よくわからないもの.swift

import UIKit

protocol Redirectable where Self: UIViewController {
    associatedtype Input
    associatedtype Result
    associatedtype Next
    
    var nextVC: Next { get set }
    
    func inject(input: Input)
    func _injectToNext(_ nextRedirectionDependency: Result) -> Next
}

extension Redirectable where Self.Result == Next.Input, Next: Redirectable {
    func redirectToNext(next: Result, isPush: Bool) {
        isPush ?
            navigationController?.pushViewController(_injectToNext(next), animated: true) :
            {
                dismiss(animated: true, completion: { [unowned self] in
                    self.present(self._injectToNext(next), animated: true, completion: nil)
                })
            }()
    }
    
    func _injectToNext(_ nextRedirectionDependency: Result) -> Next {
        nextVC.inject(input: nextRedirectionDependency)
        return nextVC
    }
}

extension Redirectable where Self.Next == Never {
    
    var nextVC: Never {
        get { fatalError() }
        set { /* Do nothing */ }
    }
    
    func redirectToNext(next: Result, isPush: Bool) {
        // Do nothing
    }
    
    func _injectToNext(_ nextRedirectionDependency: Result) -> Next {
        fatalError()
    }
}

extension Redirectable where Self.Input == Void {
    func inject(input _: Void) {
        // Do nothing
    }
}


class ViewController1: UIViewController, Redirectable {
    
    typealias Result = ViewController2.Input
    
    var nextVC: ViewController2 = ViewController2()
    private var value: Int?
    
    func inject(input: Int) {
        value = input
    }
}


class ViewController2: UIViewController, Redirectable {
    typealias Input = Void
    typealias Result = Int
    typealias Next = Never
}