pod 'Firebase/Firestore'
# Optionally, include the Swift extensions if you're using Swift.
pod 'FirebaseFirestoreSwift'

pod install

import UIKit
import Firebase

@main
class AppDelegate: UIResponder, UIApplicationDelegate {
    func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
        // Firebase初期化
        FirebaseApp.configure()
        return true
    }
}

Terminating app due to uncaught exception 'com.firebase.core', reason: '`[FIRApp configure];` (`FirebaseApp.configure()` in Swift) could not find a valid GoogleService-Info.plist in your project. Please download one from https://console.firebase.google.com/.'

import UIKit
import Firebase

struct Person {
    var name: String
    var age: Int
    var hobbys: [Hobby]
}

struct Hobby {
    var name: String
    var year: Int
}

class ViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()

        // リスト初期化
        var persons: [Person] = []

        // FirestoreのDB取得
        let db = Firestore.firestore()
        // personsコレクションを取得
        db.collection("persons").getDocuments() { collection, err in
            // エラー発生時
            if let err = err {
                print("Error getting documents: \(err)")
            } else {
                // コレクション内のドキュメントを取得
                for document in collection!.documents {
                    // hobbiesフィールドを取得
                    guard let hobbyDicList: [[String : Any]] = document.get("hobbies") as? [[String : Any]] else {
                        continue
                    }

                    // リスト初期化
                    var hobbies: [Hobby] = []

                    // hobbies内のフィールドを取得
                    for hobbyDic in hobbyDicList {
                        guard let hobbyName = hobbyDic["name"] as? String ,
                              let hobbyYear = hobbyDic["year"] as? Int else {
                            continue
                        }
                        // Hobbyを作成
                        let hobby = Hobby(name: hobbyName, year: hobbyYear)
                        // リストに追加
                        hobbies.append(hobby)
                    }

                    // Personフィールドを取得
                    guard let personName = document.get("name") as? String,
                          let personAge = document.get("age") as? Int else {
                        continue
                    }

                    // Personを作成
                    let person = Person(name: personName, age: personAge, hobbys: hobbies)

                    // リストに追加
                    persons.append(person)
                }
            }
            // コンソール出力
            print(persons)
        }
    }
}

[firestore_sample_app.Person(name: "jiro", age: 35, hobbys: [firestore_sample_app.Hobby(name: "climbing", year: 10)]), firestore_sample_app.Person(name: "taro", age: 20, hobbys: [firestore_sample_app.Hobby(name: "baseball", year: 5), firestore_sample_app.Hobby(name: "tennis", year: 8)])]