fix: #113

Defer observation until frame completion to ensure accurate result.

Author: LinXunFeng

lib/src/common/observer_widget.dart

@@ -317,27 +317,32 @@ class ObserverWidgetState<
               .contains(notification.runtimeType)) {
             final isIgnoreInnerCanHandleObserve =
                 ScrollUpdateNotification != notification.runtimeType;
-            final platform = Theme.of(context).platform;
-            if (kIsWeb ||
-                platform == TargetPlatform.windows ||
-                platform == TargetPlatform.macOS) {
+            WidgetsBinding.instance.endOfFrame.then((_) {
+              // Need to wait for frame end to avoid inaccurate observation
+              // result, reasons as follows
+              //
+              // ======================== WEB ========================
+              //
               // Getting bad observation result because scrolling in Flutter Web
               // with mouse wheel is not smooth.
               // https://github.com/flutter/flutter/issues/78708
               // https://github.com/flutter/flutter/issues/78634
               //
               // issue
               // https://github.com/LinXunFeng/flutter_scrollview_observer/issues/31
-              WidgetsBinding.instance.addPostFrameCallback((_) {
-                handleContexts(
-                  isIgnoreInnerCanHandleObserve: isIgnoreInnerCanHandleObserve,
-                );
-              });
-            } else {
+              //
+              // ======================== APP ========================
+              //
+              // When using ScrollController's animateTo with a value exceeding
+              // the maximum scroll range, it will lead to inaccurate
+              // observation result.
+              //
+              // issue
+              // https://github.com/fluttercandies/flutter_scrollview_observer/issues/113
               handleContexts(
                 isIgnoreInnerCanHandleObserve: isIgnoreInnerCanHandleObserve,
               );
-            }
+            });
           }
           return false;
         },
@@ -449,7 +454,9 @@ class ObserverWidgetState<
     bool isIgnoreInnerCanHandleObserve = true,
   }) {
     if (!isIgnoreInnerCanHandleObserve) {
-      if (!innerCanHandleObserve) return null;
+      if (!innerCanHandleObserve) {
+        return null;
+      }
       updateInnerCanHandleObserve();
     }
 
@@ -467,7 +474,9 @@ class ObserverWidgetState<
 
     final isHandlingScroll =
         widget.sliverController?.innerIsHandlingScroll ?? false;
-    if (isHandlingScroll) return null;
+    if (isHandlingScroll) {
+      return null;
+    }
 
     List<BuildContext> ctxs = fetchTargetSliverContexts();
 

test/grid_observer_test.dart

@@ -92,6 +92,7 @@ void main() {
       curve: Curves.easeInOut,
     );
     await tester.pumpAndSettle();
+    await tester.pump(observerController.observeIntervalForScrolling);
     expect(pageController.page, 3);
     expect(isCalledOnObserve, isFalse);
 
@@ -100,6 +101,8 @@ void main() {
       duration: const Duration(milliseconds: 100),
       curve: Curves.easeInOut,
     );
+    await tester.pumpAndSettle();
+    await tester.pump(observerController.observeIntervalForScrolling);
     expect(isCalledOnObserve, isTrue);
 
     scrollController.dispose();

test/list_observer_test.dart

@@ -6,15 +6,18 @@ import 'package:scrollview_observer/src/common/observer_widget_tag_manager.dart'
 
 void main() {
   Widget getListView({
+    ScrollPhysics? physics,
     ScrollController? scrollController,
     int itemCount = 100,
     bool isFixedHeight = false,
     double? cacheExtent,
     NullableIndexedWidgetBuilder? itemBuilder,
+    IndexedWidgetBuilder? separatorBuilder,
   }) {
     return Directionality(
       textDirection: TextDirection.ltr,
       child: ListView.separated(
+        physics: physics,
         controller: scrollController,
         itemBuilder: (ctx, index) {
           if (itemBuilder != null) {
@@ -31,9 +34,10 @@ void main() {
             ),
           );
         },
-        separatorBuilder: (ctx, index) {
-          return const SizedBox(height: 10);
-        },
+        separatorBuilder: separatorBuilder ??
+            (ctx, index) {
+              return const SizedBox(height: 10);
+            },
         itemCount: itemCount,
         cacheExtent: cacheExtent,
       ),
@@ -131,6 +135,7 @@ void main() {
       curve: Curves.easeInOut,
     );
     await tester.pumpAndSettle();
+    await tester.pump(observerController.observeIntervalForScrolling);
     expect(pageController.page, 3);
     expect(isCalledOnObserve, isFalse);
 
@@ -139,12 +144,56 @@ void main() {
       duration: const Duration(milliseconds: 100),
       curve: Curves.easeInOut,
     );
+    await tester.pumpAndSettle();
+    await tester.pump(observerController.observeIntervalForScrolling);
     expect(isCalledOnObserve, isTrue);
 
     scrollController.dispose();
     pageController.dispose();
   });
 
+  testWidgets('Check observation result after fast scrolling', (tester) async {
+    // Regression test for https://github.com/fluttercandies/flutter_scrollview_observer/issues/113
+    final scrollController = ScrollController();
+    final observerController = ListObserverController(
+      controller: scrollController,
+    );
+    const int itemCount = 50;
+    Widget widget = getListView(
+      scrollController: scrollController,
+      physics: const ClampingScrollPhysics(),
+      itemCount: itemCount,
+      itemBuilder: (context, index) => Text('$index'),
+      separatorBuilder: (context, index) => const SizedBox.shrink(),
+    );
+    int? lastItemIndex;
+    widget = ListViewObserver(
+      child: widget,
+      controller: observerController,
+      triggerOnObserveType: ObserverTriggerOnObserveType.directly,
+      onObserve: (result) {
+        lastItemIndex = result.displayingChildModelList.last.index;
+      },
+    );
+    await tester.pumpWidget(widget);
+
+    var result = await observerController.dispatchOnceObserve();
+    final offset = result.observeResult?.viewport.offset
+        as ScrollPositionWithSingleContext;
+    final maxScrollExtent = offset.maxScrollExtent;
+    scrollController.animateTo(
+      maxScrollExtent * 1000,
+      duration: const Duration(milliseconds: 100),
+      curve: Curves.easeOut,
+    );
+
+    await tester.pumpAndSettle();
+    await tester.pump(observerController.observeIntervalForScrolling);
+    expect(lastItemIndex, itemCount - 1);
+
+    scrollController.dispose();
+  });
+
   group('Scroll to index', () {
     testWidgets('Dynamic Height', (tester) async {
       final scrollController = ScrollController();

test/sliver_observer_test.dart

@@ -304,6 +304,7 @@ void main() {
       curve: Curves.easeInOut,
     );
     await tester.pumpAndSettle();
+    await tester.pump(observerController.observeIntervalForScrolling);
     expect(pageController.page, 3);
     expect(isCalledOnObserve, isFalse);
 
@@ -312,6 +313,8 @@ void main() {
       duration: const Duration(milliseconds: 100),
       curve: Curves.easeInOut,
     );
+    await tester.pumpAndSettle();
+    await tester.pump(observerController.observeIntervalForScrolling);
     expect(isCalledOnObserve, isTrue);
 
     scrollController.dispose();