view controller of type \(type(of: self)).")
        }

        return segueIdentifier
    }
}

import UIKit

class ViewController: UIViewController {

    override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
        switch segueIdentifier(for: segue) {
        case .showFoo:
            // prepare for segue to Foo
            break
        case .showBar:
            // prepare for segue to Bar
            break
        }
    }

}

/* We're using a protocol extension here to separate UIViewController concerns.
 See [1, 2] for more info on this.

 [1] https://developer.apple.com/library/content/documentation/Swift/Conceptual/Swift_Programming_Language/Protocols.html#//apple_ref/doc/uid/TP40014097-CH25-ID521
 [2] https://www.natashatherobot.com/using-swift-extensions/
 */

extension ViewController: SegueHandlerType {

    enum SegueIdentifier: String {
        case showFoo
        case showBar
    }
}

I think it's better to turn:

func segueIdentifierForSegue(segue: UIStoryboardSegue) -> SegueIdentifier {
        guard let identifier = segue.identifier, let segueIdentifier = SegueIdentifier(rawValue: identifier) else {
            fatalError("Couldn't handle segue identifier \(segue.identifier) for view controller of type \(type(of: self)).")
        }

        return segueIdentifier
    }

into

func segueIdentifier(forSegue segue: UIStoryboardSegue) -> SegueIdentifier {
        guard let identifier = segue.identifier, let segueIdentifier = SegueIdentifier(rawValue: identifier) else {
            fatalError("Couldn't handle segue identifier \(segue.identifier) for view controller of type \(type(of: self)).")
        }

        return segueIdentifier
    }

and also ShowFoo -> showFoo, ShowBar -> showBar
It's more Swift3 like

Even better to turn:

func segueIdentifier(forSegue segue: UIStoryboardSegue) -> SegueIdentifier {
    ...
}

into:

func segueIdentifier(for segue: UIStoryboardSegue) -> SegueIdentifier {
    ...
}

😄

You're both quite right! I've updated the Gist.

@msewell, thanks for sharing your code.

When using an unwind segue, the segue identifier is nil. The following modifications can take care of this while still catching missing SegueIdentifier cases.

func segueIdentifier(for segue: UIStoryboardSegue) -> SegueIdentifier? {
    guard let identifier = segue.identifier else { 
        return nil 
    }

    guard let segueIdentifier = SegueIdentifier(rawValue: identifier) else {
        fatalError("Couldn't handle segue identifier \(identifier) for view controller of type \(type(of: self)).")
    }

    return segueIdentifier
}

@d108

When using an unwind segue, the segue identifier is nil.

Are you sure about that? I believe you can give your unwind segue an identifier in Interface Builder.

I could be doing something wrong here (xcode 9.1 swift 4) but if i write the performSegue method as you do

    func performSegue(withIdentifier identifier: SegueIdentifier, sender: Any?) {
        performSegue(withIdentifier: identifier.rawValue, sender: sender)
    }

then when t type this
performSegue(withIdentifier: .measurements , sender: self)

I got no autocomplete after typing the period. I would have to explicitly add SegueIdentifier. to get the autocomplete to kickin

if, in the protocol extension, I change the performSegue to a different form, say by replacing the withIdentifier to _ as here
func performSegue(_ identifier: SegueIdentifier, sender: Any?) { performSegue(withIdentifier: identifier.rawValue, sender: sender) }
then I don't need to type SegueIdentifier but just start with .m and the autocomplete kicks in

the lack of Xcode autocompleting is a downside bigger than the benefit of having a method with the same parameter names Imho.

Of course, this is possibly just another Xcode bug :)

Hi, thanks for the gist !

the switch statement would be swiftier without the break statements

class ViewController: UIViewController {

    override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
        switch segueIdentifier(for: segue) {
        case .showFoo:
            // prepare for segue to Foo
        case .showBar:
            // prepare for segue to Bar
        }
    }

}