Dart on the server with Dart Frog

A fast, minimalistic backend framework for Dart 🎯

May 24, 2022
May 24, 2022
updated on
May 24, 2023
Guest Contributor

NOTE: When this blog post was published, Dart Frog was still experimental. We have since released a stable version of Dart Frog and removed the experimental label. Please see the documentation for the most up-to-date information.

We're very excited to announce that we've open sourced Dart Frog, an experimental, minimalistic backend framework for Dart 🎉

What is Dart Frog?

Our goal is to help developers effectively build backends in Dart. Dart Frog is intended to help Flutter and Dart developers maximize their productivity by having a unified tech stack that enables sharing tooling, models, and more!

Dart Frog provides a simple core with a small API surface area in order to reduce the learning curve and ramp-up time for developers. It optimizes the process of building backends which aggregate, compose, and normalize data from multiple sources.

Dart Frog is built on top of shelf and mason and is inspired by many tools including remix.run, next.js, and express.js.

Getting Started 🚀

The easiest way to get started is by installing the dart_frog_cli via pub.dev.

Installation 💻

# Install from pub.dev
dart pub global activate dart_frog_cli

At this point, dart_frog should be available as a command. You can verify by running dart_frog in your terminal.

A fast, minimalistic backend framework for Dart.

Usage: dart_frog <command> [arguments]

Global options:
-h, --help       Print this usage information.
    --version    Print the current version.

Available commands:
  build    Create a production build.
  create   Creates a new Dart Frog app.
  dev      Run a local development server.

Run "dart_frog help <command>" for more information about a command.

Create a Project 📦

Use the dart_frog create command to generate a brand new project.

# 🚀 Create a new project called "my_project"
dart_frog create my_project

Start the Dev Server 🏁

Next, we can open the newly created project and start the dev server via:

# 🏁 Start the dev server
dart_frog dev

This will start the server on localhost:8080 with hot-reload enabled ⚡️

Creating a new Route 🚏

In Dart Frog, a route consists of an onRequest function (called a route handler) exported from a dart file in the routes directory. Each endpoint is associated with a routes file based on its filename. Files named, index.dart will correspond to a / endpoint.

For example, if you create routes/hello.dart that exports an onRequest method like below, it will be accessible via the /hello endpoint.

import 'package:dart_frog/dart_frog.dart';

Response onRequest(RequestContext context) {
  return Response(body: 'Hello World');

In addition, all route handlers have access to a RequestContext which can be used to access the incoming request as well as dependencies provided to the request context (see middleware below).

import 'package:dart_frog/dart_frog.dart';

Response onRequest(RequestContext context) {
  // Access the incoming request.
  final request = context.request;

  // Return a response.
  return Response(body: 'Hello World');

We can customize the status code of the response via the statusCode parameter on the Response object:

import 'package:dart_frog/dart_frog.dart';

Response onRequest(RequestContext context) {
  return Response(statusCode: 204);

In addition, we can return JSON via the Response.json constructor:

import 'package:dart_frog/dart_frog.dart';

Response onRequest(RequestContext context) {
  return Response.json(
    body: <String, dynamic>{'hello': 'world!'},

Route handlers can be synchronous or asynchronous. To convert the above route handlers to async, we just need to update the return type from Response to Future<Response>. We can also add the async keyword in order to await futures within our handler before returning a Response.

import 'package:dart_frog/dart_frog.dart';

Future<Response> onRequest(RequestContext context) async {
  final result = await _someFuture();
  return Response.json(body: result);

Dynamic Routes 🌓

Dart Frog supports dynamic routes. For example, if you create a file called routes/posts/[id].dart, then it will be accessible at endpoints: /posts/1, /posts/2, etc.

Routing parameters are forwarded to the onRequest method as seen below.

import 'package:dart_frog/dart_frog.dart';

Response onRequest(RequestContext context, String id) {
  return Response(body: 'post id: $id');

Middleware 🍔

Middleware in Dart Frog allows you to execute code before and after a request is processed. You can modify the inbound request and outbound responses, provide dependencies, and more! This can be useful for validating authorization, adding common headers, logging, etc.

In Dart Frog, a piece of middleware consists of a middleware function exported from a _middleware.dart file within a subdirectory of the routes folder. There can only ever be one piece of middleware per route directory with routes/_middleware.dart being middleware that is executed for all inbound requests.

import 'package:dart_frog/dart_frog.dart';

Handler middleware(Handler handler) {
  return (context) async {
    // Execute code before request is handled.

    // Forward the request to the respective handler.
    final response = await handler(context);

    // Execute code after request is handled.

    // Return a response.
    return response;

We can chain built-in middleware, such as the requestLogger middleware via the use API. For example, if we create routes/_middleware.dart with the following contents, we will automatically log all requests to our server.

import 'package:dart_frog/dart_frog.dart';

Handler middleware(Handler handler) {
  return handler.use(requestLogger());

Dependency Injection 💉

Middleware can also be used to provide dependencies to a RequestContext via a provider.

provider is a type of middleware that can create and provide an instance of type T to the request context. The create callback is called lazily and the injected RequestContext can be used to perform additional lookups to access values provided upstream.

In the following example, we'll use a provider to inject a String into our request context.

import 'package:dart_frog/dart_frog.dart';

Handler middleware(Handler handler) {
  return handler
      .use(provider<String>((context) => 'Welcome to Dart Frog!'));

We can later access the provided via from within a route handler using context.read<T>():

import 'package:dart_frog/dart_frog.dart';

Response onRequest(RequestContext context) {
  final greeting = context.read<String>();
  return Response(body: greeting);

Testing 🧪

In Dart Frog, we can unit test our route handlers and middleware effectively because they are plain functions.

For example, we can test our route handler above using package:test:

import 'dart:io';

import 'package:dart_frog/dart_frog.dart';
import 'package:mocktail/mocktail.dart';
import 'package:test/test.dart';

import '../../routes/index.dart' as route;

class _MockRequestContext extends Mock implements RequestContext {}

void main() {
  group('GET /', () {
    test('responds with a 200 and greeting.', () async {
      const greeting = 'Hello World!';
      final context = _MockRequestContext();
      when(() => context.read<String>()).thenReturn(greeting);
      final response = route.onRequest(context);
      expect(response.statusCode, equals(HttpStatus.ok));
      expect(response.body(), completion(equals(greeting)));

In the above test, we're using package:mocktail to create a mock RequestContext and stub the return value when calling context.read<String>(). Then, all we need to do is call onRequest with the mocked context and we can assert that the response is what we expect. In this case, we're checking the statusCode and response body to ensure that the response is a 200 with the provided greeting.

What's Next

Check out the Dart Frog Roadmap to see where we're headed.

If you're as excited about Dart Frog as we are, please let us know! We'd love to hear any feedback from the community to help shape our direction and look forward to seeing Dart becoming more prevalent on the backend ☁️

If you're excited about Dart Frog you can show your support by ⭐️ the repository.

This article has been updated on May 25, 2022 to support dart_frog_cli v0.0.2-dev.

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