SignInActivity.java

package br.com.nglauber.livrosfirebase;

import android.content.Intent;
import android.os.Bundle;
import android.support.annotation.NonNull;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.widget.Toast;

import com.google.android.gms.auth.api.Auth;
import com.google.android.gms.auth.api.signin.GoogleSignInAccount;
import com.google.android.gms.auth.api.signin.GoogleSignInOptions;
import com.google.android.gms.auth.api.signin.GoogleSignInResult;
import com.google.android.gms.common.ConnectionResult;
import com.google.android.gms.common.api.GoogleApiClient;
import com.google.android.gms.tasks.OnCompleteListener;
import com.google.android.gms.tasks.Task;
import com.google.firebase.auth.AuthCredential;
import com.google.firebase.auth.AuthResult;
import com.google.firebase.auth.FirebaseAuth;
import com.google.firebase.auth.GoogleAuthProvider;

public class SignInActivity extends AppCompatActivity {

    private static final int RC_GOOGLE_SIGN_IN = 1;

    private GoogleApiClient mGoogleApiClient;
    private FirebaseAuth mAuth;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_sign_in);

        mAuth = FirebaseAuth.getInstance();

        initGoogleSignIn();
    }

    @Override
    public void onActivityResult(int requestCode, int resultCode, Intent data) {
        super.onActivityResult(requestCode, resultCode, data);

        if (requestCode == RC_GOOGLE_SIGN_IN) {
            GoogleSignInResult result = Auth.GoogleSignInApi.getSignInResultFromIntent(data);
            if (result.isSuccess()) {
                GoogleSignInAccount account = result.getSignInAccount();
                firebaseAuthWithGoogle(account);

            } else {
                Toast.makeText(SignInActivity.this, R.string.erro_google_sign_in, Toast.LENGTH_SHORT).show();
            }
        }
    }

    public void clickGoogleSignIn(View view) {
        signIn();
    }

    private void initGoogleSignIn() {
        GoogleSignInOptions gso = new GoogleSignInOptions.Builder(GoogleSignInOptions.DEFAULT_SIGN_IN)
                .requestIdToken(getString(R.string.google_sign_in_key))
                .requestEmail()
                .build();

        mGoogleApiClient = new GoogleApiClient.Builder(this)
                .enableAutoManage(this, new GoogleApiClient.OnConnectionFailedListener() {
                    @Override
                    public void onConnectionFailed(@NonNull ConnectionResult connectionResult) {
                        Toast.makeText(SignInActivity.this, R.string.erro_google_sign_in, Toast.LENGTH_SHORT).show();
                    }
                })
                .addApi(Auth.GOOGLE_SIGN_IN_API, gso)
                .build();
    }

    private void signIn() {
        Intent signInIntent = Auth.GoogleSignInApi.getSignInIntent(mGoogleApiClient);
        startActivityForResult(signInIntent, RC_GOOGLE_SIGN_IN);
    }

    private void firebaseAuthWithGoogle(GoogleSignInAccount acct) {
        AuthCredential credential = GoogleAuthProvider.getCredential(acct.getIdToken(), null);
        mAuth.signInWithCredential(credential)
                .addOnCompleteListener(this, new OnCompleteListener<AuthResult>() {
                    @Override
                    public void onComplete(@NonNull Task<AuthResult> task) {
                        if (!task.isSuccessful()) {
                            Toast.makeText(SignInActivity.this, R.string.erro_google_sign_in,
                                    Toast.LENGTH_SHORT).show();
                        }
                    }
                });
    }
}