🔄️ Turn the world up-side-down

dio/lib/src/adapters/io_adapter.dart

@@ -116,7 +116,7 @@ class IOHttpClientAdapter implements HttpClientAdapter {
           request.headers.set(
             key,
             value,
-            preserveHeaderCase: !options.caseInsensitiveHeaders,
+            preserveHeaderCase: options.caseSensitiveHeaders,
           );
         }
       });

dio/lib/src/dio_mixin.dart

@@ -517,7 +517,10 @@ abstract class DioMixin implements Dio {
         stream,
         cancelToken?.whenCancel,
       );
-      final headers = Headers.fromMap(responseBody.headers);
+      final headers = Headers.fromMap(
+        responseBody.headers,
+        caseSensitive: reqOpt.caseSensitiveHeaders,
+      );
       // Make sure headers and [ResponseBody.headers] are the same instance.
       responseBody.headers = headers.map;
       final ret = Response<dynamic>(
@@ -707,7 +710,10 @@ abstract class DioMixin implements Dio {
       final T? data = response.data as T?;
       final Headers headers;
       if (data is ResponseBody) {
-        headers = Headers.fromMap(data.headers);
+        headers = Headers.fromMap(
+          data.headers,
+          caseSensitive: requestOptions.caseSensitiveHeaders,
+        );
       } else {
         headers = response.headers;
       }

dio/lib/src/headers.dart

@@ -8,13 +8,13 @@ typedef HeaderForEachCallback = void Function(String name, List<String> values);
 /// The headers class for requests and responses.
 class Headers {
   Headers({
-    this.caseInsensitive = true,
+    this.caseSensitive = false,
   }) : _map = caseInsensitiveKeyMap<List<String>>();
 
   /// Create the [Headers] from a [Map] instance.
   Headers.fromMap(
     Map<String, List<String>> map, {
-    this.caseInsensitive = true,
+    this.caseSensitive = false,
   }) : _map = caseInsensitiveKeyMap<List<String>>(
           map.map((k, v) => MapEntry(k.trim(), v)),
         );
@@ -32,10 +32,10 @@ class Headers {
 
   static final jsonMimeType = MediaType.parse(jsonContentType);
 
-  /// Whether the header key should be case-insensitive.
+  /// Whether the header key should be case-sensitive.
   ///
-  /// Defaults to true.
-  final bool caseInsensitive;
+  /// Defaults to false.
+  final bool caseSensitive;
 
   final Map<String, List<String>> _map;
 
@@ -44,9 +44,6 @@ class Headers {
   /// Returns the list of values for the header named [name]. If there
   /// is no header with the provided name, [:null:] will be returned.
   List<String>? operator [](String name) {
-    if (caseInsensitive) {
-      name = name.toLowerCase();
-    }
     return _map[name.trim()];
   }
 
@@ -75,9 +72,6 @@ class Headers {
   /// cleared before the value [value] is added as its value.
   void set(String name, dynamic value) {
     if (value == null) return;
-    if (caseInsensitive) {
-      name = name.toLowerCase();
-    }
     name = name.trim();
     if (value is List) {
       _map[name] = value.map<String>((e) => '$e').toList();

