ネイティブAndroid用ログインモジュール

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

//retrofit
 implementation 'com.squareup.retrofit2:retrofit:2.9.0'
 implementation 'com.squareup.okhttp3:logging-interceptor:3.8.0'
 implementation 'com.squareup.retrofit2:converter-scalars:2.9.0'
 implementation 'com.squareup.retrofit2:converter-gson:2.9.0'

 findViewById(R.id.now_gg_login_button).setOnClickListener(new View.OnClickListener() {
      @Override
      public void onClick(View view) {
          signIn();
      }
  });

public static final String ID_TOKEN = "id_token"; // Do Not Modify
 public static final String CLIENT_ID = "<your client id>"; // Replace with your client id
 public static final String ACCOUNT_TYPE = "now.gg"; // Do Not Modify
 public static final String HOST_URL = "hostUrl"; //Do Not Modify

 private void signIn() {
     Account account = getNowggAccount();
     if (account != null) {
         Bundle bundle = new Bundle();
         bundle.putString("client_id", CLIENT_ID);
         String authTokenType = ID_TOKEN;
         AccountManager.get(getApplicationContext()).
         getAuthToken(account, authTokenType, bundle, MainActivity.this, new OnTokenAcquired(), null);
     } else {
         addNowggAccount();
     }
 }

 private Account getNowggAccount() {
     Account[] accounts = AccountManager.get(getApplicationContext()).getAccountsByType(ACCOUNT_TYPE);
     if (accounts.length > 0) {
         Log.d(TAG, "getNowggAccount: account found");
         // currently only one now.gg account can be added in a system
         return accounts[0];
     }
     return null;
 }

 private void addNowggAccount() {
     try {
         Intent intent = new Intent();
         intent.setComponent(new ComponentName("gg.now.accounts", "gg.now.accounts.AuthenticatorActivity"));
         intent.setAction("IAP_ADD_ACCOUNT");
         intent.putExtra("resultReceiver", parcelResultReceiver(resultReceiver));
         intent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
         startActivity(intent);
     } catch (ActivityNotFoundException e) {
         e.printStackTrace();
     }
 }

 ResultReceiver parcelResultReceiver(ResultReceiver actualReceiver) {
     Parcel parcel = Parcel.obtain();
     actualReceiver.writeToParcel(parcel, 0);
     parcel.setDataPosition(0);
     ResultReceiver receiverForSending = ResultReceiver.CREATOR.createFromParcel(parcel);
     parcel.recycle();
     return receiverForSending;
 }

 ResultReceiver resultReceiver = new ResultReceiver(new Handler(Looper.getMainLooper())) {
     @Override
     protected void onReceiveResult(int resultCode, Bundle resultData) {
         super.onReceiveResult(resultCode, resultData);
         Log.d(TAG, "onReceiveResult() called with: resultCode = [" + resultCode + "], resultData = [" + resultData.toString() + "]");
         // onReceiveResult
         if (resultCode == 0) { // means sign in is okay
             signIn();
         } else {
             // sign in failed
             Log.d(TAG, "onReceiveResult: sign in failed");
         }
     }
 };

      
 private class OnTokenAcquired implements AccountManagerCallback {

 @Override
  public void run(AccountManagerFuture result) {
  try {
       Bundle bundle = (Bundle) result.getResult();
       boolean success = bundle.getBoolean(AccountManager.KEY_BOOLEAN_RESULT, false);
         if (success) {
            final String token = bundle.getString(ID_TOKEN);
            // now.ggのサーバーURLを取得し、ユーザーの詳細を取得し、トークンを検証します

            final String hostUrl = bundle.getString(HOST_URL);
            verifyToken(token, hostUrl);
            }
            else {
                  // トークンの取得に失敗しました
                  // エラーの場合。開発者はエラーを表示するか、他のログイン方法を表示できます
                 Log.d(TAG, "run: get token failed " + bundle);
                 }
          } catch (AuthenticatorException | IOException | OperationCanceledException e) {
            e.printStackTrace();
          }
     }
 }

     
  private void verifyToken(String idToken, String hostUrl) {
  BackendApiCallService backendApiCallService = getBackendRetrofit(hostUrl).create(BackendApiCallService.class);

