Getting Started

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Getting Started

In this first tutorial, we will:

Download the angel3_framework package from pub.dev.
Launch Angel3 as backend server.
Add some basic routing.
Add a 404 error handler.

The source code for this example can be found at:

Prerequisite

This tutorial uses command line interface. If you are not well-versed working with it, just copy/paste the code snippets while going through this tutorial.
The Dart SDK version 2.16 or later is required. Please follow the instructions on the to download and install it.
The tool will be used to send requests to the backend server.
Note that some steps will mention Unix-specific programs, like vim. Windows users should instead use Notepad++. Alternative programs will be mentioned where relevant.

Project Setup

The first thing we'll need to do is create a new directory (folder) for our project.

mkdir hello_angel
cd hello_angel

Next, we create a pubspec.yaml file, and enter the following contents:

name: hello_angel
dependencies:
    angel3_framework: ^6.0.0

Now, just run dart pub get, which will install the angel3_framework library, and its related dependencies:

Resolving dependencies... (3.3s)
+ angel3_container 6.0.0
+ angel3_framework 6.0.0
+ angel3_http_exception 6.0.0
(... more output omitted)
Changed 33 dependencies!

Launching the Backend Server

Angel3 can speak different protocols, but more often than not, we'll want it to speak HTTP.

Create a directory named bin, and a file within bin named main.dart.

Your folder structure should now look like this:

hello_angel
    bin/
        main.dart
    pubspec.yaml

Add the following to bin/main.dart:

import 'package:angel3_framework/angel3_framework.dart';
import 'package:angel3_framework/http.dart';

void main() async {
    var app = Angel();
    var http = AngelHttp(app);
    await http.startServer('localhost', 3000);
}

Next, in your terminal, run the command dart bin/main.dart. Your server will now be running, and will listen for input until you kill it by entering Control-C (the SIGINT signal) into the terminal.

Open a new terminal window, and type the following:

curl localhost:3000 && echo

You'll just see a blank line, but the fact that you didn't see an error means that the server is indeed listening at port 3000.

Adding a Route

By adding routes to our server, we can respond to requests sent to different URL's.

Let's a handler at the root of our server, and print a simple Hello, world! message.

From this point, all new code needs to be added before the call to http.startServer (or else it will never run).

Add this code to your program:

app.get('/', (req, res) => res.write('Hello, world!'));

bin/main.dart should now look like the following:

import 'package:angel3_framework/angel3_framework.dart';
import 'package:angel3_framework/http.dart';

main() async {
    var app = Angel();
    var http = AngelHttp(app);
    app.get('/', (req, res) => res.write('Hello, world!'));
    await http.startServer('localhost', 3000);
}

(Note that this is the last time the entire file will be pasted, for the sake of brevity.)

Now, if you rerun curl localhost:3000 && echo, you'll see the message Hello, world! printed to your terminal!

Route Handlers

Let's break down the line we just added:

app.get('/', (req, res) => res.write('Hello, world!'));

It consists of the following components:

A call to app.get
A string, '/',
A closure, taking two parameters: req and res
The call res.write('Hello, world!'), which is also the return value of the aforementioned closure.

Combined with the path, '/', this signifies that whenever a request is sent to the root of our server, which in this case is the URL http://localhost:3000, the attached closure should be invoked.

The path is important because it defines the conditions under which code should run. For example, if we were to visit http://localhost:3000/foo, we'd just see a blank line printed again, because there is no route mounted corresponding to the path '/foo'.

The two parameters, req and res, hold the types RequestContext and ResponseContext, respectively. We'll briefly cover these in the next section.

Finally, we call res.write, which, as you may have surmised, prints a value to the outgoing HTTP response. That's how we are able to print Hello, world!.

Printing Headers

Just as their names suggest, the RequestContext and ResponseContext classes are abstractions used to read and write data on the Web.

app.get('/headers', (req, res) {
    req.headers.forEach((key, values) {
        res.write('$key=$values');
        res.writeln();
    });
});

Run the following:

curl -H 'X-Foo: bar' -H 'Accept-Language: en-US' \
http://localhost:3000/headers && echo

And you'll see output like the following:

user-agent=[curl/7.54.0]
accept=[*/*]
accept-language=[en-US]
x-foo=[bar]
host=[localhost:3000]

Reading Request Bodies

Web applications very often have users send data upstream, where it is then handled by the server.

Angel3 has built-in functionality for parsing bodies of three MIME types:

application/json
application/x-www-form-urlencoded
multipart/form-data

(You can also handle others, but that's beyond the scope of this demo.)

So, as long as the user sends data in one of the above forms, we can handle it in the same way.

Add the following route.It will listen on the path '/greet' for a POST request, and then attempt to parse the incoming reques t body.

Afterwards, it reads the name value from the body, and computes a greeting string.

app.post('/greet', (req, res) async {
    await req.parseBody();

    var name = req.bodyAsMap['name'] as String?;

    if (name == null) {
        throw AngelHttpException.badRequest(message: 'Missing name.');
    } else {
        res.write('Hello, $name!');
    }
});

To visit this, enter the following curl command:

curl -X POST -d 'name=Bob' localhost:3000/greet && echo

You should see Hello, Bob! appear in your terminal.

Adding an Error Handler

In the previous example, you might have noticed this line:

throw AngelHttpException.badRequest(message: 'Missing name.');

Angel3 handles errors thrown while calling route handlers, preventing your server from crashing. Ultimately, all errors are wrapped in the AngelHttpException class, or sent as-is if they are already instances of AngelHttpException.

By default, either an HTML page is printed, or a JSON message is displayed (depending on the client's Accept header). In many cases, however, you might want to do something else, i.e. rendering an error page, or logging errors through a service like Sentry.

To add your own logic, set the errorHandler of your Angel instance. It is a function that accepts 3 parameters:

AngelHttpException
RequestContext
ResponseContext

var oldErrorHandler = app.errorHandler;

app.errorHandler = (e, req, res) {
if (e.statusCode == 400) {
    res.write('Oops! You forgot to include your name.');
} else {
    return oldErrorHandler(e, req, res);
}

Note that we kept a reference to the previous error handler, so that existing logic can be reused if the case we wrote for is not handled.

To trigger a 400 Bad Request and see our error handler in action, run the following:

curl -H 'Content-Type: application/x-www-form-urlencoded' \
-X POST localhost:3000/greet && echo

You will now see 'Oops! You forgot to include your name.' printed to the console.

Conclusion

Congratulations on creating your first Angel3 backend server! Hopefully this is just one of many more to come.

The choice is now yours: either continue reading the other guides posted on this site, or tinker around and learn the ropes yourself.