fix: stacktrace in computed (#988)

* fix: preserve stacktrace in Computed and Reaction

* tests: Add test cases for testing preserving stacktrace in Computed and Observer

* chore: bump version to 2.3.1 and update changelog with bug fix for preserving stacktrace in Computed and Reaction functions

---------

Co-authored-by: Pavan Podila <[email protected]>

flutter_mobx/test/flutter_mobx_test.dart

@@ -176,6 +176,34 @@ void main() {
     expect(exception, isInstanceOf<MobXCaughtException>());
   });
 
+  testWidgets(
+    'Observer should print full stacktrace of error coming from computed',
+    (tester) async {
+      late StackTrace stackTrace;
+      final errorWrapper = await _testThrowingObserverWithStackTrace(
+        tester,
+        firstError: true,
+        actionThrows: (tester) async {
+          await tester.pumpWidget(
+            Observer(
+              builder: (context) {
+                Computed(() {
+                  try {
+                    throw Exception();
+                  } on Exception catch (e, st) {
+                    stackTrace = st;
+                    rethrow;
+                  }
+                }).value;
+              },
+            ),
+          );
+        },
+      );
+      expect(errorWrapper.stackTrace, stackTrace);
+    },
+  );
+
   testWidgets('Observer unmount should dispose Reaction', (tester) async {
     final mock = MockReaction();
     when(() => mock.hasObservables).thenReturn(true);
@@ -345,18 +373,45 @@ Future<MobXCaughtException> _testThrowingObserver(
   WidgetTester tester,
   Object errorToThrow,
 ) async {
-  late Object exception;
+  return (await _testThrowingObserverWithStackTrace(
+    tester,
+    actionThrows: (tester) async {
+      final count = Observable(0);
+      await tester.pumpWidget(FlutterErrorThrowingObserver(
+        errorToThrow: errorToThrow,
+        builder: (context) => Text(count.value.toString()),
+      ));
+      count.value++;
+    },
+  ))
+      .exception as MobXCaughtException;
+}
+
+class _ErrorWrapper {
+  final Object exception;
+  final StackTrace? stackTrace;
+
+  _ErrorWrapper({required this.exception, required this.stackTrace});
+}
+
+Future<_ErrorWrapper> _testThrowingObserverWithStackTrace(
+  WidgetTester tester, {
+  required Future<void> Function(WidgetTester tester) actionThrows,
+  bool firstError = false,
+}) async {
+  _ErrorWrapper? errorWrapper;
   final prevOnError = FlutterError.onError;
-  FlutterError.onError = (details) => exception = details.exception;
+  FlutterError.onError = (details) {
+    if (firstError && errorWrapper != null) {
+      return;
+    }
+    errorWrapper =
+        _ErrorWrapper(exception: details.exception, stackTrace: details.stack);
+  };
 
   try {
-    final count = Observable(0);
-    await tester.pumpWidget(FlutterErrorThrowingObserver(
-      errorToThrow: errorToThrow,
-      builder: (context) => Text(count.value.toString()),
-    ));
-    count.value++;
-    return exception as MobXCaughtException;
+    await actionThrows(tester);
+    return errorWrapper!;
   } finally {
     FlutterError.onError = prevOnError;
   }

mobx/CHANGELOG.md

@@ -1,3 +1,7 @@
+## 2.3.1
+
+- Fix preserving stacktrace in Computed and Reaction when exception thrown inside argument function
+
 ## 2.3.0+1
 
 - `pubspec.yaml` updated to include homepage and topics

mobx/lib/src/core/computed.dart

@@ -98,7 +98,7 @@ class Computed<T> extends Atom implements Derivation, ObservableValue<T> {
     }
 
     if (_context._hasCaughtException(this)) {
-      throw _errorValue!;
+      Error.throwWithStackTrace(_errorValue!, _errorValue!._stackTrace);
     }
 
     return _value as T;

mobx/lib/src/core/reaction.dart

@@ -91,7 +91,7 @@ class ReactionImpl with DebugCreationStack implements Reaction {
     }
 
     if (_context._hasCaughtException(this)) {
-      _reportException(_errorValue!);
+      _reportException(_errorValue!, _errorValue!.stackTrace);
     }
 
     if (notify) {
@@ -120,7 +120,7 @@ class ReactionImpl with DebugCreationStack implements Reaction {
       } on Object catch (e, s) {
         // Note: "on Object" accounts for both Error and Exception
         _errorValue = MobXCaughtException(e, stackTrace: s);
-        _reportException(_errorValue!);
+        _reportException(_errorValue!, s);
       }
     }
 
@@ -165,15 +165,18 @@ class ReactionImpl with DebugCreationStack implements Reaction {
     // Not applicable right now
   }
 
-  void _reportException(Object exception) {
+  void _reportException(Object exception, StackTrace? stackTrace) {
     if (_onError != null) {
       _onError!(exception, this);
       return;
     }
 
     if (_context.config.disableErrorBoundaries == true) {
-      // ignore: only_throw_errors
-      throw exception;
+      if (stackTrace != null) {
+        Error.throwWithStackTrace(exception, stackTrace);
+      } else {
+        throw exception;
+      }
     }
 
     if (_context.isSpyEnabled) {

mobx/lib/version.dart

@@ -1,4 +1,4 @@
 // Generated via set_version.dart. !!!DO NOT MODIFY BY HAND!!!
 
 /// The current version as per `pubspec.yaml`.
-const version = '2.3.0+1';
+const version = '2.3.1';

mobx/pubspec.yaml

@@ -1,5 +1,5 @@
 name: mobx
-version: 2.3.0+1
+version: 2.3.1
 description: "MobX is a library for reactively managing the state of your applications. Use the power of observables, actions, and reactions to supercharge your Dart and Flutter apps."
 
 repository: https://github.com/mobxjs/mobx.dart

mobx/test/exceptions_test.dart

@@ -26,4 +26,21 @@ void main() {
       expect(ex.stackTrace, isNotNull);
     }
   });
+
+  test('should preserve stacktrace', () async {
+    late StackTrace stackTrace;
+    try {
+      Computed(() {
+        try {
+          throw Exception();
+        } on Exception catch (e, st) {
+          stackTrace = st;
+          rethrow;
+        }
+      }).value;
+    } on MobXCaughtException catch (e, st) {
+      expect(st, stackTrace);
+      expect(st, e.stackTrace);
+    }
+  });
 }