Introduction

• Paytm is a leading digital payment method in India. Millions of users rely on it to pay or recharge their mobile phones.
• In this blog, I will show you how to integrate Paytm Payment Gateway in your Flutter applications using a simple example.
• This blog has a built-in sample app that uses the library: paytm_allinonesdk.
• You can find the documentation of the Paytm plugin for Flutter at :
  https://developer.paytm.com/docs/all-in-one-sdk/hybrid-apps/flutter/
• This article will show you how to make a simple Flutter app integrating payment gateway with Paytm in Flutter.
• You will get to know what all things are necessary and how to proceed with integration.

Create a merchant account

• Before we proceed with the integration part, we have to first create a merchant Paytm account.
• To do that go to Dashboard and log in with your Paytm account details.

Generating API Keys

• Click on API keys under the Developer settings in the left menu.
• Now, Click the Generate now button under the Test API Details.

Building and Deploying Backend

• Paytm requires to generate and validate the checksum.

  Checksum🤔 ??

• In a nutshell, Paytm uses the checksum signature to verify that API requests and replies transmitted between your app and Paytm over a network are not tampered with.
• Another thing is that the customer adds goods/services to the shopping/order cart and then goes to checkout on your mobile app.
• When the user proceeds to checkout, the Developer needs to call the Initiate Transaction API from the backend to generate a transaction token.

  Transaction token🤔 ??

• To start transactions in our app, as a developer we need to first call this transaction API and this API will return the txn Token, which is a mandatory parameter that we must give in the AllInOneSdk to begin transactions in our app.

• So the question is how to generate the Checksum and Transaction Token?
• Here you have two options :

  1) Use the below URL

• I have already created the backend and also deployed it on the Heroku server.
• flutter-paytm-backend.herokuapp.com/generat...
• You can simply make a POST request using http in Flutter and it will return you the Txn Token.
• However, because this URL is publicly available, I would not recommend using it if you are integrating Paytm into your production application or if you are using it in a commercial project. This URL can be used for testing purposes.

  2) Create and Deploy your own backend on your own Server. (Recommended)

• It's not as difficult as it appears.
• You only need to download the zip folder from the URL below, which contains all of the information needed to produce txnToken. As a result, you won't have to start from the beginning.
  https://github.com/red-star25/flutter-paytm-backend.git
• After downloading it. You can deploy it on the server of your own choice.

• Okay, So these all are the Pre-requisites before we begin with the integration part.

Integration Of Paytm AllInOneSdk in Flutter

Step: 1 - Add Dependency

• Inside the pubspec.yaml, add the latest paytm_allinonesdk package.

  dependencies:
      paytm_allinonesdk: ^1.1.4
  

Step: 2 - Create PaytmConfig class

• Now to sperate the business logic, create a file named paytm_config.dart under lib.

• Create PaytmConfig class and define the required variables.

  class PaytmConfig {
    final String _mid = "...";
    final String _mKey = "...";
    final String _website = "DEFAULT"; // or "WEBSTAGING" in Testing
    final String _url =
        'https://flutter-paytm-backend.herokuapp.com/generateTxnToken'; // Add your own backend URL
  
    String get mid => _mid;
    String get mKey => _mKey;
    String get website => _website;
    String get url => _url;
  }
  

• You can get the mid(Merchant Id) and mKey(Merchant Key) from the API Keys section in the Paytm Dashboard
• 
• Now create one method getMap which will return a JSON encoded string. This string contains all the information required to generate txn Token.

  String getMap(double amount, String callbackUrl, String orderId) {
    return json.encode({
      "mid": mid,
      "key_secret": mKey,
      "website": website,
      "orderId": orderId,
      "amount": amount.toString(),
      "callbackUrl": callbackUrl,
      "custId": "122", // Pass users Customer ID here
    });
  }
  

• Now, let's create a function called generateTxnToken which is used to make a POST request to the backend.

  Future<void> generateTxnToken(double amount, String orderId) async {
    final callBackUrl =
        'https://securegw.paytm.in/theia/paytmCallback?ORDER_ID=$orderId';
    final body = getMap(amount, callBackUrl, orderId);
  
    try {
      final response = await http.post(
        Uri.parse(url),
        body: body,
        headers: {'Content-type': "application/json"},
      );
      String txnToken = response.body;
      await initiateTransaction(orderId, amount, txnToken, callBackUrl);
    } catch (e) {
      print(e);
    }
  }
  

• After getting TxnToken we can now initiate the transaction. Create a function called initiateTransaction

  Future<void> initiateTransaction(String orderId, double amount,
     String txnToken, String callBackUrl) async {
   String result = '';
   try {
     var response = AllInOneSdk.startTransaction(
       mid,
       orderId,
       amount.toString(),
       txnToken,
       callBackUrl,
       false, // isStaging
       false, // restrictAppInvoke
     );
     response.then((value) {
       // Transaction successfull
       print(value);
     }).catchError((onError) {
       if (onError is PlatformException) {
         result = onError.message! + " \n  " + onError.details.toString();
         print(result);
       } else {
         result = onError.toString();
         print(result);
       }
     });
   } catch (err) {
     // Transaction failed
     result = err.toString();
     print(result);
   }
  }
  

• PaytmConfig Full Source Code : Here

Step:3 - Make Payment

• Now after all the configuration, we can make payment by calling the generateTxnToken function from our UI.

• FloatingActionButton(
        onPressed: () async {
          await paytmConfig.generateTxnToken(amount, orderId);
        },
        child: const Icon(Icons.payment),
  ),
  

Final Output:

Wrapping Up

• I hope you found this article to be beneficial. If you have any feedback/queries, leave them in the comments.
• Thank you for spending time reading this article. See you in the next article. So, until then...