Forem: Ashutosh Chauhan

Using Stream Builder in Flutter

Ashutosh Chauhan — Wed, 05 Feb 2020 11:43:00 +0000

Dart & Flutter was made with asynchronicity in mind. Dart provide really good support for Futures and Streams. For those who do not have a basic idea of asynchronous programming in dart can refer to this playlist.

We will create an App which will call an API after every second and update the page whenever the response is received, which would look something like below:

Lets start with creating a new Flutter Project.

Step 1: Create the project

You can use your favorite IDE (VS Code or any other) to start a new Flutter project. Once you have create a flutter lets edit the lib/main.dart. Keep the following code and remove everything else.

import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: Scaffold(),
    );
  }
}

Step 2: Requesting data from API

We will create a page which will show us the amount of likes a certain post has recieved. We won't be making the entire application only the Likes counter which will show us the amount of people who have liked a post. For simplicity we will use a dummy API which will repond with a random value.

Create a file lib/models.dart :

class Number {
  int value;

  Number(this.value);

  factory Number.fromJSON(Map<String, dynamic> json) {
    return Number(json['value']);
  }
}

We will use the class Number which will create a class from a json object.

Next, create another file lib/requests.dart :

import 'package:http/http.dart' as http;
import 'dart:convert';
import 'dart:async';
import 'models.dart';

Future<Number> getNumber({int num = 20}) async {
  http.Response res = await http.get(Uri.http("dummyresponse.pythonanywhere.com", "/api/$num"));
  return Number.fromJSON(json.decode(res.body));
}

Stream<Number> getNumbers(Duration refreshTime) async* {
  while (true) {
    await Future.delayed(refreshTime);
    yield await getNumber();
  }
}

The getNumber is an async method which waits for a network call to a dummy response server which generates a random number. (https://dummyresponse.pythonanywhere.com/api/31 will give random no. from 0 to 31)

The getNumbers method returns a stream which calls the API and retrieves the latest information, hence updating the information after every fixed duration.

Now, Since we have the stream let's get to the real stuff.

Step 3: Stream Builder

Create a new file lib/home_page.dart :

import 'package:flutter/cupertino.dart';
import 'package:flutter/material.dart';
import 'package:stream_builder_example/models.dart';
import 'package:stream_builder_example/requests.dart';

class HomePage extends StatefulWidget {
  HomePage({Key key}) : super(key: key);

  @override
  _HomePageState createState() => _HomePageState();
}

class _HomePageState extends State<HomePage> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(
          child: StreamBuilder(
        stream: getNumbers(Duration(seconds: 1)),
        initialData: Number(0),
        builder: (context, stream) {
          if (stream.connectionState == ConnectionState.done) {
            return Icon(
              Icons.check_circle,
              color: Colors.green,
              size: 20,
            );
          }
          if (stream.hasData) {
            return LikeCounter(stream.data.value);
          } else {
            return CircularProgressIndicator();
          }
        },
      )),
    );
  }
}

class LikeCounter extends StatelessWidget {
  static List<Color> colors = [ Colors.green, Colors.purpleAccent, Colors.deepPurple,  Colors.blueAccent, Colors.deepOrangeAccent ];
  final int num;

  LikeCounter(this.num);

  @override
  Widget build(BuildContext context) {
    return AnimatedContainer(
      duration: Duration(milliseconds: 250),
      padding: EdgeInsets.all(10),
      decoration: BoxDecoration(
        borderRadius: BorderRadius.circular(100),
        color: colors[num % colors.length],
      ),
      child: Row(
        mainAxisSize: MainAxisSize.min,
        children: <Widget>[
          Icon(Icons.favorite, color: Colors.white),
          Text(
            " $num Likes",
            style: TextStyle(color: Colors.white, fontSize: 20),
          ),
        ],
      ),
    );
  }
}

The LikeCounter is a stateless widget which shows the number of likes with different colors.

Let's see how the Stream Builder works.

