Snapshot results on iterating (#258)

* Snapshot results of RealmObjects when iteration starts. Fixes #166

CHANGELOG.md

@@ -16,6 +16,9 @@ x.x.x Release notes (yyyy-MM-dd)
 ### Compatibility
 * Dart ^2.15 on Windows, MacOS and Linux
 
+### Fixed
+* Snapshot the results collection when iterating if the items are realm objects. ([#258](https://github.com/realm/realm-dart/pull/258))
+
 
 0.2.0+alpha Release notes (2022-01-31)
 ==============================================================

flutter/realm_flutter/ios/Classes/RealmPlugin.m

@@ -43,6 +43,7 @@ void dummy(void) {
   realm_results_get_object(NULL, 0);
   realm_list_size(NULL, 0);
   realm_dart_results_add_notification_callback(NULL, NULL, NULL, NULL);
+  realm_results_snapshot(NULL);
 }
 
 @end

lib/src/native/realm_core.dart

@@ -519,6 +519,11 @@ class _RealmCore {
     return _realmLib.realm_equals(first.handle._pointer.cast(), second.handle._pointer.cast());
   }
 
+  RealmResultsHandle resultsSnapshot(RealmResults results) {
+    final resultsPointer = _realmLib.invokeGetPointer(() => _realmLib.realm_results_snapshot(results.handle._pointer));
+    return RealmResultsHandle._(resultsPointer);
+  }
+
   bool objectIsValid(RealmObject object) {
     return _realmLib.realm_object_is_valid(object.handle._pointer);
   }

lib/src/results.dart

@@ -30,8 +30,10 @@ import 'realm_class.dart';
 class RealmResults<T extends RealmObject> extends collection.IterableBase<T> {
   final RealmResultsHandle _handle;
 
-  /// The Realm isntance this collection belongs to.
+  /// The Realm instance this collection belongs to.
   final Realm realm;
+
+  final _supportsSnapshot = <T>[] is List<RealmObject?>;
 
   RealmResults._(this._handle, this.realm);
 
@@ -56,7 +58,14 @@ class RealmResults<T extends RealmObject> extends collection.IterableBase<T> {
 
   /// Returns a new `Iterator` that allows iterating the elements in this `RealmResults`.
   @override
-  Iterator<T> get iterator => _RealmResultsIterator(this);
+  Iterator<T> get iterator {
+    var results = this;
+    if (_supportsSnapshot) {
+      final handle = realmCore.resultsSnapshot(this);
+      results = RealmResultsInternal.create<T>(handle, realm);
+    }
+    return _RealmResultsIterator(results);
+  }
 
   /// The number of values in this `Results` collection.
   @override

src/realm_dart.cpp

@@ -68,6 +68,7 @@ void dummy(void) {
   realm_results_get_object(nullptr, 0);
   realm_list_size(nullptr, 0);
   realm_results_add_notification_callback(nullptr, nullptr, nullptr, nullptr, nullptr, nullptr);
+  realm_results_snapshot(nullptr);
 #if (ANDROID)
   realm_android_dummy();
 #endif

test/results_test.dart

@@ -163,6 +163,45 @@ Future<void> main([List<String>? args]) async {
     realm.close();
   });
 
+  test('Results snapshot iteration test', () {
+    var config = Configuration([Team.schema, Person.schema]);
+    var realm = Realm(config);
+
+    //Create two teams
+    realm.write(() {
+      realm.add(Team("team One"));
+      realm.add(Team("team Two"));
+    });
+
+    //Reload teams from realm and ensure they exist
+    var teams = realm.all<Team>();
+    expect(teams.length, 2);
+
+    //Adding new teams to real while iterating through them.
+    //Iterator use a snapshot of results collection and ignores newly added teams.
+    List<Team> list = [];
+    for (Team team in teams) {
+      list.add(team);
+      realm.write(() {
+        realm.add(Team("new team"));
+      });
+    }
+    //Ensure list size is the same like teams collection size was at the beginnig
+    expect(list.length, 2);
+
+    //Ensure teams collection is increased
+    expect(teams.length, 4);
+
+    //Iterating teams again will create a snapshot with the newly added items
+    list.clear();
+    for (Team team in teams) {
+      list.add(team);
+    }
+    expect(list.length, teams.length);
+
+    realm.close();
+  });
+
   test('Results query', () {
     var config = Configuration([Car.schema]);
     var realm = Realm(config);
@@ -212,7 +251,6 @@ Future<void> main([List<String>? args]) async {
     realm.close();
   });
 
-
   test('Query results with no arguments throws', () {
     var config = Configuration([Car.schema]);
     var realm = Realm(config);