Firebase Authentication with Google Sign in

1. Create a new android project
2. Set up Firebase Authentication for android

a. Using Firebase Assistant
b. Manually Add Firebase1. Add rules to the root-level build.gradle to include the google-services plugin
(classpath ‘com.google.gms:google-services:3.0.0’)
2. Enable the google-services gradle plugin in the app-level build.gradle
(apply plugin: ‘com.google.gms.google-services’)
3. Add the dependencies for firebase SDKs in the app-level build.gradle file
(com.google.firebase:firebase-auth:10.2.0)
4. Add play services authentication library
(compile ‘com.google.android.gms:play-services-auth:10.0.1’)

3. Check if the user is already logged in

-Create instances of FirebaseAuth and FirebaseAuth.AuthStateListener (e.g. mAuth and mAuthStateListener repectively)

private FirebaseAuth mAuth;
private FirebaseAUthStateListener;->Initialize these inside onCreate()

  /*get firebase auth instance*/
        mAuth=FirebaseAuth.getInstance();
        mAuthStateListener=new FirebaseAuth.AuthStateListener() {
            @Override
            public void onAuthStateChanged(@NonNull FirebaseAuth firebaseAuth) {

                FirebaseUser user=firebaseAuth.getCurrentUser();
                if(user!=null)
                {
                    /*User is logged in*/
                }
                else
                {
                    /*User not logged in*/
                }
            }
        };

-Add the mAuthStateListener to the mAuth object inside onStart()

mAuth.addAuthStateListener(mAuthStateListener) ;

-Remove the auth state listener inside onStop()

if(mAuthStateListener!=null)
   mAuth.removeAuthStateListener(mAuthListener);

4. Create a GoogleSignInOptions object and use it to build a google api client object

We need this object to configure google sign-in to request required user data

// Configure Google Sign In (inside onCreate)
  GoogleSignInOptions gso = new GoogleSignInOptions.Builder(GoogleSignInOptions.DEFAULT_SIGN_IN)
                .requestIdToken(getString(R.string.default_web_client_id))
                .requestEmail()
                .build();

 mGoogleApiClient = new GoogleApiClient.Builder(this)
      .enableAutoManage(this /* FragmentActivity */, this /* OnConnectionFailedListener */)
      .addApi(Auth.GOOGLE_SIGN_IN_API, gso)
      .build();

The parameter GoogleSignInOptions.DEFAULT_SIGN_IN gives user’s ID and basic profile

5. Add the Google Login Button to your activity’s layout

<com.google.android.gms.common.SignInButton
      android:id="@+id/sign_in_button"
      android:layout_width="wrap_content"
      android:layout_height="wrap_content" />

6. On clicking this button launch the sign in intent from google api and handle the result in onActivityResult()

SignInButton signInButton = (SignInButton) findViewById(R.id.sign_in_button);
signInButton.setOnCLickListener(new View.OnClickListener(){
      @override
      public void onClick(View view)
      {
        Intent signInIntent = Auth.GoogleSignInApi.getSignInIntent(mGoogleApiClient);
        startActivityForResult(signInIntent, RC_SIGN_IN);
      }
    })

Handle the result from this Intent in onActivityResult.
-Get the GoogleSignInResult object and check if Google sign in is successful ,
-If yes get GoogleSignInAccount instance , we shall pass this account instance to the method firebaseAuthWithGoogle(GoogleSignInAccount account); which we are going to implement in the next step.

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

        // Result returned from launching the Intent from GoogleSignInApi.getSignInIntent(...);
        if (requestCode == RC_SIGN_IN) {
            GoogleSignInResult result = Auth.GoogleSignInApi.getSignInResultFromIntent(data);
            if (result.isSuccess()) {
                // Google Sign In was successful, authenticate with Firebase
                GoogleSignInAccount account = result.getSignInAccount();
                firebaseAuthWithGoogle(account);  //We shall implement this in next steps
            } else {
                // Google Sign In failed, update UI appropriately
                // ...
            }
         }
      }

7. Implementation of firebaseAuthWithGoogle(GoogleSignInAccount account)

-Get the ID token from GoogleSignInAccount object
-Get the Firebase credentials by passing this id token to GoogleAuthProvider.getCredential()
AuthCredential credential = GoogleAuthProvider.getCredential(acct.getIdToken(), null);
-Now authenticate the user with Firebase using these credentials

mAuth.signInWithCredential(credential)
           .addOnCompleteListener(this, new OnCompleteListener<AuthResult>() {
               @Override
               public void onComplete(@NonNull Task<AuthResult> task) {
                   if (!task.isSuccessful()) {
                      //Failure
                       Toast.makeText(GoogleSignInActivity.this, "Authentication failed.",
                               Toast.LENGTH_SHORT).show();
                   }
                   else
                   {
                      //success navigate to LoggedIn Activity
                   }
               }
           });

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