StreamBuilder(
    stream: getNumbers(Duration(seconds: 1)),
    initialData: Number(0),
    builder: (context, stream) {
        if (stream.connectionState == ConnectionState.done) {
            return Icon(
                Icons.check_circle,
                color: Colors.green,
                size: 20,
            );
        }
        if (stream.hasData) {
            return LikeCounter(stream.data.value);
        } else {
            return CircularProgressIndicator();
        }
    },
)

The Stream builder needs mainly 3 parameters: stream, builder, and initialData. The initial is used as a dummy or empty element to act as the first element.

The builder method receives context and snapshot of the stream.

We can use stream.connectionState to find the status of the stream, details of state is given here.

We can use stream.hasData ad stream.data to build widget when the widget is build.

We can also use stream.hasError to build when an Error occurs.

Next, change the lib/main.dart and update HomePage() as Material App home :

import 'home_page.dart';

// ...   
return MaterialApp(
    title: 'Flutter Demo',
    theme: ThemeData(
        primarySwatch: Colors.blue,
    ),
    home: HomePage(),
);
// ...

Next Step: Use Complex Objects instead of Number

Since this is a rudimentary example we have not used a complete API. But streams provide a really good way to allow continuous chunk of data flowing through and only rebuilding the app when a value is updated. This method allows you to track values in real time without needing your user to refresh manually. This can be used in a wide variety of applications such as Social Apps, Live Sports Scores, etc.

The code for the final application can be found here:

Paytm Payment Gateway Integration in Django

Ashutosh Chauhan — Sun, 02 Feb 2020 15:27:06 +0000

Paytm Payment Gateway Integration in Django

Synopsis: This is a great way to learn how to integrate payments in your Django application. The official docs from Paytm are here.

Note: You would require a Paytm Merchant Account to be able to use the Payment Gateway

The process of sending the request to the gateway is mentioned in the following document:

Image Source: Paytm

We will create an Django App from scratch to achieve our goal.

Step 0: Setting up the Project

First we'll create a virtual environment for out project.$

# Install Django & virtualenv if not already
$ pip install Django virtualenv
# create a new Django project
$ django-admin startproject pay2me
$ cd pay2me
# Initialize a new virtualenv with Py3 since Python 2 is dead
$ virtualenv env --python=python3
# Activate the virtual environment and intall Django
$ source ./env/bin/activate
(env)$ pip install Django

You can use the command $ source ./env/bin/activate to activate the virtual environment.

We would use the default Django User models for the authentication system to save time but you can use custom User model if you wish.

Now let's add login view to our project. open pay2me/urls.py and add entry to login/logout views.

from django.contrib.auth.views import LoginView

urlpatterns = [
    path('admin/', admin.site.urls),
    path('login/', LoginView.as_view(), name='login'),
]

Now to make login and logout pages we first have to migrate our changes and create a superuser. Run the following commands in the terminal.

(env)$ python manage.py makemigrations
(env)$ python manage.py migrate
(env)$ python manage.py createsuperuser

Now we can run the server by executing python manage.py runserver and visit http://localhost:8000/login to see the login page.

The page will not load since the login page template is missing so we will create a template for LoginView. Create a directory templates in the project's root directory and another directory registration in templates. Now add a file login.html in the registration directory and add the following to it.

<!DOCTYPE html>
<head>
    <title>Login</title>
</head>
<body>
    <h2>Login</h2>
    <form method="post">
        {% csrf_token %}
        {{ form.as_p }}
        <button type="submit">Login</button>
    </form>
</body>
</html>

The login page contains contain a form and a csrf_token which will be provided by the LoginView so we don't have to worry about it.

We would also have to modify pay2me/settings.py. Open the file, Navigate to the TEMPLATES setting and add 'templates' in the DIRS list, it should look like this:

TEMPLATES = [
    {
        'BACKEND': 'django.template.backends.django.DjangoTemplates',
        'DIRS': ['templates'],
        'APP_DIRS': True,
        'OPTIONS': {
            'context_processors': [
                'django.template.context_processors.debug',
                'django.template.context_processors.request',
                'django.contrib.auth.context_processors.auth',
                'django.contrib.messages.context_processors.messages',
            ],
        },
    },
]

This would help Django locate our registration/login.html template.

