Add RealmResults.isValid (#1231)

* Add RealmResults.isValid

* Add tests

* Update CHANGELOG

CHANGELOG.md

@@ -1,6 +1,7 @@
 ## vNext (TBD)
 
 ### Enhancements
+* Add `RealmResults.isValid` ([#1231](https://github.com/realm/realm-dart/pull/1231)).
 * Support `Decimal128` datatype ([#1192](https://github.com/realm/realm-dart/pull/1192)).
 * Realm logging is extended to support logging of all Realm storage level messages. (Core upgrade).
 * Realm.logger now prints by default to the console from the first Isolate that initializes a Realm in the application. ([#1226](https://github.com/realm/realm-dart/pull/1226)).

lib/src/native/realm_core.dart

@@ -1113,6 +1113,14 @@ class _RealmCore {
     });
   }
 
+  bool resultsIsValid(RealmResults results) {
+    return using((arena) {
+      final is_valid = arena<Bool>();
+      _realmLib.invokeGetBool(() => _realmLib.realm_results_is_valid(results.handle._pointer, is_valid));
+      return is_valid.value;
+    });
+  }
+
   CollectionChanges getCollectionChanges(RealmCollectionChangesHandle changes) {
     return using((arena) {
       final out_num_deletions = arena<Size>();

lib/src/results.dart

@@ -38,6 +38,9 @@ class RealmResults<T extends Object?> extends collection.IterableBase<T> with Re
     setRealm(realm);
   }
 
+  /// Gets a value indicating whether this collection is still valid to use.
+  bool get isValid => realmCore.resultsIsValid(this);
+
   /// Returns the element of type `T` at the specified [index].
   T operator [](int index) => elementAt(index);
 

test/results_test.dart

@@ -663,6 +663,39 @@ Future<void> main([List<String>? args]) async {
     }
   });
 
+  test('RealmResults.isValid', () {
+    final config = Configuration.local([Team.schema, Person.schema]);
+    final realm = getRealm(config);
+
+    final alice = Person('Alice');
+    final bob = Person('Bob');
+    final carol = Person('Carol');
+    final dan = Person('Dan');
+
+    final team = realm.write(() {
+      return realm.add(Team('Class of 92', players: [alice, bob, carol, dan]));
+    });
+
+    final players = team.players;
+    final playersAsResults = team.players.asResults();
+
+    expect(players.isValid, isTrue);
+    expect(playersAsResults.isValid, isTrue);
+    expect(playersAsResults, [alice, bob, carol, dan]);
+
+    realm.write(() => realm.delete(team));
+
+    expect(team.isValid, isFalse); // dead object
+    expect(players.isValid, isFalse); // parent is dead
+    expect(() => players.isEmpty, throwsException); // illegal to access properties on dead object
+    expect(playersAsResults.isValid, isTrue); // Results are still valid..
+    expect(playersAsResults, isEmpty); // .. but obviously empty
+    expect(() => playersAsResults.freeze(), returnsNormally);
+    final frozeResults = playersAsResults.freeze();
+    expect(frozeResults.isFrozen, isTrue);
+    expect(frozeResults, isEmpty);
+  });
+
   test('query by condition on decimal128', () {
     final config = Configuration.local([ObjectWithDecimal.schema]);
     final realm = getRealm(config);