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Jermaine
Jermaine

Posted on • Edited on • Originally published at creativebracket.com

Build a chat application in Dart 2 (Part 1)

#dart #http #beginners #showdev

Real-time experiences rule the web today. When done correctly, this results in responsive and fluid interfaces that contribute to a good user experience.

In this three-part series, we will implement real-time functionality by building the obligatory chat app. We will be working with various libraries that come with the Dart SDK to achieve this and the Bulma CSS Framework for the UI styles.

At the end of this part, we will have the first UI view of our chat app fleshed out. Any user that visits the app will have to enter their name in a sign in view before proceeding to the chatroom.

The diagram below details the full flow:

Diagram of chat flow

The chat flow goes like this:

  1. Visitor enters their name and clicks the Join chat button. This sends the username via POST request to our backend.
  2. The backend receives and checks for the username, returning a 200 OK response if valid, else a 400 Bad Request is returned.
  3. On 200 OK the screen will be swapped to the Chatroom.
  4. From here will be a WebSocket connection listening for messages sent from the input field and broadcasted to other users on the chat.
  5. Clicking the Leave chat button will close the WebSocket connection and return to the username sign in screen.

So let's begin shall we?

1. Set up your project

Install the Stagehand package and create a web project:

$ pub global activate stagehand
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And create a web project:

$ mkdir chat_app && cd chat_app
$ stagehand web-simple # scaffold a web project
$ pub get # install dependencies in `pubspec.yaml` file
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Install the webdev tool to spin up our server for local development:

$ pub global activate webdev
$ webdev serve --live-reload # live reload is supported in Chrome
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Visiting http://localhost:8080 will show you this screen:

Dart web on localhost

2. Mark up our HTML page

We will use the UI style classes from Bulma to construct our screens.

In web/index.html, add the link tags below in the <head> before <link rel="stylesheet" href="styles.css"> to import the latest minified styles and the Font Awesome icon font dependency:

<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/bulma/0.7.1/css/bulma.min.css" integrity="sha256-zIG416V1ynj3Wgju/scU80KAEWOsO5rRLfVyRDuOv7Q=" crossorigin="anonymous" />
<link rel="stylesheet" href="https://use.fontawesome.com/releases/v5.3.1/css/all.css" integrity="sha384-mzrmE5qonljUremFsqc01SB46JvROS7bZs3IO2EmfFsd15uHvIt+Y8vEf7N7fWAU" crossorigin="anonymous" />
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In web/styles.css, replace the file contents with the below:

html,
body {
  background-color: #f5f5f5;
}
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In web/index.html, replace <div id="output"></div> with the markup for our screens:

<section class="section">
  <div class="container">

    <div id="ChatSignin">
      <h1 class="title">Chatter 🎯</h1>
      <div class="columns">
        <div class="column is-6">
          <div class="field">
            <label class="label">Please enter your name</label>
            <div class="control is-expanded has-icons-left">
              <input class="input is-medium" type="text" placeholder="Enter your name and hit ENTER" />
              <span class="icon is-medium is-left">
                <i class="fas fa-user"></i>
              </span>
            </div>
            <p class="help is-danger"></p>
          </div>
          <div class="field">
            <div class="control">
              <button class="button is-medium is-primary">
                Join chat
              </button>
            </div>
          </div>
        </div>
      </div>
    </div>

    <div id="ChatRoom" hidden>
      <h1 class="title">Success!</h1>
    </div>

  </div>
</section>
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The markup contains the views for the sign in and the chatroom screens, seen in the <div id="ChatSignin">...</div> and <div id="ChatRoom" hidden>...</div>. The #ChatRoom has a hidden attribute which hides it till we remove this later when the username is validated.

To prevent errors, remove the implementation from the main() function inside web/main.dart:

import 'dart:html';

void main() {
  // delete the line below
  // querySelector('#output').text = 'Your Dart app is running.';
}
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We should now see the output below:

Chatter sign in UI

3. Implement browser logic for sign-in flow

Let's add the logic to receive our username and send to the backend.

In web/main.dart, we will start with the content below:

import 'dart:html';

void main() {
  // Selectors
  var chatSigninBox = querySelector('#ChatSignin');
  var chatRoomBox = querySelector('#ChatRoom');
  var validationBox = chatSigninBox.querySelector('p.help');
  InputElement nameField = chatSigninBox.querySelector('input[type="text"]');
  ButtonElement submitBtn = chatSigninBox.querySelector('button');

  // Event listeners
  nameField.addEventListener('input', (evt) {
    // TODO: Run field validation
  });

  submitBtn.addEventListener('click', (evt) async { // using async/await ;)
    // TODO: Run name field validation
    // TODO: Submit name field to backend
    // TODO: Handle success response
    // TODO: Handle failure responses
  });
}
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The snippet begins with our dart:html import, which allows access to the window and document objects, including their methods.

In the above snippet, we are using the querySelector() top-level function to select our DOM elements and listen for various events on them.

An advice I'd give on defining selectors...

Specify the type whenever possible! In other words, instead of using the var keyword, replace with the type of object that is expected, so:

InputField nameField = chatSigninBox.querySelector('input[type="text"]');
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This is because the querySelector() method returns an object of type Element, which means that specific properties on particular DOM elements are not visible to the Dart analyser(that's expected), like value on nameField as it won't exist under Element. Having InputElement as a subtype allows the analyser to find the value property and prevents an error from being thrown when you try to access value under Element:

// Bad
var nameField = chatSigninBox.querySelector('input[type="text"]');
print(nameField.value); // Analyser will throw an error

// Good
InputElement nameField = chatSigninBox.querySelector('input[type="text"]');
print(nameField.value); // Analyser sees value prop in `InputElement`
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Great! Moving on...