dio/lib/src/options.dart

@@ -128,7 +128,7 @@ class BaseOptions extends _RequestConfig with OptionsMixin {
     Map<String, dynamic>? queryParameters,
     Map<String, dynamic>? extra,
     Map<String, dynamic>? headers,
-    bool? caseInsensitiveHeaders,
+    bool caseSensitiveHeaders = false,
     ResponseType? responseType = ResponseType.json,
     String? contentType,
     ValidateStatus? validateStatus,
@@ -147,7 +147,7 @@ class BaseOptions extends _RequestConfig with OptionsMixin {
           sendTimeout: sendTimeout,
           extra: extra,
           headers: headers,
-          caseInsensitiveHeaders: caseInsensitiveHeaders,
+          caseSensitiveHeaders: caseSensitiveHeaders,
           responseType: responseType,
           contentType: contentType,
           validateStatus: validateStatus,
@@ -175,7 +175,7 @@ class BaseOptions extends _RequestConfig with OptionsMixin {
     Duration? sendTimeout,
     Map<String, Object?>? extra,
     Map<String, Object?>? headers,
-    bool? caseInsensitiveHeaders,
+    bool? caseSensitiveHeaders,
     ResponseType? responseType,
     String? contentType,
     ValidateStatus? validateStatus,
@@ -196,8 +196,8 @@ class BaseOptions extends _RequestConfig with OptionsMixin {
       sendTimeout: sendTimeout ?? this.sendTimeout,
       extra: extra ?? Map.from(this.extra),
       headers: headers ?? Map.from(this.headers),
-      caseInsensitiveHeaders:
-          caseInsensitiveHeaders ?? this.caseInsensitiveHeaders,
+      caseSensitiveHeaders:
+          caseSensitiveHeaders ?? this.caseSensitiveHeaders,
       responseType: responseType ?? this.responseType,
       contentType: contentType ?? this.contentType,
       validateStatus: validateStatus ?? this.validateStatus,
@@ -221,7 +221,7 @@ class Options {
     Duration? receiveTimeout,
     this.extra,
     this.headers,
-    this.caseInsensitiveHeaders,
+    this.caseSensitiveHeaders,
     this.responseType,
     this.contentType,
     this.validateStatus,
@@ -244,7 +244,7 @@ class Options {
     Duration? receiveTimeout,
     Map<String, Object?>? extra,
     Map<String, Object?>? headers,
-    bool? caseInsensitiveHeaders,
+    bool? caseSensitiveHeaders,
     ResponseType? responseType,
     String? contentType,
     ValidateStatus? validateStatus,
@@ -281,8 +281,8 @@ class Options {
       receiveTimeout: receiveTimeout ?? this.receiveTimeout,
       extra: extra ?? effectiveExtra,
       headers: headers ?? effectiveHeaders,
-      caseInsensitiveHeaders:
-          caseInsensitiveHeaders ?? this.caseInsensitiveHeaders,
+      caseSensitiveHeaders:
+          caseSensitiveHeaders ?? this.caseSensitiveHeaders,
       responseType: responseType ?? this.responseType,
       contentType: contentType ?? this.contentType,
       validateStatus: validateStatus ?? this.validateStatus,
@@ -331,8 +331,8 @@ class Options {
       baseUrl: baseOpt.baseUrl,
       path: path,
       data: data,
-      caseInsensitiveHeaders:
-          caseInsensitiveHeaders ?? baseOpt.caseInsensitiveHeaders,
+      caseSensitiveHeaders:
+          caseSensitiveHeaders ?? baseOpt.caseSensitiveHeaders,
       sourceStackTrace: sourceStackTrace ?? StackTrace.current,
       connectTimeout: baseOpt.connectTimeout,
       sendTimeout: sendTimeout ?? baseOpt.sendTimeout,
@@ -367,15 +367,14 @@ class Options {
   /// e.g.: `content-type` and `Content-Type` will be treated as the same key.
   Map<String, dynamic>? headers;
 
-  /// Whether the header keys should be case-insensitive.
-  /// e.g.: `Content-Type` will be sent as `content-type` eventually.
+  /// Whether the header keys should be case-sensitive.
   ///
-  /// Defaults to true.
+  /// Defaults to false.
   ///
   /// This option WILL NOT take effect on these circumstances:
   /// - XHR ([HttpRequest]) does not support handling this explicitly.
   /// - The HTTP/2 standard only supports lowercase header keys.
-  bool? caseInsensitiveHeaders;
+  bool? caseSensitiveHeaders;
 
