Query - reimplement findFirst() using DataVisitor to avoid side-effects

Author: Ivan Dlugos

objectbox/lib/src/native/bindings/data_visitor.dart

@@ -33,6 +33,10 @@ import 'bindings.dart';
 ///     visitor.userData, offset, limit);
 ///   visitor.close(); // make sure to close the visitor
 ///   checkObx(err);
+///
+/// TODO do we actually need to go through these hoops?
+/// Why don't we create a `Pointer.fromFunction((...) => ...), 0)` as needed?
+/// Or better yet, create them only once and reuse (where possible)?
 
 int _lastId = 0;
 final _callbacks = <int, bool Function(Pointer<Uint8> dataPtr, int length)>{};
@@ -49,12 +53,14 @@ int _forwarder(Pointer<Void> callbackId, Pointer<Void> dataPtr, int size) {
   return callback(dataPtr.cast<Uint8>(), size) ? 1 : 0;
 }
 
+final Pointer<NativeFunction<obx_data_visitor>> _forwarderPtr =
+    Pointer.fromFunction(_forwarder, 0);
+
 /// A data visitor wrapper/forwarder to be used where obx_data_visitor is expected.
 class DataVisitor {
   final Pointer<Int64> _idPtr = malloc<Int64>();
 
-  Pointer<NativeFunction<obx_data_visitor>> get fn =>
-      Pointer.fromFunction(_forwarder, 0);
+  Pointer<NativeFunction<obx_data_visitor>> get fn => _forwarderPtr;
 
   Pointer<Void> get userData => _idPtr.cast<Void>();
 

objectbox/lib/src/native/query/query.dart

@@ -611,7 +611,7 @@ class _ConditionGroupAll extends _ConditionGroup {
 /// Use [property] to only return values or an aggregate of a single Property.
 class Query<T> {
   final Pointer<OBX_query> _cQuery;
-  final Store store;
+  final Store store; // TODO make private
   final EntityDefinition<T> _entity;
 
   int get entityId => _entity.model.id.id;
@@ -672,15 +672,23 @@ class Query<T> {
   void close() => checkObx(C.query_close(_cQuery));
 
   /// Finds Objects matching the query and returns the first result or null
-  /// if there are no results.
-  /// Warning: this implicitly sets offset=0 & limit=1 and leaves them set.
-  /// In the future, this behaviour will change.
+  /// if there are no results. Note: [offset] and [limit] are respected, if set.
   T? findFirst() {
-    // TODO move to the core to avoid side-effects
-    offset(0);
-    limit(1);
-    final list = find();
-    return (list.isEmpty ? null : list[0]);
+    T? result;
+    final visitor = DataVisitor((Pointer<Uint8> dataPtr, int length) {
+      result = _entity.objectFromFB(store, dataPtr.asTypedList(length));
+      return false; // we only want to visit the first element
+    });
+
+    try {
+      store.runInTransaction(TxMode.read, () {
+        checkObx(C.query_visit(_cQuery, visitor.fn, visitor.userData));
+      });
+    } finally {
+      visitor.close();
+    }
+
+    return result;
   }
 
   /// Finds Objects matching the query and returns their IDs.