Google sign in process

AndroidManifest.xml

<application>
  ....
<meta-data
            android:name="com.google.android.geo.API_KEY"
            android:value="@string/google_maps_key" />

</application>

<uses-permission android:name="android.permission.INTERNET" />

build.gradle (module)

implementation 'com.google.android.gms:play-services-auth:19.0.0'

build.gradle (project)

dependencies {
...
classpath 'com.google.gms:google-services:4.3.4'
}

GSI1.java

private GoogleSignInClient mGoogleSignInClient;

GSI2.java

GoogleSignInOptions gso = new GoogleSignInOptions.Builder(GoogleSignInOptions.DEFAULT_SIGN_IN)
                .requestIdToken(getString(R.string.server_client_id))
                .requestEmail()
                .build();
mGoogleSignInClient = getClient(this, gso);

GSI3.java

@Override
public void onActivityResult(int requestCode, int resultCode, Intent data) {
    super.onActivityResult(requestCode, resultCode, data);
    if (requestCode == RC_SIGN_IN) {
        Task<GoogleSignInAccount> task = GoogleSignIn.getSignedInAccountFromIntent(data);
        handleSignInResult(task);
    }
}

GSI4.java

private void handleSignInResult(Task<GoogleSignInAccount> completedTask) {
    try {
        GoogleSignInAccount account = completedTask.getResult(ApiException.class);
        updateUI(account);
    } catch (ApiException e) {
        e.printStackTrace();
        updateUI(null);
    }
}

@Override
public void onStart() {
    super.onStart();
    GoogleSignInAccount account = getLastSignedInAccount(this);
    updateUI(account);
}

GSI5.java

private void updateUI(@Nullable GoogleSignInAccount account) {
    if (account != null) {
        String idToken = account.getIdToken();
        // call function with the token
        callGcpFunction(idToken);
    } else {
        signIn();
    }
}

private void signIn() {
    Intent signInIntent = mGoogleSignInClient.getSignInIntent();
    startActivityForResult(signInIntent, RC_SIGN_IN);
}