Binding Handler for Shelf

Build Status Pub Version


Provides shelf middleware that lets you use ordinary Dart functions as Shelf Handlers.

shelf_bind frees you to:

  • use your own functions without worrying about the shelf boilerplate
  • focus on writing the business logic with your own classes and let shelf_bind deal with fitting it in to shelf

shelf_bind favours convention over configuration so that you can write the minimal code necessary but still be able to override defaults as needed.

shelf_bind is a powerful binding framework that supports:

  • binding to simple types * including type conversion

  • binding to your own domain objects via property setters via constructors

  • binding from request path, query, body and header fields
  • injecting your own custom arguments like http clients
  • seamless integration with shelf_route (and bundled with mojito and shelf_rest)
  • automatic parameter validation with constrain
  • automatic conversion between snake_case and camelCase for query params and between kebab-case and camelCase for headers
  • sensible defaults that mean that no annotations are needed for the majority of cases but annotations are there for when you need.

It can be used as a standalone shelf component or as part of framework that integrates it with other components.

The simplest way to use it with shelf_route is to use mojito or shelf_rest, as their routers have already wired in shelf_bind.

If you are starting out then I recommend looking at mojito first and use this README as more detailed information about handler binding.

Using Standalone

You can skip this standalone usage section if you are using shelf_bind with mojito or shelf_rest.

The bind function creates a shelf Handler from a normal dart function.

var handler = bind(() => "Hello World");

This creates a shelf Handler equivalent of

var handler = (Request request) => new Response.ok("Hello World");

If the function returns a Future this will be mapped to a Future<Response>

bind(() => new Future.value("Hello World"))

Now you can set up a shelf-io server to bring your much needed greeting the world (awthanks)

io.serve(bind(() => "Hello World"), 'localhost', 8080);

Path Parameters

Any simple type parameters you add to your function will match to path parameters of the same name.

Names will automatically be converted between snake_case and camelCase

(String name) => "Hello $name"

shelf_bind supports binding to any path parameters including:

  • path segments like /greeting/fred
  • query parameters like /greeting?name=fred

It accesses the path parameters using shelf_path which means it will work with any middleware (such as shelf_route) that uses shelf_path to store path parameters in the Request context property.

This also means it is not tied to any particular format for representing paths. For example it doesn't matter if the paths are defined like /greeting/:name or /greeting/{name} or /person{?name} or whatever.

Simple Types

You can also bind to simple types like int

(String name, int age) => "Hello $name of age $age"


  • num
  • int
  • double
  • bool
  • DateTime
  • Uri

Please file a feature request (or pull request) if you want a new type supported

Optional Named Parameters

You can use optional named parameters with defaults too.

(String name, {int age: 20}) => "Hello $name of age $age"

If a named parameter is not provided (or is null) in the context, then the default value will be used.

Binding multiple path parameters into your Classes

You can bind multiple path parameters into your own classes. This is described in the advanced section.

Request Body

Handler parameters that are not simple types are by default assumed to come from the body.

This includes:

  • Map
  • List
  • any of your classes (that are not registered as custom objects).

For example, the handler parameters in the following will all be assumed to come from the request body.

(Map myMap) => ...

(List myList) => ...

(Person myMap) => ...

shelf_bind currently supports both JSON and FORM encoded bodies.

By default, shelf_bind attempts to determine the encoding the request content-type as follows:

  • if none is present the body is assumed to be JSON
  • if the content-type is set and is FORM or JSON then it will be handled as that type
  • if it is any other content-type then a 400 response will be returned

You can override this behaviour by using the @RequestBody annotation. If the @RequestBody annotation is present then the content will be treated as the type provided in the annotation.

For example the following will be treated as FORM encoded regardless of the request content-type

(@RequestBody(format: ContentType.FORM) Map myMap) => ...

Shelf Request Object

You can access the shelf Request object simply by adding that as a parameter to your function.

Note: since you can access all the parts of the request directly, including headers, you rarely need to do this.

(String name, Request request) => "Hello $name ${request.method}"


Response Body

By default the return value of your function is encoded as JSON by calling JSON.encode.

So for example you can return a map

() => { "greeting" : "Hello World" }

This will work for anything that can be encoded as JSON including any of your custom classes

class SayHello {
  String greeting;

  Map toJson() => { 'greeting': greeting };

SayHello myGreeter() => new SayHello()..greeting = "Hello World"

Response Status

You can override the default status code as described in the section on Annotations.