  Call < TokenVerifyResponse > tokenVerifyResponseCall = backendApiCallService.verifyIdToken(new TokenVerifyRequest(ID_TOKEN, idToken, CLIENT_ID));

  tokenVerifyResponseCall.enqueue(new Callback < TokenVerifyResponse > () {
    @Override
    public void onResponse(Call < TokenVerifyResponse > call, Response < TokenVerifyResponse > response) {
      if (response.isSuccessful()) {
        Log.d(TAG, "onResponse: " + response.body().toString());
      } else {
        Gson gson = new Gson();
        TokenVerifyResponse error = gson.fromJson(response.errorBody().charStream(), TokenVerifyResponse.class);

        if (!error.isSuccess() && ("EXPIRED_TOKEN".equals(error.getCode()) || "INVALID_TOKEN".equals(error.getCode()))) {
          // retry the verify token call, after getting a new TOKEN
        }
      }
    }
    @Override
    public void onFailure(Call call, Throwable t) {
      Log.d(TAG, "onFailure() called with: call = [" + call + "], t = [" + t + "]");
    }
  });

}


 public Retrofit getBackendRetrofit(String hostUrl) {
   OkHttpClient okHttpClient = new OkHttpClient.Builder().build();
   return new Retrofit.Builder()
     .baseUrl(hostUrl)
     .addConverterFactory(ScalarsConverterFactory.create())
     .addConverterFactory(GsonConverterFactory.create())
     .client(okHttpClient)
     .build();
 }

 public interface BackendApiCallService {
   @POST("/accounts/oauth2/v1/verify-token")
   Call < TokenVerifyResponse > verifyIdToken(@Body TokenVerifyRequest authRequest);
 }

 public class TokenVerifyRequest {
   String token_type;
   String token;
   String client_id;

   public TokenVerifyRequest(String token_type, String token, String client_id) {
     this.token_type = token_type;
     this.token = token;
     this.client_id = client_id;
   }
 }

 public class TokenVerifyResponse {
   boolean success;
   String code;
   String msg;
   @SerializedName("decodedData")
   UserDataVerified userDataVerified;

   public TokenVerifyResponse(boolean success, String code, String msg, UserDataVerified userDataVerified) {
     this.success = success;
     this.code = code;
     this.msg = msg;
     this.userDataVerified = userDataVerified;
   }

   public UserDataVerified getUserDataVerified() {
     return userDataVerified;
   }

   public boolean isSuccess() {
     return success;
   }
   public String getCode() {
    return code;
   }

   @Override
   public String toString() {
     return "TokenVerifyResponse{" +
       "success=" + success +
       ", code='" + code + '\'' +
       ", msg='" + msg + '\'' +
       ", userDataVerified=" + userDataVerified +
       '}';
   }
 }

UserDataVerified userData = tokenVerifyResponse.getUserDataVerified();
 String userId = userData.getUserId();
 String name = userData.getName();
 String email = userData.getEmail();
 String picture = userData.getPicture();
 
 public class UserDataVerified {
   String iss;
   String sub;
   String aud;
   String exp;
   String iat;
   String auth_time;
   String tokenId;
   String sessionId;
   String scope;

   String email;
   String name;
   String picture;
   String mobile;
   String userId;

   public UserDataVerified(String iss, String sub, String aud, String exp, String iat,
                           String auth_time, String email, String mobile, String userId,
                           String tokenId, String sessionId, String scope, String name, String picture) {
       this.iss = iss;
       this.sub = sub;
       this.aud = aud;
       this.exp = exp;
       this.iat = iat;
       this.auth_time = auth_time;
       this.email = email;
       this.mobile = mobile;
       this.userId = userId;
       this.tokenId = tokenId;
       this.sessionId = sessionId;
       this.scope = scope;
       this.name = name;
       this.picture = picture;
  }

   @Override
   public String toString() {
       return "UserDataVerified{" +
               "iss='" + iss + '\'' +
               ", sub='" + sub + '\'' +
               ", aud='" + aud + '\'' +
               ", exp='" + exp + '\'' +
               ", iat='" + iat + '\'' +
               ", auth_time='" + auth_time + '\'' +
               ", tokenId='" + tokenId + '\'' +
               ", sessionId='" + sessionId + '\'' +
               ", scope='" + scope + '\'' +
               ", email='" + email + '\'' +
               ", name='" + name + '\'' +
               ", picture='" + picture + '\'' +
               ", mobile='" + mobile + '\'' +
               ", userId='" + userId + '\'' +
               '}';
   }
 }