w_transport

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Platform-agnostic transport library for sending and receiving data over HTTP and WebSocket. HTTP support includes plain-text, JSON, form-data, and multipart data, as well as custom encoding. WebSocket support includes native WebSockets in the browser and the VM with the option to use SockJS in the browser.

Importing

import 'package:w_transport/w_transport.dart';

This main entry point depends on neither dart:html nor dart:io - it'splatform-independent!

With this, you have access to all of our transport classes necessary for sending HTTP requests and establishing WebSocket connections while remaining platform-independent. This means you can use the w_transport library to build components, libraries, or APIs that will be reusable in the browser and on the Dart VM.

The end consumer will make the decision between browser and VM, most likely in a main() block.

Platforms

Browser

import 'package:w_transport/w_transport_browser.dart'
    show configureWTransportForBrowser;

void main() {
  configureWTransportForBrowser();
}

Dart VM

import 'package:w_transport/w_transport_vm.dart'
    show configureWTransportForVM;

void main() {
  configureWTransportForVM();
}

Tests

import 'package:w_transport/w_transport_mock.dart'
    show configureWTransportForTest;

void main() {
  configureWTransportForTest();
}

HTTP

Static Request Methods

For one-off or simple requests, use the static methods on the Http class:

await Http.get(Uri.parse('/ping'));
await Http.post(Uri.parse('/tasks/2'), body: 'new task');

These standard HTTP methods are supported:

  • DELETE : Http.delete()
  • GET : Http.get()
  • HEAD : Http.head()
  • OPTIONS : Http.options()
  • PATCH : Http.patch()
  • POST : Http.post()
  • PUT : Http.put()

If you need to send a request with a non-standard HTTP method, use send():

await Http.send('COPY', Uri.parse('/tasks/4'));

Headers

Map headers = {
  'authorization': 'Bearer sometoken',
  'x-custom': 'value'
};
await Http.get(Uri.parse('/notes/'), headers: headers);

Plain-Text Body

await Http.post(Uri.parse('/notes/'), body: 'testing..');

Plain-text request bodies default to UTF8 encoding.

Request Classes

The above works well for simple requests, but what if you need to send JSON? Or use a different encoding? Send a multi-part request? Or maybe you just want more explicit control over the request. In these cases, you'll want to use one of the exposed request classes:

  • Request
  • FormRequest
  • JsonRequest
  • MultipartRequest
  • StreamedRequest

Every request class extends from BaseRequest.

Common Request API

Though each request type has some idiosyncrasies, they share a common underlying API. We'll cover this common API using Request as the example.

Creating a Request

Request request = new Request();

At this point, there's nothing special about this request, and you'd use it almost exactly like you'd use the top-level request API from above:

Request request = new Request();
await request.post(uri: Uri.parse('/notes/'),
                   headers: {'x-auth-token': 'a390bn'},
                   body: 'A note.');

As you'll notice, all of the parameters are optional - even the uri - because they can be set on the Request directly before being sent:

Request request = new Request()
  ..uri = Uri.parse('/notes/')
  ..headers = {'x-auth-token': 'a390bn'}
  ..body = 'A note.';
await request.post();

For simpler requests, it's recommended that you set the uri, headers, and/or body when dispatching the request (in other words, when you call .get(), .post(), etc).

For requests with more configuration or where the configuration may need to be broken up into several steps, set these properties on the request object directly.

Again, all of the standard HTTP methods are supported and each has its own method (get(), post(), etc). If you need to send a request with a custom HTTP method, use send():

Request request = new Request();
await request.send('COPY', uri: Uri.parse('/notes/6'));

Canceling a Request

All of the request classes support cancellation. At any time, the .abort() method can be called. If the request has not completed yet, it will be canceled. If it has already completed, it has no effect.

Request request = new Request();
request.get(Uri.parse('/notes/'));
...
request.abort();

Credentials (browser only)

HTTP requests made from a browser have an added restriction - secure cookies are not sent by default on cross-origin requests. To include these secure cookies when sending a request, set withCredentials to true. Although this only applies to browsers, it's included in the platform-independent API because it has no effect on the other platforms.

Request request = new Request()
  ..uri = Uri.parse('https://otherhost.com/notes/')
  ..withCredentials = true;
await request.get();

Content-Length, Content-Type and Encoding

All of the request classes set the content-type automatically based on the type of data being sent in the request body, and the charset parameter is set using the encoding's name.

