1. add adapter demo

2. add DioHttpHeaders class

README-ZH.md

@@ -587,6 +587,9 @@ HttpClientAdapter是 Dio 和 HttpClient之间的桥梁。2.0抽象出adapter主
 
 Dio 使用`DefaultHttpClientAdapter`作为其默认HttpClientAdapter，`DefaultHttpClientAdapter`使用`dart:io:HttpClient` 来发起网络请求。
 
+[这里](https://github.com/flutterchina/dio/blob/master/example/adapter.dart) 有一个简单的自定义Adapter的示例，读者可以参考。另外本项目的自动化测试用例全都是通过一个自定义的[MockAdapter](https://github.com/flutterchina/dio/blob/master/package_src/test/mock_adapter.dart)来模拟服务器返回数据的。
+
+
 
 
 ### 设置Http代理

README.md

@@ -580,7 +580,7 @@ We can use any HttpClient not just `dart:io:HttpClient` to make the Http request
 dio.httpClientAdapter = new DefaultHttpClientAdapter();
 ```
 
-
+[Here](https://github.com/flutterchina/dio/blob/master/example/adapter.dart) is a simple example to custom adapter. 
 
 ### Using proxy
 

example/adapter.dart

@@ -0,0 +1,26 @@
+import 'package:dio/dio.dart';
+
+class MyAdapter extends HttpClientAdapter {
+  DefaultHttpClientAdapter defaultHttpClientAdapter =
+      DefaultHttpClientAdapter();
+
+  @override
+  Future<ResponseBody> fetch(RequestOptions options,
+      Stream<List<int>> requestStream, Future cancelFuture) async {
+    Uri uri = options.uri;
+    // hook requests to  google.com
+    if (uri.host == "google.com") {
+      return ResponseBody.fromString("Too young too simple!", 200, null);
+    }
+    return defaultHttpClientAdapter.fetch(options, requestStream, cancelFuture);
+  }
+}
+
+main() async {
+  var dio = new Dio();
+  dio.httpClientAdapter = MyAdapter();
+  Response response = await dio.get("https://google.com");
+  print(response);
+  response = await dio.get("https://baidu.com");
+  print(response);
+}

example/test.dart

@@ -9,6 +9,7 @@ main() async {
     ),
   );
   dio.interceptors.add(LogInterceptor());
+
   await dio.get(
     "/test",
     queryParameters: {"kk": "tt"},

package_src/lib/dio.dart

@@ -17,3 +17,4 @@ export 'src/upload_file_info.dart';
 export 'src/interceptors/log.dart';
 export 'src/interceptors/cookie_mgr.dart';
 export 'src/adapter.dart';
+export 'src/dio_http_headers.dart';

package_src/lib/src/adapter.dart

@@ -50,11 +50,7 @@ abstract class HttpClientAdapter {
 }
 
 class ResponseBody {
-  ResponseBody(
-    this.stream,
-    this.statusCode,
-    this.headers,
-  );
+  ResponseBody(this.stream, this.statusCode, [this.headers]);
 
   /// The response stream
   Stream<List<int>> stream;
@@ -65,11 +61,11 @@ class ResponseBody {
   /// Http status code
   int statusCode;
 
-  ResponseBody.fromString(String text, this.statusCode, this.headers)
+  ResponseBody.fromString(String text, this.statusCode, [this.headers])
       : stream =
             Stream.fromIterable(utf8.encode(text).map((e) => [e]).toList());
 
-  ResponseBody.fromBytes(List<int> bytes, this.statusCode, this.headers)
+  ResponseBody.fromBytes(List<int> bytes, this.statusCode, [this.headers])
       : stream = Stream.fromIterable(bytes.map((e) => [e]).toList());
 }
 
@@ -118,7 +114,7 @@ class DefaultHttpClientAdapter extends HttpClientAdapter {
     }
 
     HttpClientResponse responseStream = await request.close();
-    return new ResponseBody(
+    return ResponseBody(
       responseStream,
       responseStream.statusCode,
       responseStream.headers,

package_src/lib/src/dio.dart

@@ -10,6 +10,7 @@ import 'options.dart';
 import 'response.dart';
 import 'transformer.dart';
 import 'adapter.dart';
+import 'dio_http_headers.dart';
 
 /// Callback to listen the progress for sending/receiving data.
 ///
@@ -719,6 +720,9 @@ class Dio {
         stream,
         cancelToken?.whenCancel,
       );
+      if(responseBody.headers==null){
+        responseBody.headers=DioHttpHeaders();
+      }
       Response ret = new Response(
           headers: responseBody.headers,
           request: options,