Let's implement the event listener for the username field:

nameField.addEventListener('input', (evt) {
  if (nameField.value.trim().isNotEmpty) {
    nameField.classes
      ..removeWhere((className) => className == 'is-danger')
      ..add('is-success');
    validationBox.text = '';
  } else {
    nameField.classes
      ..removeWhere((className) => className == 'is-success')
      ..add('is-danger');
  }
});
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This will validate the field and add the appropriate classes for success and failure states. These class names(is-success and is-danger) are provided by Bulma.

Let's implement the event listener for the submit button:

submitBtn.addEventListener('click', (evt) async {
  // 1. Validate name field
  if (nameField.value.trim().isEmpty) {
    nameField.classes.add('is-danger');
    validationBox.text = 'Please enter your name';
    return;
  }

  submitBtn.disabled = true;

  try {
    // 2. Submit name to backend via POST
    var response = await HttpRequest.postFormData(
      'http://localhost:9780/signin', // TODO: Endpoint to be created in next step
      {
        'username': nameField.value,
      },
    );

    // 3. Handle success response and switch view
    chatSigninBox.hidden = true;
    chatRoomBox.hidden = false;
  } catch (e) {
    // 4. Handle failure response
    submitBtn
      ..disabled = false
      ..text = 'Failed to join chat. Try again?';
  }
});
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Here's what's happening once the Join chat button is clicked:

  1. We check the username field for a valid entry. If not valid, we display an error message and stop executing any further. If valid, then we move to step 2.
  2. We send the form data to the endpoint at http://localhost:9780/signin
  3. On successful response, we will hide the chat sign in UI and reveal the chatroom UI
  4. If the backend returns an error, we will replace the text of the Join chat button, and encourage the user to try again

Here's what you should now have:

Chat signin form

Submitting the username displays the failure message, since we have not implemented the backend yet.

4. Implement logic for the backend

We now need the backend to receive our request containing the username.

Create a bin/server.dart file and create a server:

import 'dart:io';
import 'dart:convert';

main() async {
  // TODO: Get port from environment variable
  var port = 9780;
  var server = await HttpServer.bind(InternetAddress.loopbackIPv4, port);

  print('Server listening on port $port');

  await for (HttpRequest request in server) {
    // TODO: Handle requests
  }
}
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Learn more on writing HTTP servers and handling POST requests in Dart.

Replace the // TODO: Handle requests line with the below:

// TODO: Only needed in local development. Will be removed in future
request.response.headers.add('Access-Control-Allow-Origin', 'http://localhost:8080');

switch (request.uri.path) {
  case '/signin':
    String payload = await request.transform(Utf8Decoder()).join();
    var username = Uri.splitQueryString(payload)['username'];

    if (username != null && username.isNotEmpty) {
      // TODO: Check username is unique
      request.response
        ..write(username)
        ..close();
    } else {
      request.response
        ..statusCode = 400
        ..write('Please provide a valid user name')
        ..close();
    }
    break;
  case '/ws':
    // TODO: Upgrade request to Websocket connection
    break;
  default:
    // TODO: Forward to static file server
}
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The snippet begins with a CORS header allowing connections from our webdev server. This will only be needed for local development. When building for production, we will refactor this line.

Then we have a switch statement to execute the appropriate logic based on the path of the request. The relevant area right now is the case for /signin. It validates the username and sends the right response.

Run this file with the command below:

$ dart bin/server.dart
  Server listening on port 9780 # You should see this message
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Return to http://localhost:8080 and try again:

Successful username sign in

Conclusion

And this concludes Part 1 of the series. As always, I hope this was insightful and you learnt something new today. And the working solution is here.

One thing before your go...Subscribe to my Youtube channel for the latest videos on Dart. Thanks!

Like, share and follow me 😍 for more content on Dart.

Continue reading:

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Further reading

  1. dart:html library
  2. How to use JavaScript libraries in your Dart applications
  3. Free Dart screencasts on Egghead.io

Top comments (6)

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ratulotron profile image
Ratul Minhaz •

Hi Jermaine, I faced a small problem while re-doing your tutorial. Even though this worked the first time, now the code in the server.dart file is giving weird result. Apparently the payload in the request is being converted wrongly:

    String data = await request.transform(Utf8Decoder()).join();
    var data = Uri.splitQueryString(payload);

    /* Prints this:
        {{
            "username": "Ratul"
        }: }
    */

As you can see, Uri.splitQueryString is making the whole payload the key of the map it returns. I did fix the problem by using json.decode() method, but can you kindly explain what is happening here?

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creativ_bracket profile image
Jermaine •

Hey Ratul, the snippet you're providing looks different. You have String data instead of String payload. What does your code for the section in question look like?

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ratulotron profile image
Ratul Minhaz •

I think I misused a function down the line and this happened. I reverted everything back and rewrote the signin handler and it fixed the issue. I was trying out posting JSON data instead of form data and may have used a wrong method.
Oh and I just renamed payload to data for my convenience.

Thanks for the reply :D Eagerly waiting for the third part of the series!

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ratulotron profile image
Ratul Minhaz •

This post made me interested in Dart! Thanks for writing it! I am eagerly waiting for the second part.

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creativ_bracket profile image
Jermaine •

Thanks Rahul.

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creativ_bracket profile image
Jermaine •

Part 2 is now published.

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