By default, UTF8 encoding is used for requests.

The content-type of a request is available as .contentType and is of type MediaType from the http_parser package.

This property is read-only and is updated based on the type of data in the request body and the encoding. The exception to this is a streamed request where the request body is asynchronous and thus not known ahead of time.

Additionally, the content-length is set automatically for all Request classes since the length of the body in bytes is known before sending. The content-length of a request is available as the read-only property .contentLength and is the number of bytes of the request body when encoded.

Again, the exception to this is StreamedRequest since the body is sent asynchronously.

Consider the following plain-text request:

Request request = new Request()
  ..body = 'Hello World ®';

print(request.contentType.toString());
// content-type: text/plain; charset=utf-8
print(request.contentLength);
// 14

As you can see, the content-type is text/plain because the request body is plain-text and the charset is utf-8 because UTF8 encoding is used by default.

Let's change the encoding:

Request request = new Request()
  ..body = 'Hello World ®'
  ..encoding = LATIN1;

print(request.contentType.toString());
// content-type: text/plain; charset=iso-8859-1
print(request.contentLength);
// 13

The content-type value will change based on the type of request being sent. For plain Requests, it will always be text/plain. The other supported request types will set the content-type as follows:

  • FormRequest: application/x-www-form-urlencoded
  • JsonRequest: application/json
  • MultipartRequest: multipart/form-data

Timeout Threshold

A timeout threshold can be set on any request. If the request takes longer than the set duration, the request will be canceled.

Request request = new Request()
  ..uri = Uri.parse('/notes/')
  ..timeoutThreshold = new Duration(seconds: 15);
request.get();
// This will throw if the request takes longer than 15 seconds.

Request & Response Interception

All of the request classes have hooks that allow request and response interception.

Although hooks for interception are available on the request classes, their purpose is to enable an API on the Client class for intercepting every request created by the client, which is much more useful.

Request interception occurs right before the request is dispatched, at which point changes (async if necessary) to the request instance can be made.

Request request = new Request();

// Register a hook to intercept the request.
request.requestInterceptor = (Request request) async {
  // Modify the request as necessary.
};

Response interception occurs after the response is received but before it is delivered to the caller. At this point, a finalized version of the request can be inspected and the response instance can be modified, augmented, or replaced. Additionally, a RequestException instance will be available if one occurred. Again, this interception can be async if necessary.

Request request = new Request();

// Register a hook to intercept the response.
request.responseInterceptor =
    (FinalizedRequest request, BaseResponse response,
    [RequestException exception]) async {
  // Return a `BaseResponse` instance, modified as necessary.
};

Note that while response interceptors can replace the response instance (and thus are expected to return a BaseResponse instance), request interceptors cannot do this because the request creator's reference would then be incorrect. For this reason, request interceptors must modify the request in place.

Automatic Request Retrying

All of the request classes have an autoRetry API for enabling and configuring automatic request retrying. For example, the following request is configured to automatically retry up to 3 times for non-mutation requests that fail with a 500 or a 502:

Request request = new Request();
request.autoRetry
  ..enabled = true
  ..maxRetries = 3
  ..forHttpMethods = ['GET', 'HEAD', 'OPTIONS']
  ..forStatusCodes = [500, 502];

See the documentation for more information.

Request Types

Now that we've established the API common across all of our Request classes, let's dive into the different types of requests that are supported.

  • JsonRequest
  • FormRequest
  • MultipartRequest
  • Request
  • StreamedRequest

JsonRequest

A JsonRequest sets the content-type to application/json and accepts JSON-encodable Maps or Lists for the request body.

var note = {
  'title': 'My Note',
  'contents': '...',
  'date': new DateTime.now().toString()
};
JsonRequest request = new JsonRequest()
  ..uri = Uri.parse('/notes/')
  ..body = note;
await request.post();

Prior to sending a JsonRequest, the request body will be encoded to an appropriate format (text or bytes, depending on the platform).

FormRequest

A FormRequest sets the content-type to application/x-www-form-urlencoded and accepts a Map<String, String> for the request body where each key-value pair represents a form field's name and value.

By default, a FormRequest's body is an empty Map, allowing you to incrementally set each field.

