MockWebServer for Dart/Flutter
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A flexible Dart web server that can be used to script tests and web server interactions


The best way to do integration tests, or to reproduce specific edge cases, is to be able to script the interaction between the software that is being tested and the web server.

MockWebServer aims to facility testing by offering a flexible stand alone Server that will respond with a given script (or forward the request to your dispatcher).

MockWebServer is based on the library of the same name created by Square for Java.


Starting it

MockWebServer can run in a given port or an ephemeral one

new MockWebServer(); // Will use an ephemeral port picked by the system
new MockWebServer(port: 8081); // Will use 8081

To start it just do

MockWebServer server = new MockWebServer();

Once you have the server running you will want to get the url of it, so that you can configure your app to connect there

server.url; // will return
server.port; // 8081; //

Adding responses to the queue

Once the server is started, you can queue the responses that you want. The response queue is First In First Out

// Enqueue an empty body with response code 401
server.enqueue(httpCode: 401);

// Response code defaults to 200, so this will be a 200 with the given json as the body
server.enqueue(body: '{ "message" : "hi"}');

// HTTP 200 with empty body and some header
Map<String, String> headers = new Map();
headers["X-Server"] = "MockDart";
server.enqueue(headers: headers);

// All the parameters are optional so you can mix and match according to what you need
server.enqueue(httpCode: 201, body: "answer", headers: headers, duration: duration);

// You can always call enqueueResponse() to directly enqueue a MockResponse
Map<String, String> headers = new Map();
headers["X-Server"] = "MockDart";

var mockResponse = new MockResponse()
  ..httpCode = 201
  ..body = "Created"
  ..headers = headers
  ..delay = new Duration(seconds: 2);


Delaying the response

To test timeouts or race conditions you may want to have the server take some time

server.enqueue(delay: new Duration(seconds: 2), httpCode: 201);
Stopwatch stopwatch = new Stopwatch();

HttpClientResponse response = request(path: "");

expect(stopwatch.elapsed.inMilliseconds, greaterThanOrEqualTo(2000));
expect(response.statusCode, 201);

Validating that the request was correct

You may want to check that your app is sending the correct requests. To do so you can obtain the requests that have been made to the server. The request queue is Last In First Out

server.enqueue(body: "a");
server.enqueue(body: "b");
server.enqueue(body: "c");

request(path: "first");
request(path: "second");
request(path: "third");

// takeRequest is FIFO
// You should probably assign takeRequest() to a var so that you can 
// validate multiple things.
expect(server.takeRequest().headers['x-header'], "nosniff");
expect(server.takeRequest().method, "GET");
expect(server.takeRequest().uri.path, "/third");

Using a dispatcher for fine-grained routing

If you want more control than what the FIFO queue offers, you can set a dispatcher and set the logic there.

var dispatcher = (HttpRequest request) {
  if (request.uri.path == "/users") {
    return new MockResponse()
      ..httpCode = 200
      ..body = "working";
  } else if (request.uri.path == "/users/1") {
    return new MockResponse()..httpCode = 201;

  return new MockResponse()..httpCode = 404;

server.dispatcher = dispatcher;

HttpClientResponse response = request(path: "unknown");
expect(response.statusCode, 404);

response = request(path: "users");
expect(response.statusCode, 200);
expect(read(response), "working");

response = request(path: "users/1");
expect(response.statusCode, 201);


You can start the server using TLS by passing the certificate parameter when creating the instance of MockWebServer. For example using the included certificates you would do

Certificate certificate = Certificate.make(serverChain: 'your_server_chain.pem', serverKey: 'your_server_key.pem')
  ..password = "dartdart"
  ..key = key
  ..chain = chain;

// To use the included certificates:
// Certificate certificate = Certificate.included();

MockWebServer _server =
    new MockWebServer(certificate: certificate);

If you do so, and your client validates the certs, you will need to use a proper SecurityContext, for example using the included trusted_certs.pem

var certRes =
    new File('your_trusted_certs.pem');
List<int> cert = await certRes.readAsBytes();

SecurityContext clientContext = new SecurityContext()

// To use the included trusted_certs.pem:
// SecurityContext clientContext = DefaultSecurityContext();

var client = new HttpClient(context: clientContext);

Please check the tests of MockWebServer to see a complete example of this.


If want to use IPv6, you can pass addressType: InternetAddressType.IP_V6 in the constructor to have the server use it. Keep in mind that the host property will then be ::1 instead of

MockWebServer _server =
        new MockWebServer(port: 8030, addressType: InternetAddressType.IP_V6);

Setting a default response

In some scenarions, if there's nothing on queue and there's no dispatcher, you may want the server to default to a response (e.g 404) instead of throwing an exception.

_server.defaultResponse = MockResponse()..httpCode = 404;


During the tearDown of your tests you should stop the server. server.shutdown() will do.