feat: Add flush method to boxes

hive/lib/src/backend/js/storage_backend_js.dart

@@ -205,4 +205,7 @@ class StorageBackendJs extends StorageBackend {
         : WorkerGlobalScope.instance.indexedDB;
     return indexDB!.deleteDatabase(_db.name!);
   }
+
+  @override
+  Future<void> flush() => Future.value();
 }

hive/lib/src/backend/storage_backend.dart

@@ -34,6 +34,9 @@ abstract class StorageBackend {
 
   /// Clear database and delete from disk
   Future<void> deleteFromDisk();
+
+  /// Flush all changes to disk
+  Future<void> flush();
 }
 
 /// Abstract database manager

hive/lib/src/backend/storage_backend_memory.dart

@@ -67,4 +67,7 @@ class StorageBackendMemory extends StorageBackend {
   Future<void> deleteFromDisk() {
     throw UnsupportedError('This operation is unsupported for memory boxes.');
   }
+
+  @override
+  Future<void> flush() => Future.value();
 }

hive/lib/src/backend/vm/storage_backend_vm.dart

@@ -226,4 +226,11 @@ class StorageBackendVm extends StorageBackend {
       await _file.delete();
     });
   }
+
+  @override
+  Future<void> flush() {
+    return _sync.syncWrite(() async {
+      await writeRaf.flush();
+    });
+  }
 }

hive/lib/src/box/box_base.dart

@@ -124,4 +124,7 @@ abstract class BoxBase<E> {
   ///
   /// In the browser, the IndexedDB database is being removed.
   Future<void> deleteFromDisk();
+
+  /// Flushes all pending changes of the box to disk.
+  Future<void> flush();
 }

hive/lib/src/box/box_base_impl.dart

@@ -254,4 +254,7 @@ class _NullBoxBase<E> implements BoxBase<E> {
 
   @override
   Never watch({key}) => throw UnimplementedError();
+
+  @override
+  Never flush() => throw UnimplementedError();
 }

hive/lib/src/box/box_impl.dart

@@ -103,4 +103,9 @@ class BoxImpl<E> extends BoxBaseImpl<E> implements Box<E> {
     }
     return map;
   }
+
+  @override
+  Future<void> flush() async {
+    await backend.flush();
+  }
 }

hive/lib/src/box/lazy_box_impl.dart

@@ -92,4 +92,9 @@ class LazyBoxImpl<E> extends BoxBaseImpl<E> implements LazyBox<E> {
 
     await performCompactionIfNeeded();
   }
+
+  @override
+  Future<void> flush() async {
+    await backend.flush();
+  }
 }