Since the template is all set, now restart the server and go to http://localhost:8000/login and the login form should look like this:

Let's continue to the next step.

Step 1: Making the UI

Since we are keeping this concise we will only have two pages:

Payment page : Where we will enter the amount we have to pay
Callback Page : The response received from Paytm carrying payment status

Lets start by making a new app to organize these features.

(env)$ python manage.py startapp payments

First of all let's add the payments app to the project. To do that we will add 'payments' in the INSTALLED_APPS list in pay2me/settings.py .

INSTALLED_APPS = [
    'django.contrib.admin',
    'django.contrib.auth',
    'django.contrib.contenttypes',
    'django.contrib.sessions',
    'django.contrib.messages',
    'django.contrib.staticfiles',
    'payments',
]

Now lets add the template files pay.html and callback.html in templates/payments in the payments app directory.

pay.html

<!DOCTYPE html>

<head>
    <title>Payment Home</title>
</head>

<body>
    <h2>Payment Home</h2>
    {% if error %}
    <h3>{{ error }}</h3>
    {% endif %}
    <form method="post">
        {% csrf_token %}
        <label for="username">Username: </label>
        <input type="text" name="username" required>
        <label for="password">Password: </label>
        <input type="text" name="password" required>
        <label for="amount">Amount: </label>
        <input type="number" name="amount" value="100">
        <input type="submit" name="submit" value="Submit" required>
    </form>
</body>

</html>

callback.html

<!DOCTYPE html>
<head>
    <title>Callback</title>
</head>
<body>
    <h2>Callback Messsage: </h2>                    <br>
    <h3> Checksum Verification: {{ message }} </h3> <br>
    MID: {{ MID }}                                  <br>
    TXNID: {{ TXNID }}                              <br>
    ORDERID: {{ ORDERID }}                          <br>
    BANKTXNID: {{ BANKTXNID }}                      <br>
    TXNAMOUNT: {{ TXNAMOUNT }}                      <br>
    CURRENCY: {{ CURRENCY }}                        <br>
    <h3> STATUS: {{ STATUS }} </h3>                 <br>
    RESPCODE: {{ RESPCODE }}                        <br>
    RESPMSG: {{ RESPMSG }}                          <br>
    TXNDATE: {{ TXNDATE }}                          <br>
    GATEWAYNAME: {{ GATEWAYNAME }}                  <br>
    BANKNAME: {{ BANKNAME }}                        <br>
    BIN_NAME: {{ BIN_NAME }}                        <br>
    PAYMENTMODE: {{ PAYMENTMODE }}                  <br>
    CHECKSUMHASH: {{ CHECKSUMHASH }}
</body>
</html>

The Payment page asks for user login information and payment amount, whereas Callback page shows the values of a lot of parameters which would be provided by Paytm upon completion of payment.

Step 2: Make Transaction model

Since any payment related application would require transactions we would create a Transaction model in payments/models.py as follows:

from django.db import models
from django.contrib.auth import get_user_model

User = get_user_model()

class Transaction(models.Model):
    made_by = models.ForeignKey(User, related_name='transactions', 
                                on_delete=models.CASCADE)
    made_on = models.DateTimeField(auto_now_add=True)
    amount = models.IntegerField()
    order_id = models.CharField(unique=True, max_length=100, null=True, blank=True)
    checksum = models.CharField(max_length=100, null=True, blank=True)

    def save(self, *args, **kwargs):
        if self.order_id is None and self.made_on and self.id:
            self.order_id = self.made_on.strftime('PAY2ME%Y%m%dODR') + str(self.id)
        return super().save(*args, **kwargs)

The transaction model is defined in such a way that the order_id is unique and generated based on date of transaction. We can see there is a checksum which will store the checksum generated by python file.

We overrode the save method to automatically generate order_id from the date and time of the transaction.

Let's run the migrations again to add the Transaction model in the database.

