[Xcode/SwiftUI] FirebaseAuthで簡単なログイン or 新規登録画面を作る

import SwiftUI

struct HomeView: View {
    @State private var email: String = ""
    @State private var password: String = ""
    @StateObject var authManager = AuthManager()

    var body: some View {
        VStack(spacing: 16) {
            if authManager.isLoggedIn {
                Text("ログイン成功！")
                Image("img_yoshi") // 画像は個人で好きなものを
                    .resizable()
                    .frame(width: 300, height: 300)
                Button {
                    authManager.logout()
                } label: {
                    Text("ログアウト")
                }
            } else {
                TextField("メールアドレス", text: $email)
                    .textFieldStyle(RoundedBorderTextFieldStyle())
                    .padding()
                SecureField("パスワード", text: $password)
                    .textFieldStyle(RoundedBorderTextFieldStyle())
                    .padding()
                Button {
                    authManager.login(email: email, password: password)
                } label: {
                    Text("ログイン")
                }
                Button {
                    authManager.register(email: email, password: password)
                } label: {
                    Text("新規登録")
                }
            }
            if let errorMessage = authManager.errorMessage {
                Text(errorMessage)
                    .foregroundColor(.red)
            }
        }
        .padding()
    }
}

import SwiftUI
import FirebaseAuth

final class AuthManager: ObservableObject {
    @Published var errorMessage: String?
    @Published var isLoggedIn = false

    // ログイン
    func login(email: String, password: String) {
        Auth.auth().signIn(withEmail: email, password: password) { [weak self] result, error in
            if let error = error {
                self?.errorMessage = error.localizedDescription
            } else {
                self?.isLoggedIn = true
                self?.errorMessage = nil
            }
        }
    }

    // 新規登録
    func register(email: String, password: String) {
        Auth.auth().createUser(withEmail: email, password: password) { [weak self] result, error in
            if let error = error {
                self?.errorMessage = error.localizedDescription
            } else {
                self?.isLoggedIn = true
                self?.errorMessage = nil
            }
        }
    }

    // ログアウト
    func logout() {
        do {
            try Auth.auth().signOut()
            isLoggedIn = false
        } catch {
            errorMessage = error.localizedDescription
        }
    }
}