Implement authenticator

chopper/lib/src/authenticator.dart

@@ -0,0 +1,9 @@
+import 'dart:async';
+
+import 'package:chopper/chopper.dart';
+
+/// This method should return a [Request] that includes credentials to satisfy an authentication challenge received in
+/// [response]. It should return `null` if the challenge cannot be satisfied.
+abstract class Authenticator {
+  FutureOr<Request> authenticate(Request request, Response response);
+}

chopper/lib/src/base.dart

@@ -1,4 +1,5 @@
 import 'dart:async';
+import 'package:chopper/src/authenticator.dart';
 import 'package:meta/meta.dart';
 import 'package:http/http.dart' as http;
 import 'constants.dart';
@@ -37,6 +38,10 @@ class ChopperClient {
   /// the request and response interceptors are called respectively.
   final Converter converter;
 
+  /// The [Authenticator] that can provide reactive authentication for a
+  /// request.
+  final Authenticator authenticator;
+
   /// The [ErrorConverter] that handles response transformation before the
   /// response interceptors are called, but only on error responses
   /// (statusCode < 200 || statusCode >= 300\).
@@ -109,6 +114,7 @@ class ChopperClient {
     this.baseUrl = '',
     http.Client client,
     Iterable interceptors = const [],
+    this.authenticator,
     this.converter,
     this.errorConverter,
     Iterable<ChopperService> services = const [],
@@ -316,6 +322,14 @@ class ChopperClient {
     final response = await http.Response.fromStream(streamRes);
     dynamic res = Response(response, response.body);
 
+    if (authenticator != null) {
+      var updatedRequest = authenticator.authenticate(request, res);
+
+      if (updatedRequest != null) {
+        res = await send(updatedRequest);
+      }
+    }
+
     if (_responseIsSuccessful(response.statusCode)) {
       res = await _handleSuccessResponse<BodyType, InnerType>(
         res,