(env)$ python manage.py makemigration
(env)$ python manage.py migrate

Step 3: Adding Paytm Settings

Add the following settings to pay2me/settings.py :

PAYTM_MERCHANT_ID = '<your_merchant_id>'
PAYTM_SECRET_KEY = '<your_paytm_secret_key>'
PAYTM_WEBSITE = 'WEBSTAGING'
PAYTM_CHANNEL_ID = 'WEB'
PAYTM_INDUSTRY_TYPE_ID = 'Retail'

Step 4: Create Views for Payments

Paytm has provided a repository for the checksum generation code here but the code is using a deprecated library pycrypto, the following code contains the modified file compatible with the latest pycryptodome library. So download the file and save it in your payments app with file name paytm.py.

Now lets begin the create the initiate_payment view that would receive the username, password and amount. Open payments/views.py and add the following code.

from django.shortcuts import render
from django.contrib.auth import authenticate, login as auth_login
from django.conf import settings
from .models import Transaction
from .paytm import generate_checksum, verify_checksum


def initiate_payment(request):
    if request.method == "GET":
        return render(request, 'payments/pay.html')
    try:
        username = request.POST['username']
        password = request.POST['password']
        amount = int(request.POST['amount'])
        user = authenticate(request, username=username, password=password)
        if user is None:
            raise ValueError
        auth_login(request=request, user=user)
    except:
        return render(request, 'payments/pay.html', context={'error': 'Wrong Accound Details or amount'})

    transaction = Transaction.objects.create(made_by=user, amount=amount)
    transaction.save()
    merchant_key = settings.PAYTM_SECRET_KEY

    params = (
        ('MID', settings.PAYTM_MERCHANT_ID),
        ('ORDER_ID', str(transaction.order_id)),
        ('CUST_ID', str(transaction.made_by.email)),
        ('TXN_AMOUNT', str(transaction.amount)),
        ('CHANNEL_ID', settings.PAYTM_CHANNEL_ID),
        ('WEBSITE', settings.PAYTM_WEBSITE),
        # ('EMAIL', request.user.email),
        # ('MOBILE_N0', '9911223388'),
        ('INDUSTRY_TYPE_ID', settings.PAYTM_INDUSTRY_TYPE_ID),
        ('CALLBACK_URL', 'http://127.0.0.1:8000/callback/'),
        # ('PAYMENT_MODE_ONLY', 'NO'),
    )

    paytm_params = dict(params)
    checksum = generate_checksum(paytm_params, merchant_key)

    transaction.checksum = checksum
    transaction.save()

    paytm_params['CHECKSUMHASH'] = checksum
    print('SENT: ', checksum)
    return render(request, 'payments/redirect.html', context=paytm_params)

Lets understand the view part by part:

def initiate_payment(request):
    if request.method == "GET":
        return render(request, 'payments/pay.html')

The first step is to check the method of the request. When the request method is GET then we will just return the payment page.

try:
        username = request.POST['username']
        password = request.POST['password']
        amount = int(request.POST['amount'])
        user = authenticate(request, username=username, password=password)
        if user is None:
            raise ValueError
        auth_login(request=request,     user=user)
    except:
        return render(request, 'payments/pay.html', context={'error': 'Wrong Account Details or amount'})

Then we verify the username and password entered by the user in the form and tries to log in the user. If the login details are invalid then the view returns back the login page with an error message.

    transaction = Transaction.objects.create(made_by=user, amount=amount)
    transaction.save()
    merchant_key = settings.PAYTM_SECRET_KEY

Next we create a Transaction object for our user and get our merchant key from settings.py.

    params = (
        ('MID', settings.PAYTM_MERCHANT_ID),
        ('ORDER_ID', str(transaction.order_id)),
        ('CUST_ID', str(transaction.made_by.email)),
        ('TXN_AMOUNT', str(transaction.amount)),
        ('CHANNEL_ID', settings.PAYTM_CHANNEL_ID),
        ('WEBSITE', settings.PAYTM_WEBSITE),
        # ('EMAIL', request.user.email),
        # ('MOBILE_N0', '9911223388'),
        ('INDUSTRY_TYPE_ID', settings.PAYTM_INDUSTRY_TYPE_ID),
        ('CALLBACK_URL', 'http://127.0.0.1:8000/callback/'),
        # ('PAYMENT_MODE_ONLY', 'NO'),
    )
    paytm_params = dict(params)

Then we create a dictionary for all the settings for the Paytm Checksum Generator to create a checksum.

    checksum = generate_checksum(paytm_params, merchant_key)

    transaction.checksum = checksum
    transaction.save()

    paytm_params['CHECKSUMHASH'] = checksum

Next after generating the checksum we add the checksum to the paytm_params dictionary as well as the Transaction object.

    return render(request, 'payment/redirect.html', context=paytm_params)

At the end we return the redirect page.