   /// Timeout when sending data.
   ///
@@ -493,7 +492,7 @@ class RequestOptions extends _RequestConfig with OptionsMixin {
     String? baseUrl,
     Map<String, dynamic>? extra,
     Map<String, dynamic>? headers,
-    bool caseInsensitiveHeaders = true,
+    bool? caseSensitiveHeaders,
     ResponseType? responseType,
     String? contentType,
     ValidateStatus? validateStatus,
@@ -513,7 +512,7 @@ class RequestOptions extends _RequestConfig with OptionsMixin {
           receiveTimeout: receiveTimeout,
           extra: extra,
           headers: headers,
-          caseInsensitiveHeaders: caseInsensitiveHeaders,
+          caseSensitiveHeaders: caseSensitiveHeaders,
           responseType: responseType,
           contentType: contentType,
           validateStatus: validateStatus,
@@ -546,7 +545,7 @@ class RequestOptions extends _RequestConfig with OptionsMixin {
     CancelToken? cancelToken,
     Map<String, dynamic>? extra,
     Map<String, dynamic>? headers,
-    bool? caseInsensitiveHeaders,
+    bool? caseSensitiveHeaders,
     ResponseType? responseType,
     String? contentType,
     ValidateStatus? validateStatus,
@@ -583,8 +582,8 @@ class RequestOptions extends _RequestConfig with OptionsMixin {
       cancelToken: cancelToken ?? this.cancelToken,
       extra: extra ?? Map.from(this.extra),
       headers: headers ?? Map.from(this.headers),
-      caseInsensitiveHeaders:
-          caseInsensitiveHeaders ?? this.caseInsensitiveHeaders,
+      caseSensitiveHeaders:
+          caseSensitiveHeaders ?? this.caseSensitiveHeaders,
       responseType: responseType ?? this.responseType,
       validateStatus: validateStatus ?? this.validateStatus,
       receiveDataWhenStatusError:
@@ -662,7 +661,7 @@ class _RequestConfig {
     String? method,
     Map<String, dynamic>? extra,
     Map<String, dynamic>? headers,
-    bool? caseInsensitiveHeaders,
+    bool? caseSensitiveHeaders,
     String? contentType,
     ListFormat? listFormat,
     bool? followRedirects,
@@ -678,7 +677,7 @@ class _RequestConfig {
         assert(sendTimeout == null || !sendTimeout.isNegative),
         _sendTimeout = sendTimeout,
         method = method ?? 'GET',
-        caseInsensitiveHeaders = caseInsensitiveHeaders ?? true,
+        caseSensitiveHeaders = caseSensitiveHeaders ?? false,
         listFormat = listFormat ?? ListFormat.multi,
         extra = extra ?? {},
         followRedirects = followRedirects ?? true,
@@ -718,7 +717,7 @@ class _RequestConfig {
     }
   }
 
-  late bool caseInsensitiveHeaders;
+  late bool caseSensitiveHeaders;
 
   Duration? get sendTimeout => _sendTimeout;
   Duration? _sendTimeout;

dio/lib/src/response.dart

@@ -19,7 +19,8 @@ class Response<T> {
     this.redirects = const [],
     Map<String, dynamic>? extra,
     Headers? headers,
-  })  : headers = headers ?? Headers(),
+  })  : headers = headers ??
+            Headers(caseSensitive: requestOptions.caseSensitiveHeaders),
         extra = extra ?? <String, dynamic>{};
 
   /// The response payload in specific type.

dio/test/basic_test.dart

@@ -48,9 +48,11 @@ void main() {
           'content-length': ['200'],
           'Test': ['1', '2'],
         },
-        caseInsensitive: false,
+        caseSensitive: true,
       );
       expect(headers['SET-COOKIE']?.length, 2);
+      // Although it's case-sensitive, we still use case-insensitive map.
+      expect(headers['set-cookie']?.length, 2);
       expect(headers['content-length']?.length, 1);
       expect(headers['Test']?.length, 2);
     });