Firebase Android Snippets

To avoid contention when many clients update the same document simultaneously, use a distributed counter pattern. This involves creating a root counter document that holds the number of shards, and a subcollection of shards where each shard document holds a portion of the total count.

1. Data Structure:

   • Root document: counters/${ID} containing numShards.
   • Shard documents: counters/${ID}/shards/${NUM} containing count.

2. Operations:

   • Create: Initialize the root document and all shard documents with an initial count of 0.
   • Increment: Randomly select one shard from the subcollection and use FieldValue.increment(1) to update its count. This distributes the write load across multiple documents.
   • Get Count: Retrieve all shard documents in the subcollection and sum their count values locally.

// Example usage pattern for distributed counters
val db = FirebaseFirestore.getInstance()
val counterRef = db.collection("counters").document("my_counter")
val solution = SolutionCounters(db)

// 1. Create the counter with 5 shards
solution.createCounter(counterRef, 5)
    .addOnSuccessListener { /* Counter initialized */ }

// 2. Increment the counter
solution.incrementCounter(counterRef, 5)

// 3. Get the total count
solution.getCount(counterRef)
    .addOnSuccessListener { totalCount -> println("Total count: $totalCount") }

To implement Google Sign-In in an Android app using Firebase, you combine the Android CredentialManager API with Firebase Authentication. The process involves three main steps:

1. Configure a Google ID Option: Use GetGoogleIdOption.Builder to specify your server's client ID (not the Android client ID) and decide whether to filter by authorized accounts.
2. Retrieve the Credential: Use CredentialManager.getCredential() to launch the system UI and retrieve a CustomCredential containing a Google ID token.
3. Sign in to Firebase: Convert the retrieved credential into a GoogleAuthProvider credential using the ID token, then call auth.signInWithCredential(credential).

Note: When signing out, you should call both auth.signOut() and credentialManager.clearCredentialState(ClearCredentialStateRequest()) to ensure the user is fully logged out from both Firebase and the system credential provider.

// 1. Create the Google ID option
val googleIdOption = GetGoogleIdOption.Builder()
    .setServerClientId(getString(R.string.default_web_client_id))
    .setFilterByAuthorizedAccounts(true)
    .build()

// 2. Create the request
val request = GetCredentialRequest.Builder()
    .addCredentialOption(googleIdOption)
    .build()

// 3. Launch Credential Manager and handle result
lifecycleScope.launch {
    try {
        val result = credentialManager.getCredential(baseContext, request)
        val credential = result.credential
        
        if (credential is CustomCredential && credential.type == TYPE_GOOGLE_ID_TOKEN_CREDENTIAL) {
            val googleIdTokenCredential = GoogleIdTokenCredential.createFrom(credential.data)
            val idToken = googleIdTokenCredential.idToken
            
            // 4. Sign in to Firebase
            val firebaseCredential = GoogleAuthProvider.getCredential(idToken, null)
            auth.signInWithCredential(firebaseCredential)
                .addOnCompleteListener { task -> 
                    /* Handle success/failure */ 
                }
        }
    } catch (e: GetCredentialException) {
        // Handle error
    }
}

To authenticate a user using a custom token (minted by your backend server), use the signInWithCustomToken(token) method on a FirebaseAuth instance. This method returns a Task that you can monitor using addOnCompleteListener to handle successful authentication or failures.

For detailed architectural guidance on how to mint these tokens, refer to the official documentation: https://firebase.google.com/docs/auth/android/custom-auth

// Assuming 'auth' is an initialized FirebaseAuth instance and 'customToken' is your minted string
customToken?.let {
    auth.signInWithCustomToken(it)
        .addOnCompleteListener(this) { task ->
            if (task.isSuccessful) {
                // Sign in success
                val user = auth.currentUser
                updateUI(user)
            } else {
                // Sign in failure
                Log.w(TAG, "signInWithCustomToken:failure", task.exception)
                Toast.makeText(baseContext, "Authentication failed.", Toast.LENGTH_SHORT).show()
                updateUI(null)
            }
        }
}

To protect your own backend services using Firebase App Check, you must retrieve an App Check token from the Firebase SDK and include it in your API requests. The standard way to do this is by adding the token to a custom HTTP header, typically named X-Firebase-AppCheck.

1. Use Firebase.appCheck.getAppCheckToken(false) to retrieve the current token.
2. Extract the token string using .token.
3. Pass this string into your network client (e.g., Retrofit) via an HTTP header.

// 1. Define your service with the App Check header
interface YourExampleBackendService {
    @GET("yourExampleEndpoint")
    fun exampleData(
        @Header("X-Firebase-AppCheck") appCheckToken: String,
    ): Call<List<String>>
}

// 2. Retrieve the token and make the call
Firebase.appCheck.getAppCheckToken(false).addOnSuccessListener {\<appCheckToken ->
    val token = appCheckToken.token
    val apiCall = yourExampleBackendService.exampleData(token)
    // ... execute call
}

To authenticate a user with Facebook in an Android app using Firebase, you must first obtain a Facebook AccessToken using the Facebook SDK, then convert that token into a Firebase credential using FacebookAuthProvider.getCredential(). Finally, pass that credential to FirebaseAuth.signInWithCredential().

Workflow

1. Initialize Firebase Auth: Get an instance of FirebaseAuth via Firebase.auth.
2. Handle Facebook Login: Use the Facebook SDK's LoginButton and CallbackManager to handle the login flow and retrieve an AccessToken.
3. Convert to Firebase Credential: Use FacebookAuthProvider.getCredential(token.token) to create a credential from the Facebook token.
4. Sign In: Call auth.signInWithCredential(credential) to complete the Firebase authentication process.

// 1. Get the Facebook AccessToken from the Facebook SDK callback
// 2. Create the Firebase credential
val credential = FacebookAuthProvider.getCredential(token.token)

// 3. Sign in to Firebase
auth.signInWithCredential(credential)
    .addOnCompleteListener(this) { task ->
        if (task.isSuccessful) {
            val user = auth.currentUser
            // Sign in success
        } else {
            // Sign in failure
        }
    }