FormRequest request = new FormRequest()
  ..uri = Uri.parse('/notes/')
  ..fields['title'] = 'My Note'
  ..fields['contents'] = '...'
  ..fields['date'] = new DateTime.now().toString();
await request.post();

MultipartRequest

A MultipartRequest sets the content-type to multipart/form-data and accepts both fields and files for the request body. The MultipartRequest class takes care of generating a unique boundary string used to separate each part of the request body.

The fields are key-value pairs representing a form field's name and value, just like the FormRequest:

MultipartRequest request = new MultipartRequest()
  ..uri = Uri.parse('/notes/')
  ..fields['title'] = 'My Note'
  ..fields['date'] = new DateTime.now().toString();

The files are also key-value pairs, but each pair represents a file's name and object. The actual file object can be several different types.

This is one area where the API is not entirely platform-independent because the APIs for file I/O in the browser are so restricted that they cannot easily be abstracted.

This library includes a MultipartFile class as an option for a platform-independent file abstraction, but it requires that you have access to a byte stream to construct an instance.

The files map accepts the following types:

  • MultipartFile (any platform)
  • dart:html.File (browser)
  • dart:html.Blob (browser)
Stream<List<int>> byteStream;
int length;
MultipartFile file = new MultipartFile(byteStream, length);

MultipartRequest request = new MultipartRequest()
  ..uri = Uri.parse('/notes/')
  ..fields['title'] = 'My Note'
  ..files['attachment'] = file;

Request (plain-text)

A Request sets the content-type to text/plain and accepts either a String or a list of bytes (List<int>) as the body.

// Request body as string
Request request = new Request()
  ..uri = Uri.parse('/notes/')
  ..body = 'My notes.';

// Request body as bytes
Request request = new Request()
  ..uri = Uri.parse('/notes/')
  ..bodyBytes = UTF8.encode('My notes.');

The latter approach is useful if you are already dealing with encoded data - no need to translate back and forth between bytes and text just to fit the API.

Be sure to set encoding if using something other than the default UTF8.

StreamedRequest

A StreamedRequest accepts a byte stream (Stream<List<int>>) as the request body. When a StreamedRequest is sent, the headers will be sent immediately, but the request body will be sent as items are added to the stream. Once the stream has been closed and has finished, the request will end.

List<int> encoded = UTF8.encode('data');
Stream<List<int>> byteStream = new Stream.fromIterable(encoded);

StreamedRequest request = new StreamedRequest()
  ..uri = Uri.parse('/bytes/')
  ..body = byteStream;

Note: the stream you supply should be a single-subscription stream (not a broadcast stream) to avoid losing data.

HTTP Client

An HTTP client acts as a single point from which many requests can be constructed. All requests constructed from a client will inherit headers, the withCredentials flag, and the timeout threshold.

On the server, the Dart VM will also be able to take advantage of cached network connections between requests that share a client.

Client client = new Client()
  ..headers['x-xsrf-token'] = 'ab93c...'
  ..withCredentials = true;

// This request will inherit the above header and withCredentials value.
// Once created, it can be used and dispatched as expected.
Request request = client.newRequest();

If you know that a client will no longer be used, or if you'd like to cancel all outstanding requests from a client, you should close the client. On the server, this ensures that cached network connections are closed.

Client client = new Client();
...
client.close();

Intercepting Requests & Responses from a Client

The request classes have hooks for intercepting the request and the response which the Client class leverages to provide an API for registering a chain of interceptors that will be applied to all requests and subsequent responses created by the client.

class HeaderInterceptor extends HttpInterceptor {
  @override
  Future<RequestPayload> interceptRequest(RequestPayload payload) async {
    payload.request.headers['x-foo'] = 'bar';
    return payload;
  }
}

class QueryParamInterceptor extends HttpInterceptor {
  @override
  Future<RequestPayload> interceptRequest(RequestPayload payload) async {
    payload.request.updateQuery({'baz': 'bar'});
    return payload;
  }
}

main() {
  Client client = new Client()
    ..addInterceptor(new HeaderInterceptor())
    ..addInterceptor(new QueryParamInterceptor());

  // The client will create a request interceptor that chains together the logic
  // from both of the interceptors registered above. This will be set on the
  // request, meaning that this request will have an `x-foo: bar` header and a
  // `baz=bar` query parameter.
  client.newRequest().get(uri: Uri.parse('...'));

Obviously these examples are contrived, but this pattern enables some powerful functionality. Consider the following interceptors as possibilities:

  • Analytics: records types of requests, request duration, request failures, etc.
  • CSRF: sets a header for CSRF verification on outgoing mutation requests and updates said token if the response headers include a new one.
  • OAuth2: sets the Authorization header to a valid OAuth2 token.
  • Session monitoring: watches for failures due to invalid session, like an HTTP 401 failure.

