如何在SignInWithCredentialAsync时检测FirebaseUser是否已存在

问题核心 #

使用Firebase Authentication结合GoogleSignIn实现登录时，后端API需要明确当前用户是首次创建还是已存在的老用户。尤其在用户卸载重装应用后，本地user变量为空，重新执行SignInWithCredentialAsync流程时，无法直接判断用户在Firebase服务器上的状态。

解决方案 #

Firebase的AuthResult对象包含AdditionalUserInfo.IsNewUser属性，可直接判断该用户是否是首次通过当前凭证登录Firebase。利用这个属性就能精准区分新用户注册和老用户登录场景。

修改后的代码 #

public void OnSignIn()
{
    if (fireBaseReady == true)
    {
        if (user != null)
        {
            LoggedInAlready("login", user);
        }
        else
        {
            loginButton.SetActive(false);
            GoogleSignIn.Configuration = configuration;
            GoogleSignIn.Configuration.UseGameSignIn = false;
            GoogleSignIn.Configuration.RequestIdToken = true;
            GoogleSignIn.Configuration.RequestEmail = true;
            GoogleSignIn.DefaultInstance.SignIn().ContinueWithOnMainThread(OnAuthenticationFinished);
        }
    }
}

internal void OnAuthenticationFinished(System.Threading.Tasks.Task<GoogleSignInUser> task)
{
    if (task.IsCanceled)
    {
        warning.text = "SignIn got canceled";
        anWarning.Play("Base Layer.FontButtonFadeLong", 0, 0);
    }
    else if (task.IsFaulted)
    {
        warning.text = "SignIn was Faulted";
        anWarning.Play("Base Layer.FontButtonFadeLong", 0, 0);
    }
    else
    { 
        userToken = task.Result.IdToken;
        Credential credential = Firebase.Auth.GoogleAuthProvider.GetCredential(userToken, null);
        auth.SignInWithCredentialAsync(credential).ContinueWithOnMainThread(authTask =>
        {
            if (authTask.IsCanceled)
            {
                warning.text = "Authentification is canceled";
                anWarning.Play("Base Layer.FontButtonFadeLong", 0, 0);
            }
            else if (authTask.IsFaulted)
            {
                warning.text = "Authentification is faulted";
                anWarning.Play("Base Layer.FontButtonFadeLong", 0, 0);
            }
            else
            {
                var authResult = authTask.Result;
                user = authResult.User;
                // 通过AdditionalUserInfo判断是否为新用户
                string actionType = authResult.AdditionalUserInfo.IsNewUser ? "register" : "login";
                LoggedInAlready(actionType, user);
            }
        });
    }
}

关键说明 #

• authResult.AdditionalUserInfo.IsNewUser：该属性由Firebase服务器返回，不受本地应用卸载重装影响。只要用户在Firebase服务器上已存在（即使本地无缓存），该值会返回false；首次登录则返回true。
• 此方案无需额外维护本地存储或后端数据库状态，直接利用Firebase Auth的原生能力即可完成判断。

内容的提问来源于stack exchange，提问作者user5441400