feat(graphql): Support custom equality function for cache comparison

- The current equality check has received some dissatisfaction due to performance (and is a source of jank/missed frames in my application)
- This is an unopinionated way to allow those consumers to provide their own equality function with better performance via optimizedDeepEquals

Author: kvenn

packages/graphql/lib/client.dart

@@ -9,4 +9,5 @@ export 'package:graphql/src/graphql_client.dart';
 
 export 'package:graphql/src/links/links.dart';
 
-export 'package:graphql/src/utilities/helpers.dart' show gql;
+export 'package:graphql/src/utilities/helpers.dart'
+    show gql, optimizedDeepEquals;

packages/graphql/lib/src/cache/fragment.dart

@@ -1,10 +1,10 @@
 import 'dart:convert';
+import "package:graphql/client.dart";
 import "package:meta/meta.dart";
 import "package:collection/collection.dart";
 
 import "package:gql/ast.dart";
 import 'package:gql/language.dart';
-import "package:gql_exec/gql_exec.dart";
 import 'package:normalize/utils.dart';
 
 /// A fragment in a [document], optionally defined by [fragmentName]
@@ -32,9 +32,7 @@ class Fragment {
   bool operator ==(Object o) =>
       identical(this, o) ||
       (o is Fragment &&
-          const ListEquality<Object?>(
-            DeepCollectionEquality(),
-          ).equals(
+          gqlDeepEquals(
             o._getChildren(),
             _getChildren(),
           ));
@@ -89,9 +87,7 @@ class FragmentRequest {
   bool operator ==(Object o) =>
       identical(this, o) ||
       (o is FragmentRequest &&
-          const ListEquality<Object?>(
-            DeepCollectionEquality(),
-          ).equals(
+          gqlDeepEquals(
             o._getChildren(),
             _getChildren(),
           ));

packages/graphql/lib/src/core/_base_options.dart

@@ -104,9 +104,7 @@ abstract class BaseOptions<TParsed extends Object?> {
       identical(this, other) ||
       (other is BaseOptions &&
           runtimeType == other.runtimeType &&
-          const ListEquality<Object?>(
-            DeepCollectionEquality(),
-          ).equals(
+          gqlDeepEquals(
             other.properties,
             properties,
           ));

packages/graphql/lib/src/core/policies.dart

@@ -307,9 +307,7 @@ class DefaultPolicies {
   bool operator ==(Object o) =>
       identical(this, o) ||
       (o is DefaultPolicies &&
-          const ListEquality<Object?>(
-            DeepCollectionEquality(),
-          ).equals(
+          gqlDeepEquals(
             o._getChildren(),
             _getChildren(),
           ));

packages/graphql/lib/src/core/query_manager.dart

@@ -1,5 +1,6 @@
 import 'dart:async';
 
+import 'package:graphql/src/utilities/helpers.dart';
 import 'package:graphql/src/utilities/response.dart';
 import 'package:meta/meta.dart';
 import 'package:collection/collection.dart';
@@ -19,20 +20,29 @@ import 'package:graphql/src/scheduler/scheduler.dart';
 
 import 'package:graphql/src/core/_query_write_handling.dart';
 
-bool Function(dynamic a, dynamic b) _deepEquals =
-    const DeepCollectionEquality().equals;
+typedef DeepEqualsFn = bool Function(dynamic a, dynamic b);
+
+/// The equality function used for comparing cached and new data.
+///
+/// You can alternatively provide [optimizedDeepEquals] for a faster
+/// equality check. Or provide your own via [GqlClient] constructor.
+DeepEqualsFn gqlDeepEquals = const DeepCollectionEquality().equals;
 
 class QueryManager {
   QueryManager({
     required this.link,
     required this.cache,
     this.alwaysRebroadcast = false,
+    DeepEqualsFn? deepEquals,
     bool deduplicatePollers = false,
   }) {
     scheduler = QueryScheduler(
       queryManager: this,
       deduplicatePollers: deduplicatePollers,
     );
+    if (deepEquals != null) {
+      gqlDeepEquals = deepEquals;
+    }
   }
 
   final Link link;
@@ -551,7 +561,8 @@ class QueryManager {
   ) =>
       cachedData != null &&
       query.latestResult != null &&
-      (alwaysRebroadcast || !_deepEquals(query.latestResult!.data, cachedData));
+      (alwaysRebroadcast ||
+          !gqlDeepEquals(query.latestResult!.data, cachedData));
 
   void setQuery(ObservableQuery<Object?> observableQuery) {
     queries[observableQuery.queryId] = observableQuery;

packages/graphql/lib/src/graphql_client.dart

@@ -26,12 +26,14 @@ class GraphQLClient implements GraphQLDataProxy {
     required this.cache,
     DefaultPolicies? defaultPolicies,
     bool alwaysRebroadcast = false,
+    DeepEqualsFn? deepEquals,
     bool deduplicatePollers = false,
   })  : defaultPolicies = defaultPolicies ?? DefaultPolicies(),
         queryManager = QueryManager(
           link: link,
           cache: cache,
           alwaysRebroadcast: alwaysRebroadcast,
+          deepEquals: deepEquals,
           deduplicatePollers: deduplicatePollers,
         );
 

packages/graphql/lib/src/utilities/helpers.dart

@@ -90,3 +90,46 @@ SanitizeVariables variableSanitizer(
                 toEncodable: sanitizeVariables,
               ),
             ) as Map<String, dynamic>;
+
+/// Compare two json-like objects for equality
+/// [a] and [b] must be one of
+/// - null
+/// - num
+/// - bool
+/// - String
+/// - List of above types
+/// - Map of above types
+///
+/// This is an alternative too [DeepCollectionEquality().equals],
+/// which is very slow and has O(n^2) complexity:
+bool optimizedDeepEquals(Object? a, Object? b) {
+  if (identical(a, b)) {
+    return true;
+  }
+  if (a == b) {
+    return true;
+  }
+  if (a is Map) {
+    if (b is! Map) {
+      return false;
+    }
+    if (a.length != b.length) return false;
+    for (var key in a.keys) {
+      if (!b.containsKey(key)) return false;
+      if (!optimizedDeepEquals(a[key], b[key])) return false;
+    }
+    return true;
+  }
+  if (a is List) {
+    if (b is! List) {
+      return false;
+    }
+    final length = a.length;
+    if (length != b.length) return false;
+    for (var i = 0; i < length; i++) {
+      if (!optimizedDeepEquals(a[i], b[i])) return false;
+    }
+    return true;
+  }
+  return false;
+}