Shelf Response

If you want full control over the response you can simply return a Shelf Reponse directly

() => new Response.ok("Hello World")

Error Response

shelf_bind doesn't do any specific formatting for errors. Instead it leaves it to upstream middleware to handle, such as shelf_exception_handler.

This allows all your error handling to be kept in one location.

import 'package:http_exception/http_exception.dart';

() => throw new BadRequestException()

Sprinkling in some shelf_exception_handler middleware

var handler = const Pipeline()
    .addHandler(bind(() => throw new BadRequestException()));

we get a handler that will return a 400 response.

Tweaking with Annotations

Path Parameters

To tweak how the binding from the request path parameters is performed, use the @PathParam annotation.

You can change the default mapping for the path name. For example if you have a handler parameter called argOne, by default this would map to a request path parameter called arg_one

If you instead want this to map to arg1 you can specify that as follows

(@PathParam(pathName: 'arg1') String argOne) => ...

Request Body

To tweak how the binding from the request body is performed, use the @RequestBody annotation.

Note, only one handler parameter can be mapped to the body.


To force the body to always be interpreted as JSON set the format as follows

bind(@RequestBody(format: ContentType.JSON) Person person) => "Hello ${}")


bind(@RequestBody(format: ContentType.FORM) Person person) => "Hello ${}")

Response Headers

You can override the default status (200) that is set on a successful return of the handler method using the ResponseHeaders annotation. You can also have the location header set to the incoming request url.

String _create(String name) => "Hello $name";

final handler = bind(_create);

You can set the status to anything you like

@ResponseHeaders(successStatus: 204)
String _whatever(String name) => "Hello $name";

When setting the location field on a POST, the primary key field on the return object is used for the last segment of the path.

By default the primary key field is id, but this can be overridden by specifying the idField parameter.

@ResponseHeaders.created(idField: #name)
Person _create(@RequestBody() Person person) => person;

The name field is now used for the last segment. For example if a POST is made to http://localhost/person and the name is fred, the location will be set as

location: http://localhost/person/fred

Using With Shelf Route

One of the main uses of shelf_bind is with a router like shelf_route.

The easiest way to do that is simply to use mojito or shelf_rest as they provide this out of the box

As bind returns a Handler you can simply pass that handler into the shelf_route's Router methods

var myRouter = router()
  ..get('/', bind(() => "Hello World"));

Couldn't be much easier. However, having to wrap all your handler's in the bind adds a bit of noise. To avoid that we can install a HandlerAdapter into the router first. shelf_bind provides one out of the box.

var myRouter = router(handlerAdapter: handlerAdapter())
  ..get('/', () => "Hello World");


The following is shows all the example handlers from above using shelf_route as the router

import 'package:shelf/shelf.dart' as shelf;
import 'package:shelf/shelf_io.dart' as io;
import 'package:shelf_route/shelf_route.dart' as route;
import 'package:shelf_bind/shelf_bind.dart';
import 'package:http_exception/http_exception.dart';
import 'package:shelf_exception_handler/shelf_exception_handler.dart';
import 'dart:async';

void main() {
  var router = route.router(handlerAdapter: handlerAdapter())
      ..get('/', () => "Hello World")
      ..get('/later', () => new Future.value("Hello World"))
      ..get('/map', () => {"greeting": "Hello World"})
      ..get('/object', () => new SayHello()..greeting = "Hello World")
      ..get('/ohnoes', () => throw new BadRequestException())
      ..get('/response', () => new Response.ok("Hello World"))
      ..get('/greeting/{name}', (String name) => "Hello $name")
          (String name, int age) => "Hello $name of age $age")
      ..get('/greeting3/{name}', (Person person) => "Hello ${}")
          (String name, Request request) => "Hello $name ${request.method}")'/greeting6', (Person person) => "Hello ${}")
          (@PathParams() Person person) => "Hello ${}");

  var handler = const shelf.Pipeline()


  io.serve(handler, 'localhost', 8080).then((server) {
    print('Serving at http://${}:${server.port}');

class SayHello {
  String greeting;

  Map toJson() => { 'greeting': greeting };

class Person {
  final String name;{});

  Person.fromJson(Map json) : = json['name'];

  Map toJson() => { 'name': name };

See more detailed example in the project at example/binding_example.dart

Advanced Usage

Binding multiple path parameters into your Classes

You can bind path variables to properties of your classes by using the @PathParams annotation.

class Person {
  String name;

bind((@PathParams() Person person) => "Hello ${}")

If you prefer immutable classes then you can bind to a constructor

class Person {
  final String name;{});

The constructor must use named arguments for all the properties and the names must match the request path parameter names.

