hwnn_1_4.swift

// Composition root
let animals: [Animal] = [
    Dog(name: "Doggy", age: 3),
    Horse(name: "Horsy", age: 5)
]

let pageDetails = PageDetails()

let vc = ListView(pageDetails: pageDetails, animals: animals)

// Then, once one animal is selected:
// ListView.swift
func tableView(_ tableView: UITableView,
               didSelectRowAt indexPath: IndexPath) {
    let animal = animals[indexPath.row]
    pageDetails.animal = animal
    present(pageDetails, animated: true)
}