Step 5: Create a Redirect Page

Create a redirect page in payments/templates/payments/redirect.html:

<html>
    <head>
        <title>Merchant Check Out Page</title>
    </head>
    <body>
        <h1>Please do not refresh this page...</h1>
        <form method="post" action="https://securegw-stage.paytm.in/order/process/" name="f1">
            <table>
                <tbody>
                    <input type="hidden" name="MID" value="{{ MID }}">
                    <input type="hidden" name="WEBSITE" value="{{ WEBSITE }}">
                    <input type="hidden" name="ORDER_ID" value="{{ ORDER_ID }}">
                    <input type="hidden" name="CUST_ID" value="{{ CUST_ID }}">
                    <input type="hidden" name="INDUSTRY_TYPE_ID" value="{{ INDUSTRY_TYPE_ID }}">
                    <input type="hidden" name="CHANNEL_ID" value="{{ CHANNEL_ID }}">
                    <input type="hidden" name="TXN_AMOUNT" value="{{ TXN_AMOUNT }}">
                    <input type="hidden" name="CALLBACK_URL" value="{{ CALLBACK_URL }}">
                    <input type="hidden" name="CHECKSUMHASH" value="{{ CHECKSUMHASH }}">
                </tbody>
            </table>
        <script type="text/javascript">
            document.f1.submit();
        </script>
        </form>
    </body>
</html>

The redirect page contains a simple form that contains the fields from the dictionary. We used the javascript code to automatically post the form to the paytm gateway.

Step 6: Create the Callback View

The callback view is the view which would be called when paytm will respond with the status of the Transaction.

Add the following callback view in the payments/views.py

from django.views.decorators.csrf import csrf_exempt

@csrf_exempt
def callback(request):
    if request.method == 'POST':
        received_data = dict(request.POST)
        paytm_params = {}
        paytm_checksum = received_data['CHECKSUMHASH'][0]
        for key, value in received_data.items():
            if key == 'CHECKSUMHASH':
                paytm_checksum = value[0]
            else:
                paytm_params[key] = str(value[0])
        # Verify checksum
        is_valid_checksum = verify_checksum(paytm_params, settings.PAYTM_SECRET_KEY, str(paytm_checksum))
        if is_valid_checksum:
            received_data['message'] = "Checksum Matched"
        else:
            received_data['message'] = "Checksum Mismatched"
            return render(request, 'payments/callback.html', context=received_data)
        return render(request, 'payments/callback.html', context=received_data)

The callback view receives a post request from the paytm server. Then we retrieve the received data in a dictionary and and verify the checksum sent from paytm as to prevent forged requests. We then return the page with the details of the transaction and the message.

Step 7: Creating the routes.

As the final step, Let's make the URLs for the views and get this long tutorial over. Create/open urls.py in the payments app. Add the following routes.

from django.urls import path
from .views import initiate_payment, callback

urlpatterns = [
    path('pay/', initiate_payment, name='pay'),
    path('callback/', callback, name='callback'),
]

Now let's include these URLs in the pay2me/urls.py

from django.urls import include
urlpatterns = [
    # include 
    path('', include('payments.urls'))
]

Test Run: Try out a Payment

Now let's run python manage.py runserver and go to http://localhost:8000/pay/ to create a payment.

Fill the form.

This will redirect to the paytm payment gateway. Use any dummy payment methods.

You can also choose the status (Success/Failure) of the Transaction

Then you will finally land on the callback page which will show you the status of the transaction.

Github: Sample

Here is the link to the above sample code on Github.