Firebaseのドキュメントにも書いてある通りだが、ViewControllerにFirebaseをimportしない方法で記述する。

FirestoreContoroller.swift

import UIKit

class FirestoreViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()
        
    }
    

    @IBAction func addAction(_ sender: Any) {
        Database.addTest()
      }

      @IBAction func readAction(_ sender: Any) {
        Database.readTest() { users in
          for user in users {
            print("\(user.first), \(user.last), \(user.born)")
          }
        }
      }
    }

UserModel.swift

import Foundation
import FirebaseAuth

struct UserModel: Codable {

  private var _first: String?
  var first: String { _first ?? "" }
  private var _last: String?
  var last: String { _last ?? "" }
  private var _born: Int?
  var born: Int { _born ?? 0 }

  private var _weight: Double?
  var weight: Double { _weight ?? 0 }

  private var _height: String?
  var height: String { _height ?? "" }

  private let auth = Auth.auth()
  var isEmailValidation: Bool {
    auth.currentUser?.isEmailVerified ?? false
  }

  private enum CodingKeys: String, CodingKey {
    case _first = "first"
    case _last = "last"
    case _born = "born"
  }
}

Database.swift

import Foundation
import Firebase
import FirebaseFirestore
import FirebaseFirestoreSwift


class Database {
  private static let db = Firestore.firestore()

  static func addTest() {

    var ref: DocumentReference? = nil
    ref = db.collection("users").addDocument(data: [
      "first": "Ada",
      "last": "Lovelace",
      "born": 1815
    ]) { err in
      if let err = err {
        print("Error adding document: \(err)")
      } else {
        print("Document added with ID: \(ref!.documentID)")
      }
    }
  }

  static func readTest(completion: (([UserModel]) -> Void)? = nil) {

    let _completion: ((QuerySnapshot?, Error?) -> Void) = { (querySnapshot, err) in

      var users: [UserModel] = []

      if let err = err {
        print("Error getting documents: \(err)")
        completion?(users)
        return
      }

      for document in querySnapshot!.documents {
        if let _user = try? Firestore.Decoder().decode(UserModel.self, from: document.data()) {
          users.append(_user)
        }
      }

      completion?(users)
    }

    db.collection("users").getDocuments(completion: _completion) //全部終わるまで待つ
  }
}