This interceptor logic is asynchronous, which means that you can get really creative. Let's take the CSRF interceptor example and consider a scenario where the initial request requires a CSRF token but one is not known at the time. We can preempt the request and send a separate request to obtain a token:

class CsrfInterceptor extends HttpInterceptor {
  Uri csrfEndpointUri = ...;
  String token;

  @override
  Future<RequestPayload> interceptRequest(RequestPayload payload) async {
    if (token == null) {
      token = await fetchNewToken();
    }
    payload.request.headers['x-xsrf-token'] = token;
    return payload;
  }

  // Assuming we have an endpoint to retrieve a CSRF token.
  Future<String> fetchNewToken() async {
    Response response = await Http.get(csrfEndpointUri);
    return response.body.asJson()['token'];
  }
}

Responses

Every request, once sent, is asynchronous and eventually returns a response. By default, the entire response is loaded into memory and made available to you in three different formats:

Bytes

The response is left in its encoded state and returned directly to you as a list of bytes.

Response response = await Http.get(Uri.parse('/file'));
Uint8List body = response.body.asBytes();

Text

The response's content-type header is inspected for a charset parameter. If found and if valid, the corresponding encoding will be used to decode the response body to text. Otherwise, the default LATIN1 encoding will be used.

Response response = await Http.get(Uri.parse('/file'));
String body = response.body.asString();

JSON

The response is decoded to text (using the above process) and then decoded into either a Map or a List.

This will throw if the response body is not valid JSON.

Response response = await Http.get(Uri.parse('/file'));
Map body = response.body.asJson();

Streamed Responses

As mentioned above, every response is loaded in its entirety into memory by default. This can be problematic for extremely large responses. The solution is to request the response as a stream of data so that it's loaded asynchronously and - if large enough - in chunks.

To request a streamed response, use the corresponding stream method. For example, instead of get(), use streamGet().

StreamedResponse response = await Http.streamGet(Uri.parse('/file'));
response.body.byteStream.listen((List<int> bytes) { ... });

WebSocket

The WebSocket API mirrors the dart:io.WebSocket class to keep things simple. If you've used the VM's WebSocket class before, then you're ready to go. The benefit is that this same API works in the browser as well (even when configured to use SockJS), which is a big improvement over the dart:html.WebSocket class.

The WSocket class included in this library is a Stream and a StreamSink, so sending and receiving data is as simple as adding items to it like a sink and listening to it like a stream.

Establishing a Connection

WSocket webSocket = await WSocket.connect(Uri.parse('ws://echo.websocket.org'));

The connect() method will throw if a connection cannot be established.

Receiving Data

WSocket webSocket = await WSocket.connect(Uri.parse('ws://echo.websocket.org'));

webSocket.listen((data) {
  // Handle message.
}, onError: (error) {
  // Handle error (if desired).
  // The socket will close immediately after this.
}, onDone: () {
  // Perform any cleanup if desired.
});

Sending Data

WSocket webSocket = await WSocket.connect(Uri.parse('ws://echo.websocket.org'));
webSocket.add('message');
await webSocket.addStream(new Stream.fromIterable([...]));

Listening for Completion

WSocket webSocket = await WSocket.connect(Uri.parse('ws://echo.websocket.org'));
webSocket.done.then((_) {
  // Perform cleanup, reopen socket, etc.
}).catchError((error) {
  // Handle socket error, reopen socket, etc.
});

Using SockJS

Sockets can be configured to use SockJS under the hood instead of native WebSockets. This configuration must occur on a per-socket basis.

