Override first, last and single on RealmResults (#1267)

* Override first, last and single on RealmResults

* Update CHANGELOG

* Add tests

* Address PR feedback

CHANGELOG.md

@@ -4,6 +4,7 @@
 * Added support binary data type. ([#1320](https://github.com/realm/realm-dart/pull/1320))
 * Extended `ClientResetError` to return the `backupFilePath` where the backup copy of the realm will be placed once the client reset process has completed. ([#1291](https://github.com/realm/realm-dart/pull/1291))
 * Added `CompensatingWriteError` containing detailed error information about the writes that have been reverted by the server due to permissions or subscription view restrictions. The `Configuration.flexibleSync.syncErrorHandler` will be invoked with this error type when this error occurs ([#1291](https://github.com/realm/realm-dart/pull/1291)).
+* Improve performance of elementAt, first, single and last on RealmResults ([#1261](https://github.com/realm/realm-dart/issues/1261), [#1262](https://github.com/realm/realm-dart/pull/1262), [#1267](https://github.com/realm/realm-dart/pull/1267)).
 
 ### Fixed
 * The constructors of all `SyncError` types are deprecated. The sync errors will be created only internally ([#1291](https://github.com/realm/realm-dart/pull/1291)).

lib/src/results.dart

@@ -16,7 +16,6 @@
 //
 ////////////////////////////////////////////////////////////////////////////////
 import 'dart:async';
-import 'dart:collection' as collection;
 import 'dart:ffi';
 
 import 'collections.dart';
@@ -28,7 +27,7 @@ import 'realm_object.dart';
 /// added to or deleted from the Realm that match the underlying query.
 ///
 /// {@category Realm}
-class RealmResults<T extends Object?> extends collection.IterableBase<T> with RealmEntity implements Finalizable {
+class RealmResults<T extends Object?> extends Iterable<T> with RealmEntity implements Finalizable {
   final RealmObjectMetadata? _metadata;
   final RealmResultsHandle _handle;
 
@@ -75,6 +74,34 @@ class RealmResults<T extends Object?> extends collection.IterableBase<T> with Re
   @override
   int get length => realmCore.getResultsCount(this);
 
+  @override
+  T get first {
+    if (length == 0) {
+      throw RealmStateError('No element');
+    }
+    return this[0];
+  }
+
+  @override
+  T get last {
+    if (length == 0) {
+      throw RealmStateError('No element');
+    }
+    return this[length - 1];
+  }
+
+  @override
+  T get single {
+    final l = length;
+    if (l > 1) {
+      throw RealmStateError('Too many elements');
+    }
+    if (l == 0) {
+      throw RealmStateError('No element');
+    }
+    return this[0];
+  }
+
   /// Creates a frozen snapshot of this query.
   RealmResults<T> freeze() {
     if (isFrozen) {

test/results_test.dart

@@ -741,4 +741,53 @@ Future<void> main([List<String>? args]) async {
     final items = realm.query<Product>(ids);
     expect(items.length, 0);
   });
+
+  test('RealmResults.first throws on empty', () async {
+    final config = Configuration.local([Dog.schema, Person.schema]);
+    final realm = getRealm(config);
+
+    final empty = Iterable<int>.empty();
+    // To illustrate the behavior of Iterable.single on a regular iterable
+    expect(() => empty.first, throws<StateError>('No element'));
+
+    expect(() => realm.all<Dog>().first, throws<StateError>('No element'));
+    expect(() => realm.all<Dog>().first, throws<RealmStateError>('No element'));
+  });
+
+  test('RealmResults.last throws on empty', () async {
+    final config = Configuration.local([Dog.schema, Person.schema]);
+    final realm = getRealm(config);
+
+    final empty = Iterable<int>.empty();
+    // To illustrate the behavior of Iterable.single on a regular iterable
+    expect(() => empty.last, throws<StateError>('No element'));
+
+    expect(() => realm.all<Dog>().last, throws<StateError>('No element'));
+    expect(() => realm.all<Dog>().last, throws<RealmStateError>('No element'));
+  });
+
+  test('RealmResult.single throws on empty', () {
+    final config = Configuration.local([Dog.schema, Person.schema]);
+    final realm = getRealm(config);
+
+    final empty = Iterable<int>.empty();
+    // To illustrate the behavior of Iterable.single on a regular iterable
+    expect(() => empty.single, throws<StateError>('No element'));
+
+    expect(() => realm.all<Dog>().single, throws<StateError>('No element'));
+    expect(() => realm.all<Dog>().single, throws<RealmStateError>('No element'));
+  });
+
+  test('RealmResult.single throws on two items', () {
+    final config = Configuration.local([Dog.schema, Person.schema]);
+    final realm = getRealm(config);
+
+    final twoItems = [1, 2];
+    // To illustrate the behavior of Iterable.single on a regular list
+    expect(() => twoItems.single, throws<StateError>('Too many elements'));
+
+    realm.write(() => realm.addAll([Dog('Fido'), Dog('Spot')]));
+    expect(() => realm.all<Dog>().single, throws<StateError>('Too many elements'));
+    expect(() => realm.all<Dog>().single, throws<RealmStateError>('Too many elements'));
+  });
 }