Added filtering support for collections

lib/query.dart

@@ -1,6 +1,7 @@
 library query;
 
 export 'package:json_api/src/query/fields.dart';
+export 'package:json_api/src/query/filter.dart';
 export 'package:json_api/src/query/include.dart';
 export 'package:json_api/src/query/page.dart';
 export 'package:json_api/src/query/query_parameters.dart';

lib/src/query/filter.dart

@@ -0,0 +1,30 @@
+import 'package:json_api/src/query/query_parameters.dart';
+
+/// Query parameters defining Filter
+/// @see https://jsonapi.org/recommendations/#filtering
+class Filter extends QueryParameters {
+  /// The [filter] argument maps the resource type to a list of filters.
+  ///
+  /// Example:
+  /// ```dart
+  /// Filter({'articles': ['title', 'body'], 'people': ['name']}).addTo(url);
+  /// ```
+  /// encodes to
+  /// ```
+  /// ?filter[articles]=title,body&filter[people]=name
+  /// ```
+  Filter(Map<String, List<String>> filter)
+      : _filter = {...filter},
+        super(filter.map((k, v) => MapEntry('filter[$k]', v.join(','))));
+
+  /// Extracts the requested filter from the [uri].
+  static Filter fromUri(Uri uri) => Filter(uri.queryParameters
+      .map((k, v) => MapEntry(_regex.firstMatch(k)?.group(1), v.split(',')))
+        ..removeWhere((k, v) => k == null));
+
+  List<String> operator [](String key) => _filter[key];
+
+  static final _regex = RegExp(r'^filter\[(.+)\]$');
+
+  final Map<String, List<String>> _filter;
+}

test/unit/query/filter_test.dart

@@ -0,0 +1,23 @@
+import 'package:json_api/src/query/filter.dart';
+import 'package:test/test.dart';
+
+void main() {
+  test('Can decode url', () {
+    final uri = Uri.parse(
+        '/articles?include=author&filter%5Barticles%5D=title%2Cbody&filter%5Bpeople%5D=name');
+    final filter = Filter.fromUri(uri);
+    expect(filter['articles'], ['title', 'body']);
+    expect(filter['people'], ['name']);
+  });
+
+  test('Can add to uri', () {
+    final filter = Filter({
+      'articles': ['title', 'body'],
+      'people': ['name']
+    });
+    final uri = Uri.parse('/articles');
+
+    expect(filter.addToUri(uri).toString(),
+        '/articles?filter%5Barticles%5D=title%2Cbody&filter%5Bpeople%5D=name');
+  });
+}