Add responseHeaders extension on XMLHttpRequest (#298)

* Add `responseHeaders` extension on `XMLHttpRequest`

Move this extension from `package:http` to here, it might also be used in `package:grpc`.

* Add test

* Reformat

* Add changelog

* Use LineSplitter

* Use `update`

* Use split instead of indexOf

web/CHANGELOG.md

@@ -10,6 +10,7 @@
   `getter`s and `setter`s, respectively.
 - Exposed constants with primitive values as non-`external` so they can be
   `switch`ed over.
+- Add an extension `responseHeaders` to `XMLHttpRequest`.
 
 ## 1.0.0
 

web/lib/src/helpers/extensions.dart

@@ -21,6 +21,7 @@
 ///  * conversions: for example to wrap a `TouchList` as a `List<Touch>`
 library;
 
+import 'dart:convert';
 import 'dart:js_interop';
 import 'dart:math' show Point;
 
@@ -103,3 +104,33 @@ extension TouchListConvert on TouchList {
   @Deprecated('Use JSImmutableListWrapper<TouchList, Touch> directly instead.')
   List<Touch> toList() => JSImmutableListWrapper<TouchList, Touch>(this);
 }
+
+/// Returns all response headers as a key-value map.
+///
+/// Multiple values for the same header key can be combined into one,
+/// separated by a comma and a space.
+///
+/// See: http://www.w3.org/TR/XMLHttpRequest/#the-getresponseheader()-method
+extension XMLHttpRequestGlue on XMLHttpRequest {
+  Map<String, String> get responseHeaders {
+    // from Closure's goog.net.Xhrio.getResponseHeaders.
+    final headers = <String, String>{};
+    final headersString = getAllResponseHeaders();
+    final headersList =
+        LineSplitter.split(headersString).where((header) => header.isNotEmpty);
+    for (final header in headersList) {
+      final split = header.split(': ');
+      if (split.length <= 1) {
+        continue;
+      }
+      final key = split[0].toLowerCase();
+      final value = split.skip(1).join(': ');
+      headers.update(
+        key,
+        (oldValue) => '$oldValue, $value',
+        ifAbsent: () => value,
+      );
+    }
+    return headers;
+  }
+}

web/test/helpers_test.dart

@@ -36,4 +36,23 @@ void main() {
     // Ensure accessing any arbitrary item in the list does not throw.
     expect(() => dartList[0], returnsNormally);
   });
+
+  test('responseHeaders transforms headers into a map', () async {
+    final request = XMLHttpRequest()
+      ..open('GET', 'www.google.com')
+      ..send();
+
+    await request.onLoad.first;
+
+    expect(
+      request.responseHeaders,
+      allOf(
+        containsPair('content-length', '10'),
+        containsPair('content-type', 'text/plain; charset=utf-8'),
+        containsPair('x-content-type-options', 'nosniff'),
+        containsPair('x-frame-options', 'SAMEORIGIN'),
+        containsPair('x-xss-protection', '1; mode=block'),
+      ),
+    );
+  });
 }