Support efficient skip on RealmResults

realm · Sep 1, 2023 · 4f8eca8 · 4f8eca8
1 parent 29ff9f1
commit 4f8eca8
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Showing 2 changed files with 43 additions and 7 deletions.
diff --git a/lib/src/results.dart b/lib/src/results.dart
@@ -30,11 +30,13 @@ import 'realm_object.dart';
 class RealmResults<T extends Object?> extends Iterable<T> with RealmEntity implements Finalizable {
   final RealmObjectMetadata? _metadata;
   final RealmResultsHandle _handle;
+  final int _skipOffset; // to support skip efficiently
 
   final _supportsSnapshot = <T>[] is List<RealmObjectBase?>;
 
-  RealmResults._(this._handle, Realm realm, this._metadata) {
+  RealmResults._(this._handle, Realm realm, this._metadata, [this._skipOffset = 0]) {
     setRealm(realm);
+    assert(length >= 0);
   }
 
   /// Gets a value indicating whether this collection is still valid to use.
@@ -47,11 +49,11 @@ class RealmResults<T extends Object?> extends Iterable<T> with RealmEntity imple
   @override
   T elementAt(int index) {
     if (this is RealmResults<RealmObjectBase>) {
-      final handle = realmCore.resultsGetObjectAt(this, index);
+      final handle = realmCore.resultsGetObjectAt(this, _skipOffset + index);
       final accessor = RealmCoreAccessor(metadata, realm.isInMigration);
       return RealmObjectInternal.create(T, realm, handle, accessor) as T;
     } else {
-      return realmCore.resultsGetElementAt(this, index) as T;
+      return realmCore.resultsGetElementAt(this, _skipOffset + index) as T;
     }
   }
 
@@ -72,7 +74,7 @@ class RealmResults<T extends Object?> extends Iterable<T> with RealmEntity imple
 
   /// The number of values in this `Results` collection.
   @override
-  int get length => realmCore.getResultsCount(this);
+  int get length => realmCore.getResultsCount(this) - _skipOffset;
 
   @override
   T get first {
@@ -102,6 +104,12 @@ class RealmResults<T extends Object?> extends Iterable<T> with RealmEntity imple
     return this[0];
   }
 
+  @override
+  RealmResults<T> skip(int count) {
+    RangeError.checkValueInInterval(count, 0, length, "count");
+    return RealmResults<T>._(_handle, realm, _metadata, _skipOffset + count);
+  }
+
   /// Creates a frozen snapshot of this query.
   RealmResults<T> freeze() {
     if (isFrozen) {
@@ -211,9 +219,9 @@ class _RealmResultsIterator<T extends Object?> implements Iterator<T> {
   int _index;
   T? _current;
 
-  _RealmResultsIterator(RealmResults<T> results)
+  _RealmResultsIterator(RealmResults<T> results, {int index = 0})
       : _results = results,
-        _index = -1;
+        _index = index - 1;
 
   @override
   T get current => _current ??= _results[_index];

diff --git a/test/results_test.dart b/test/results_test.dart
@@ -18,8 +18,10 @@
 
 // ignore_for_file: unused_local_variable
 
+import 'dart:math';
 import 'dart:typed_data';
 
+import 'package:ffi/ffi.dart';
 import 'package:test/test.dart' hide test, throws;
 import '../lib/realm.dart';
 import 'test.dart';
@@ -832,7 +834,7 @@ Future<void> main([List<String>? args]) async {
 
     queryWithListArg("uuidProp", [uid_1, uid_2, null], expected: 2);
     queryWithListArg("nullableUuidProp", [null], expected: 3);
-  
+
     queryWithListArg("intProp", [1, 2, null], expected: 2);
     queryWithListArg("nullableIntProp", [null], expected: 3);
 
@@ -911,4 +913,30 @@ Future<void> main([List<String>? args]) async {
     expect(result.length, 1);
     expect(result.first.name, 'secondary school 1');
   });
+
+  test('RealmResults.skip', () {
+    final config = Configuration.local([Task.schema]);
+    final realm = getRealm(config);
+    const max = 10;
+    realm.write(() {
+      realm.addAll(List.generate(max, (_) => Task(ObjectId())));
+    });
+
+    final results = realm.all<Task>();
+    for (var i = 0; i < max; i++) {
+      expect(results.skip(i).length, max - i);
+      for (var j = 0; j < max - i; j++) {
+        expect(results.skip(i)[j], results[i + j]);
+        expect(results.skip(i).skip(j)[0], results[i + j]); // chained skip
+      }
+      expect(
+        () => results.skip(max + i + 1),
+        throwsA(isA<RangeError>().having((e) => e.invalidValue, 'count', max + i + 1)),
+      );
+      expect(
+        () => results.skip(-i - 1),
+        throwsA(isA<RangeError>().having((e) => e.invalidValue, 'count', -i - 1)),
+      );
+    }
+  });
 }