By default the constructor must be called build. This will be overridable with annotations in the future.


shelf_bind integrates with the powerful Constrain package to support automatic validation of your handler function parameters.

Enable validation via the validateParameters property to the bind function

bind((Person person) => "Hello ${}", validateParameters: true)

Or when using with Shelf Route you can set it on handlerAdapter to apply to all routes (see section on Shelf Route integration below)

handlerAdapter: handlerAdapter(validateParameters: true)

Now lets spice up the Person class with a few (contrived) constraints.

class Person {
  @Ensure(nameIsAtLeast3Chars, description: 'name must be at least 3 characters')
  final String name;

  @Ensure(allStreetsStartWith15, description: "All streets must start with 15")
  List<Address> addresses;{});

  Person.fromJson(Map json) : = json['name'],
    this.addresses = _addressesFromJson(json['addresses']);

  static List<Address> _addressesFromJson(json) {
    if (json == null || json is! List) {
      return null;

    return => new Address.fromJson(a)).toList(growable: false);

  Map toJson() => { 'name': name, 'addresses':  addresses };

  String toString() => 'Person[name: $name]';

class Address {
  String street;

  Address.fromJson(Map json) : this.street = json['street'];

  Map toJson() => { 'street': street };

  String toString() => 'Address[street: $street]';

// The constraint functions

Matcher nameIsAtLeast3Chars() => hasLength(greaterThan(3));

bool allStreetsStartWith15(List<Address> addresses) =>
  addresses.every((a) => a.street == null || a.street.startsWith("15"));

Matcher streetIsAtLeast10Characters() => hasLength(greaterThanOrEqualTo(10));

Now whenever the handler is invoked, the Person object will be validated before it is passed to your Dart function. If it fails validation a BadRequestException (from the http_exception package) will be thrown containing the detailed constraint violations.

If you've configured shelf_exception_handler correctly you will get responses like

HTTP/1.1 400 Bad Request
content-type: application/json

    "errors": [
            "constraint": {
                "description": "all streets must start with 15",
                "group": "DefaultGroup",
                "type": "Ensure"
            "details": null,
            "invalidValue": {
                "type": "List",
                "value": [
                    "Address[street: blah blah st]"
            "leafObject": {
                "type": "Person",
                "value": "Person[name: fred]"
            "message": "Constraint violated at path addresses\nall streets must start with 15\n",
            "propertyPath": "addresses",
            "reason": null,
            "rootObject": {
                "type": "Person",
                "value": "Person[name: fred]"
    "message": "Bad Request",
    "status": 400

Response Validation

Similar to handler function parameter validation, you can enable validation of your responses using the constrain package. This is to ensure you never send out invalid data.

Enable response validation via the validateReturn property to the bind function

(String name) => new Person(name)

A HttpException (from http_exception package) with a 500 status will be thrown if validation fails as this means you have messed up your code ;-).

See the Validation part of the Path Parameters section for more detailed explanation of validation.

Injecting Custom Parameters

In addition to the normal request related data like path parameters, body and headers, shelf_bind also supports injecting arbitrary objects to handler functions. These are referred to as Custom Objects.

Typically these objects are instantiated from data relating to the request but this is not a requirement.

A common usage is to inject clients to remote services such HTTP clients and database clients. These services may need to be invoked as the authenticated user.

Use the customObjects parameter to handlerAdapter or bind to inject your own factories for these objects

bind((String name, PersonLookupClient client) => client.lookup(name),
    customObjects: customObjects);
var adapter = handlerAdapter(customObjects: customObjects);

The customObjects parameter is a just a map from the type to the factory. The factory takes a Request argument.

var customObjects = {
    PersonLookupClient: (req) => new Future.value(new PersonLookupClient())

class PersonLookupClient {
  Future<Person> lookup(String name) =>
      new Future.value(new name));

Factories may return Future's in which case the future will be resolved before passing the resolved object to the handler method.

Packages like mojito and shelf_rest inject their own custom objects

More Information

See the wiki for more details on all the options


See open issues.


Contributions are welcome. Please:

  1. fork the repo and implement your changes with good unit test coverage of your changes
  2. create a pull request and include enough detail in the description