Creative Bracket

How to handle the POST request body without a framework

Following the positive response from a similar article based on Node, I found it fitting that I demonstrate how you can handle the payload and parameters sent as part of a POST request made to a Dart server backend. Long story short, I found it’s simpler.

This example works where the Content-Type of the request payload is application/x-www-form-urlencoded, which essentially means that the form data is formatted as a query string when sent to the server.

1. Create our server

Create a main.dart file and enter the snippet below:

import 'dart:io';

void main() async {
  var server = await HttpServer.bind('localhost', 9000);

  await for (HttpRequest req in server) {
    req.response
      ..headers.set('Content-Type', 'text/html')
      ..write('''
        <!doctype html>
        <html>
        <body>
          <form action="/" method="post">
            <input type="text" name="fname" /><br />
            <input type="number" name="age" /><br />
            <input type="file" name="photo" /><br />
            <button>Save</button>
          </form>
        </body>
        </html>
      ''')
      ..close();
  }
}

This bootstraps our server and responds with a form when a request is made to http://localhost:9000. The snippet begins by importing the dart:io library, since it contains the classes we’ll need to create our server. The whole bootstrapping process happens in a main() function, which is needed to run our Dart app.

To run this file, type the below command in the terminal:

dart main.dart

And you should be greeted with a form in the browser:

2. Capture the POSTed payload

Let’s now ensure that we are dealing with a POST request:

...
...
await for (HttpRequest req in server) {
  if (req.method == 'POST') {
   // deal with the payload  
  } else {
    req.response
      ..headers.set('Content-Type', 'text/html')
      ..write('''
        <!doctype html>
        <html>
        <body>
          <form action="/" method="post">
            <input type="text" name="fname" /><br />
            <input type="number" name="age" /><br />
            <input type="file" name="photo" /><br />
            <button>Save</button>
          </form>
        </body>
        </html>
      ''')
      ..close();
  }
}

Requests to the server are treated as a Stream, which means that we can use the request’s transform method to decode the content.

if (req.method == 'POST') {
  var content = await req.transform().join();
} ...

This won’t work straightaway because the transform method requires a StreamTransformer. A StreamTransformer contains a bind method that allows it to manipulate the streaming data somehow. We need one to transform our streaming request data into a readable format, and afterwards use the join method to combine our transformed chunks.

Beneath our dart:io import, let’s require the dart:convert library:

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

And use the Utf8Decoder as our transformer:

var content = await req.transform(Utf8Decoder()).join();

Printing out content will give you the below result, provided you filled in the form with the relevant details:

fname=John&age=30&photo=file.jpg

However, we still need to extract our key/value pairs of information from the query string. Fortunately, we have a Uri class in the core Dart SDK:

var content = await req.transform(Utf8Decoder()).join();
var queryParams = Uri(query: content).queryParameters;

3. Respond to the POST

Now let’s send back a response:

var content = await req.transform(Utf8Decoder()).join();
var queryParams = Uri(query: content).queryParameters;

req.response
  ..write('Parsed data belonging to ${queryParams['fname']}')
  ..close();

UPDATE 20/10/2018: Tobe Osakwe(Creator of Angel) helpfully pointed out the use of splitQueryString static method to extract the query parameters:

// var queryParams = Uri(query: content).queryParameters;
var queryParams = Uri.splitQueryString(content);

Our query params are returned as a Map object, allowing us to pull our values like this:

queryParams['fname'];
queryParams['age'];
queryParams['photo'];

Here’s the full solution:

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

void main() async {
  var server = await HttpServer.bind('127.0.0.1', 9000);

  await for (HttpRequest req in server) {
    if (req.method == 'POST' && req.headers.contentType.toString() == 'application/x-www-form-urlencoded') {
      var content = await req.transform(Utf8Decoder()).join();
      var queryParams = Uri(query: content).queryParameters;
      req.response
        ..write('Parsed data belonging to ${queryParams['fname']}')
        ..close();
    } else {
      req.response
        ..headers.set('Content-Type', 'text/html')
        ..write('''
          <!doctype html>
          <html>
          <body>
            <form action="/" method="post">
              <input type="text" name="fname" /><br />
              <input type="number" name="age" /><br />
              <input type="file" name="photo" /><br />
              <button>Save</button>
            </form>
          </body>
          </html>
        ''')
        ..close();
    }
  }
}

In closing…

Like the related article, this works for application/x-www-form-urlencoded content types and would require a different approach if parsing other content types. We’ll look at this in a future article.

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


Further reading

  1. Hello world server in Dart
  2. Creating Streams in Dart
  3. Free Dart Screencasts on Egghead.io

Jermaine Oppong

Hello 👋, I show programmers how to build full-stack web applications with the Dart SDK. I am passionate about teaching others, having received tremendous support on sites like dev.to and medium.com for my articles covering various aspects of the Dart language and ecosystem.