Uri uri = Uri.parse('ws://echo.websocket.org');
WSocket webSocket = await WSocket.connect(uri,
   useSockJS: true, sockJSProtocolsWhitelist: ['websocket', 'xhr-streaming']);

Testing & Mocks

Just like the browser or the Dart VM, tests are considered a platform for which this library can be configured. By configuring w_transport for tests, mock implementations of all classes will be used.

import 'package:w_transport/w_transport_mock.dart';

main() {
  configureWTransportForTest();
}

That's it. No changes to your source code are necessary! Once configured for test, you are in control of every HTTP request and every WebSocket connection. The APIs for controlling these transports are exported with the w_transport_mock.dart entry point as static APIs on a MockTransports class.

Resetting Mocks:

At any point, you can reset all mock expectations and handlers, giving you a clean state to begin a new mock setup:

MockTransports.reset();

Mocking HTTP

Expecting a request (and returning a 200 OK response by default)

MockTransports.http.expect('GET', Uri.parse('/resource'));

Expecting a request and providing a custom response

Response response = new MockResponse.unauthorized(
    body: 'Invalid access token',
    headers: {'x-session': 'ab93s...'});
MockTransports.http.expect(
    'GET',
    Uri.parse('/resource'),
    respondWith: response);

Expecting a request and causing a failure (exception)

MockTransports.http.expect(
    'GET',
    Uri.parse('/resource'),
    failWith: new Exception('Unexpected error...'));

Registering a handler (expecting multiple requests to the same URI)

MockTransports.http.when(
    Uri.parse('/resource'),
    (FinalizedRequest request) async {
      if (request.method == 'GET') {
        return new MockResponse.ok(body: '...');
      }
      if (request.method == 'DELETE') {
        return new MockResponse(204);
      }
      ...
    });

Registering a handler (expecting multiple requests with the same URI and method)

MockTransports.http.when(
    Uri.parse('/resource'),
    (FinalizedRequest request) async => new MockResponse.ok(body: '...'),
    method: 'GET');

Verifying there are no unresolved requests and/or unsatisfied expectations

MockTransports.verifyNoOutstandingExceptions();

This will throw if an expected request has yet to occur or if a request was made for which no applicable handler was found and was not otherwise expected.

Mocking WebSockets

Expecting and accepting a websocket connection

var ws = new MockWSocket();
MockTransports.webSocket.expect(
    Uri.parse('/ws'),
    connectTo: ws);

Expecting and rejecting a websocket connection

MockTransports.webSocket.expect(Uri.parse('/ws'), reject: true);

This will cause the WSocket.connect(...) clause to throw.

Listening to outgoing data from a mock websocket connection

var ws = new MockWSocket();
ws.onOutgoing((data) { ... });

MockTransports.webSocket.expect(
    Uri.parse('/ws'),
    connectTo: ws);

Sending data to the client from a mock websocket connection

var ws = new MockWSocket();
// Echo.
ws.onOutgoing((data) {
  ws.addIncoming(data);
});

MockTransports.webSocket.expect(
    Uri.parse('/ws'),
    connectTo: ws);

// Create a WSocket instance here...

// Connected message.
ws.add('Connected.');

Triggering a close event for a mock websocket connection

var ws = new MockWSocket();
MockTransports.webSocket.expect(
    Uri.parse('/ws'),
    connectTo: ws);

new Timer(new Duration(seconds: 5), () {
  ws.triggerServerClose();
});

Credits

This library was influenced in many ways by the http package, especially with regard to multipart requests, and served as a useful source for references to pertinent IETF RFCs.

Development

This project leverages the dart_dev package for most of its tooling needs, including static analysis, code formatting, running tests, collecting coverage, and serving examples. Check out the dart_dev readme for more information.

Note: to run integration tests, you'll need two JS dependencies for a SockJS server. Run an npm install to download them.

Libraries

w_transport

Platform-agnostic transport library for sending and receiving data over HTTP and WebSocket. HTTP support includes plain-text, JSON, form-data, and multipart data, as well as custom encoding. WebSocket support includes native WebSockets in the browser and the VM with the option to use SockJS in the browser.

w_transport.w_transport_browser

Transport for the browser. Exposes a single configuration method that must be called before instantiating any of the transport classes.

w_transport.w_transport_mock

Easily mock out the platform-specific details of w_transport. Exposes a single configuration method that must be called before instantiating any of the transport classes.

w_transport.w_transport_vm

Transport for the server. This exposes a single configuration method that must be called before instantiating any of the transport classes.