Add restorePosition method.

Author: mgenware

CHANGELOG.md

@@ -1,3 +1,7 @@
+## 4.2.0
+
+- Add `restorePosition` method.
+
 ## 4.1.0
 
 - Add `readInto` method.

lib/src/random_access_source.dart

@@ -59,4 +59,14 @@ abstract class RandomAccessSource {
 
   /// Closes the source.
   Future<void> close();
+
+  /// Restores the position after executing the given [action].
+  Future<void> restorePosition<T>(Future<T> Function() action) async {
+    final currentPosition = await position();
+    try {
+      await action();
+    } finally {
+      await seek(currentPosition);
+    }
+  }
 }

pubspec.yaml

@@ -1,6 +1,6 @@
 name: random_access_source
 description: A shared interface for common random access data.
-version: 4.1.0
+version: 4.2.0
 repository: https://github.com/flutter-cavalry/random_access_source
 
 environment:

test/raf_test.dart

@@ -109,4 +109,29 @@ void main() {
     expect(await src.readByte(), -1);
     await src.close();
   });
+
+  test('RestorePosition', () async {
+    final src = await rafSource();
+    await src.seek(10);
+    expect(await src.position(), 10);
+
+    await src.restorePosition(() async {
+      expect(await src.position(), 10);
+      expect(await src.readByte(), flutterIcon[10]);
+      expect(await src.position(), 11);
+    });
+    expect(await src.position(), 10);
+
+    try {
+      await src.restorePosition(() async {
+        await src.seek(20);
+        throw Exception('Test Exception');
+      });
+    } catch (e) {
+      // Expected exception
+    }
+    expect(await src.position(), 10);
+
+    await src.close();
+  });
 }