fix executeUpdate

Author: hoc081098

lib/src/impl/real_storage.dart

@@ -330,19 +330,28 @@ class RealRxStorage<Key extends Object, Options,
   // Get streams (implements [RxStorage])
   //
 
+  @experimental
   @nonVirtual
   @override
   Future<void> executeUpdate<T extends Object>(
-      Key key, Decoder<T?> decoder, Encoder<T?> encoder,
-      [Options? options]) {
+    Key key,
+    Decoder<T?> decoder,
+    Transformer<T?> transformer,
+    Encoder<T?> encoder, [
+    Options? options,
+  ]) {
     assert(_debugAssertNotDisposed());
 
     return _enqueueWritingTask<void>(
       key,
       () async {
+        // Read
         final value =
             await _useStorage((s) => s.read<T>(key, decoder, options));
-        await _writeWithoutSynchronization(key, value, encoder, options);
+        // Modify
+        final transformed = transformer(value);
+        // Write
+        await _writeWithoutSynchronization(key, transformed, encoder, options);
       },
     );
   }

lib/src/interface/rx_storage.dart

@@ -1,10 +1,15 @@
 import 'dart:async';
 
+import 'package:meta/meta.dart';
+
 import '../../rx_storage.dart';
 import '../impl/real_storage.dart';
 import '../logger/logger.dart';
 import 'storage.dart';
 
+/// Transform a value to another value with same type.
+typedef Transformer<T> = T Function(T);
+
 /// Get [Stream]s by key from persistent storage.
 abstract class RxStorage<Key extends Object, Options>
     implements Storage<Key, Options> {
@@ -22,11 +27,15 @@ abstract class RxStorage<Key extends Object, Options>
 
   /// `Read–modify–write`.
   ///
-  /// Read value by [key], then decode with [decoder], then encode with [encoder]
-  /// and finally save computed value to persistent storage.
+  /// Read value by [key], then decode with [decoder],
+  /// then transform by [transformer],
+  /// then encode with [encoder]
+  /// and finally save decoded value to persistent storage.
+  @experimental
   Future<void> executeUpdate<T extends Object>(
     Key key,
     Decoder<T?> decoder,
+    Transformer<T?> transformer,
     Encoder<T?> encoder, [
     Options? options,
   ]);

test/storage/streams_test.dart

@@ -374,7 +374,8 @@ void main() {
       await rxStorage.executeUpdate<String>(
         'String',
         (s) => s as String?, // read
-        (s) => 'Transformed $s', // write
+        (s) => 'Transformed $s', // modify,
+        (s) => s, // write
       );
       expect(
         await rxStorage.getString('String'),
@@ -388,10 +389,10 @@ void main() {
       await rxStorage.executeUpdate<User>(
         'User',
         jsonStringToUser, // read
-        (user) => user == null
-            ? null
-            : userToJsonString(
-                user.withName('Transformed ${user.name}')), // write
+        (user) =>
+            user != null ? user.withName('Transformed ${user.name}') : null,
+        // modify
+        userToJsonString, // write
       );
       expect(
         await rxStorage.readUser(),
@@ -405,18 +406,19 @@ void main() {
         rxStorage.executeUpdate<String>(
           'String',
           (s) => s as String?, // read
-          (s) => 'Transformed $s', // write
+          (s) => 'Transformed $s', // modify,
+          (s) => s, // write
         ),
         throwsException,
       );
       expect(
         rxStorage.executeUpdate<User>(
           'User',
           jsonStringToUser, // read
-          (user) => user == null
-              ? null
-              : userToJsonString(
-                  user.withName('Transformed ${user.name}')), // write
+          (user) =>
+              user != null ? user.withName('Transformed ${user.name}') : null,
+          // modify
+          userToJsonString, // write
         ),
         throwsException,
       );