Simple Protocol Witness Example

ProtocolWitness.swift

import UIKit

struct ViewModel1 {
    let title = "Hello"
}

struct ViewModel2 {
    let message = "some message"
}

struct CellConfiguring<M, C: UICollectionViewCell> {
    let config: (M, C) -> Void
}

class Cell1: UICollectionViewCell {
    var label = UILabel()
    var setup: CellConfiguring<ViewModel1, Cell1>?
}

class Cell2: UICollectionViewCell {
    var message = UILabel()
    var setup: CellConfiguring<ViewModel2, Cell2>?
}

extension CellConfiguring where M == ViewModel1, C == Cell1 {
    static func witness() -> Self {
        .init { d, c in
            c.label.text = d.title
            print(#line, d.title)
        }
    }
}

extension CellConfiguring where M == ViewModel2, C == Cell2 {
    static func witness() -> Self {
        .init { d, c in
            c.message.text = d.message
            print(#line, d.message)
        }
    }
}

let cell1 = Cell1(frame: .zero)
let model1 = ViewModel1()
CellConfiguring.witness().config(model1, cell1)
let cell2 = Cell2(frame: .zero)
let model2 = ViewModel2()
CellConfiguring.witness().config(model2, cell2)