You can let your users authenticate with Firebase using their Twitter accounts
by integrating Twitter authentication into your app.

## Before you begin

1. [Add Firebase to your C++ project](https://firebase.google.com/docs/cpp/setup.md.txt#note_select_platform).
2. In the Firebase console, go to **Security** \> [**Authentication**](https://console.firebase.google.com/project/_/authentication/).
3. In the **Sign-in method** tab, enable the **Twitter** sign-in provider.
4. Add the **API key** and **API secret** from that provider's developer console to the provider configuration:
   1. [Register your app](https://apps.twitter.com/) as a developer application on Twitter and get your app's OAuth **API key** and **API secret**.
   2. Make sure your Firebase **OAuth redirect URI** (for example, `my-app-12345.firebaseapp.com/__/auth/handler`) is set as your **Authorization callback URL** in your app's settings page on your [Twitter app's config](https://apps.twitter.com/).
5. Click **Save**.

## Access the `firebase::auth::Auth` class

The `Auth` class is the gateway for all API calls.

1. Add the Auth and App header files:

   ```c++
   #include <cassert>
   #include <cinttypes>
   #include <cstdint>

   #include "firebase/app.h"
   #include "firebase/auth.h"
   ```
2. In your initialization code, create a [`firebase::App`](https://firebase.google.com/docs/reference/cpp/class/firebase/app.md.txt) class.

   ```c++
   #if defined(__ANDROID__)
     firebase::App* app =
         firebase::App::Create(firebase::AppOptions(), my_jni_env, my_activity);
   #else
     firebase::App* app = firebase::App::Create(firebase::AppOptions());
   #endif  // defined(__ANDROID__)
   ```
3. Acquire the `firebase::auth::Auth` class for your `firebase::App`. There is a one-to-one mapping between `App` and `Auth`.

   ```c++
   firebase::auth::Auth* auth = firebase::auth::Auth::GetAuth(app);
   ```

## Authenticate with Firebase

1. Follow the [Sign in with Twitter](https://developer.twitter.com/en/docs/twitter-for-websites/log-in-with-twitter/guides/implementing-sign-in-with-twitter) documentation to get an OAuth access token and an OAuth secret.
2. After a user successfully signs in, exchange the token and secret for a Firebase credential, and authenticate with Firebase using the Firebase credential:

   ```c++
   firebase::auth::Credential credential =
       firebase::auth::TwitterAuthProvider::GetCredential(token, secret);
   firebase::Future<firebase::auth::AuthResult> result =
       auth->SignInAndRetrieveDataWithCredential(credential);
   ```
3. If your program has an update loop that runs regularly (say at 30 or 60 times per second), you can check the results once per update with `Auth::SignInAndRetrieveDataWithCredentialLastResult`:

   ```c++
   firebase::Future<firebase::auth::AuthResult> result =
       auth->SignInAndRetrieveDataWithCredentialLastResult();
   if (result.status() == firebase::kFutureStatusComplete) {
     if (result.error() == firebase::auth::kAuthErrorNone) {
       firebase::auth::AuthResult auth_result = *result.result();
       printf("Sign in succeeded for `%s`\n",
              auth_result.user.display_name().c_str());
     } else {
       printf("Sign in failed with error '%s'\n", result.error_message());
     }
   }
   ```
   Or, if your program is event driven, you may prefer to [register a callback on the
   Future](https://firebase.google.com/docs/auth/cpp/twitter-login#register_callback_on_future).

## Register a callback on a Future

Some programs have `Update` functions that are called 30 or 60 times per second. For example, many games follow this model. These programs can call the `LastResult` functions to poll asynchronous calls. However, if your program is event driven, you may prefer to register callback functions. A callback function is called upon completion of the Future.

```c++
void OnCreateCallback(const firebase::Future<firebase::auth::User*>& result,
                      void* user_data) {
  // The callback is called when the Future enters the `complete` state.
  assert(result.status() == firebase::kFutureStatusComplete);

  // Use `user_data` to pass-in program context, if you like.
  MyProgramContext* program_context = static_cast<MyProgramContext*>(user_data);

  // Important to handle both success and failure situations.
  if (result.error() == firebase::auth::kAuthErrorNone) {
    firebase::auth::User* user = *result.result();
    printf("Create user succeeded for email %s\n", user->email().c_str());

    // Perform other actions on User, if you like.
    firebase::auth::User::UserProfile profile;
    profile.display_name = program_context->display_name;
    user->UpdateUserProfile(profile);

  } else {
    printf("Created user failed with error '%s'\n", result.error_message());
  }
}

void CreateUser(firebase::auth::Auth* auth) {
  // Callbacks work the same for any firebase::Future.
  firebase::Future<firebase::auth::AuthResult> result =
      auth->CreateUserWithEmailAndPasswordLastResult();

  // `&my_program_context` is passed verbatim to OnCreateCallback().
  result.OnCompletion(OnCreateCallback, &my_program_context);
}
```
The callback function can also be a lambda, if you prefer.

```c++
void CreateUserUsingLambda(firebase::auth::Auth* auth) {
  // Callbacks work the same for any firebase::Future.
  firebase::Future<firebase::auth::AuthResult> result =
      auth->CreateUserWithEmailAndPasswordLastResult();

  // The lambda has the same signature as the callback function.
  result.OnCompletion(
      [](const firebase::Future<firebase::auth::User*>& result,
         void* user_data) {
        // `user_data` is the same as &my_program_context, below.
        // Note that we can't capture this value in the [] because std::function
        // is not supported by our minimum compiler spec (which is pre C++11).
        MyProgramContext* program_context =
            static_cast<MyProgramContext*>(user_data);

        // Process create user result...
        (void)program_context;
      },
      &my_program_context);
}
```

## Next steps

After a user signs in for the first time, a new user account is created and
linked to the credentials---that is, the user name and password, phone
number, or auth provider information---the user signed in with. This new
account is stored as part of your Firebase project, and can be used to identify
a user across every app in your project, regardless of how the user signs in.

- In your apps, you can get the user's basic profile information from the
  [`firebase::auth::User`](https://firebase.google.com/docs/reference/cpp/class/firebase/auth/user.md.txt) object:

  ```c++
  firebase::auth::User user = auth->current_user();
  if (user.is_valid()) {
    std::string name = user.display_name();
    std::string email = user.email();
    std::string photo_url = user.photo_url();
    // The user's ID, unique to the Firebase project.
    // Do NOT use this value to authenticate with your backend server,
    // if you have one. Use firebase::auth::User::Token() instead.
    std::string uid = user.uid();
  }
  ```
- In your Firebase Realtime Database and Cloud Storage
  [Security Rules](https://firebase.google.com/docs/database/security/user-security), you can
  get the signed-in user's unique user ID from the `auth` variable,
  and use it to control what data a user can access.

You can allow users to sign in to your app using multiple authentication
providers by [linking auth provider credentials to an
existing user account.](https://firebase.google.com/docs/auth/cpp/account-linking)

To sign out a user, call [`SignOut()`](https://firebase.google.com/docs/reference/cpp/class/firebase/auth/auth.md.txt#signout):

```c++
auth->SignOut();
```