alsobroken.swift

//
//  ContentView.swift
//  AddressBook
//
//  Created by Chris Eidhof on 13.07.20.
//

import SwiftUI

final class Contact: ObservableObject, Identifiable {
    let id = UUID()
    @Published var name: String
    @Published var city: String

    init(name: String, city: String) {
        self.name = name
        self.city = city
    }
}

var contacts: [Contact] = [
    Contact(name: "John Appleseed", city: "Cupertino"),
    Contact(name: "Kate Bell", city: "San Francisco")
]

struct PersonCell: View {
    @Binding var selectedPerson: Contact?
    @ObservedObject var person: Contact
    var body: some View {
        Button(person.name) {
            selectedPerson = person
        }
    }
}

struct PersonDetail: View {
    @StateObject var person: Contact
    var body: some View {
        return VStack {
            Text(person.name).bold()
            Text(person.city)
        }
    }
}

struct ContentView: View {
    @State var selectedPerson: Contact?
    var body: some View {
        NavigationView {
            List(contacts) { person in
                PersonCell(selectedPerson: $selectedPerson, person: person)
            }.navigationTitle("People")
            if let p = selectedPerson {
                PersonDetail(person: p)
            }
        }
    }
}

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}

broken.swift

//
//  ContentView.swift
//  AddressBook
//
//  Created by Chris Eidhof on 13.07.20.
//

import SwiftUI

final class Person: ObservableObject, Identifiable {
    let id = UUID()
    @Published var name: String
    @Published var city: String

    init(name: String, city: String) {
        self.name = name
        self.city = city
    }
}

var people: [Person] = [
    Person(name: "John Appleseed", city: "Cupertino"),
    Person(name: "Kate Bell", city: "San Francisco")
]

struct PersonCell: View {
    @Binding var selectedPersonId: UUID
    @ObservedObject var person: Person
    var body: some View {
        Button(person.name) {
            selectedPersonId = self.person.id
        }
    }
}

struct PersonDetail: View {
    @StateObject var person: Person
    init(id: UUID) {
        let p = people.first { $0.id == id }!
        print(p.name)
        self._person = StateObject(wrappedValue: p)
    }
    var body: some View {
        return VStack {
            Text(person.name).bold()
            Text(person.city)
        }
    }
}

struct ContentView: View {
    @State var selectedPersonId = people[0].id
    var body: some View {
        NavigationView {
            List(people) { person in
                PersonCell(selectedPersonId: $selectedPersonId, person: person)
            }.navigationTitle("People")
            PersonDetail(id: selectedPersonId)
            // to make this work, either add `.id(selectedPersonId)` to the `PersonDetail` view, or use the solution below.
        }
    }
}

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}

working.swift

//
//  ContentView.swift
//  AddressBook
//
//  Created by Chris Eidhof on 13.07.20.
//

import SwiftUI

final class Person: ObservableObject, Identifiable {
    let id = UUID()
    @Published var name: String
    @Published var city: String

    init(name: String, city: String) {
        self.name = name
        self.city = city
    }
}

var people: [Person] = [
    Person(name: "John Appleseed", city: "Cupertino"),
    Person(name: "Kate Bell", city: "San Francisco")
]

struct PersonCell: View {
    @StateObject var person: Person
    var body: some View {
        NavigationLink(person.name, destination: PersonDetail(person: person))
    }
}

struct PersonDetail: View {
    @StateObject var person: Person
    var body: some View {
        return VStack {
            Text(person.name).bold()
            Text(person.city)
        }
    }
}

struct ContentView: View {
    var body: some View {
        NavigationView {
            List(people) { person in
                PersonCell(person: person)
            }.navigationTitle("People")
            Text("Select a person")
        }
    }
}

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}