implementation_snippet.java

    /**
     * This function talks to server and helps logging a returning user in. Here we will not be
     * doing any checks on username and password strings, as they have already been checked
     * and tailored for this function.
     * @param username the username entered by user
     * @param password password as entered by user
     */
    private void login(String username ,String password) {
        /* Get client with the help of helper function of your own */
        ApolloClient client = ApolloUtils.getApolloClient(this);
        /*
         * Classes like LoginQuery will be auto generated when you add your .graphql files and
         * build your project. Debugging graphql files is not straightforward. Android
         * studio won't help you much in description of errors in your .graphql files.
         * Therefore, I would recommend you to please check queries/mutations in GraphQL
         * Playground for errors, then copy-paste from there.
         */
        if (client != null) {
            client.query(
                    LoginQuery.builder()
                            .username(username) // pass parameters
                            .password(password)
                            .build())
                    .enqueue(new ApolloCall.Callback<LoginQuery.Data>() {
                        @Override
                        public void onResponse(@NotNull Response<LoginQuery.Data> response) {
                            /*
                             * Make note that these callbacks are not on UI Thread for obvious 
                             * reasons. We don't have to explicitly handle thread, but 
                             * all interactions with UI Thread must be enclosed as below.
                             */
                            runOnUiThread(() -> {
                                if (response.hasErrors()) handleErrors(response);
                                else finishLogin(response);
                            });
                            Log.i(TAG, "onResponse: " + response.data());
                        }

                        @Override
                        public void onFailure(@NotNull ApolloException e) {
                            runOnUiThread(() -> {
                                showToast("Failed to connect with server");
                            });
                        }
                    });
        }
    }


    /**
     * In case of successful login, we would require some information to be extracted from
     * response. This information will be used in further queries. We would be storing this
     * information as persistent data using SharedPreferences.
     * @param response the response sent by server
     */
    private void finishLogin(@NotNull Response<LoginQuery.Data> response) {
        SharedPreferenceUtils prefUtils = new SharedPreferenceUtils(this);
        try {
            /* Let us get auth token */
            String authToken = response.data().loginDetails().token();
            prefUtils.setAuthToken(authToken);
            /* Let us now get login id */
            int loginId = response.data().loginDetails().login().loginId();
            prefUtils.setLoginId(loginId);
            /* Safe point to navigate the user further */
            Intent intent = new Intent(LoginActivity.this, DeviceInfoActivity.class);
            startActivity(intent);
            finish();
        }
         catch (NullPointerException e) {
            e.printStackTrace();
            prefUtils.setAuthToken(SHARED_PREF_DEFAULT_VALUE_STRING);
            prefUtils.setLoginId(SHARED_PREF_DEFAULT_VALUE_INT);
            showToast("Something is not right");
            showProgress(false);
         }

    }





    /*
     *********************************************************************************************
     * Below code is from another class.
     * This below code helps us to get hold of our apollo client. 
     * You might use these functions in different activities/fragments, so it would be
     * better if they are placed in a utility class and made static.
     *********************************************************************************************
     */


    private static final String TAG = ApolloUtils.class.getSimpleName();

    public static ApolloClient getApolloClient(Context context){
        SharedPreferenceUtils prefUtils = new SharedPreferenceUtils(context);
        if (prefUtils.getBaseUrl().equals(SHARED_PREF_DEFAULT_VALUE_STRING)) {
            Intent intent = new Intent(context, LoginActivity.class);
            context.startActivity(intent);
            return null;
        }
        /* This is what makes the query for us */
        return ApolloClient.builder()
                .serverUrl(prefUtils.getBaseUrl())
                .okHttpClient(createOkHttpWithValidToken(context))
                .build();
    }

    private static OkHttpClient createOkHttpWithValidToken(Context context) {
        String authToken = new SharedPreferenceUtils(context).getAuthToken();
        Log.i(TAG, "createOkHttpWithValidToken: " + authToken);
        /*  ApolloClient uses OkHttp under the hood for handling network requests. */
        return new OkHttpClient.Builder()
                .addNetworkInterceptor(chain ->
                        chain.proceed(chain.request()
                                .newBuilder()
                                .header(HEADER_AUTHORIZATION,
                                HEADER_AUTHORIZATION_VALUE + authToken)
                                .build()))
                .build();
    }