feat(ChatScrollObserver): add customAdjustPosition

Author: LinXunFeng

lib/src/utils/src/chat/chat_observer_scroll_physics_mixin.dart

@@ -37,6 +37,27 @@ mixin ChatObserverScrollPhysicsMixin on ScrollPhysics {
       ));
       return adjustPosition;
     }
+
+    // Customize the adjustPosition.
+    double? customAdjustPosition = observer.customAdjustPosition?.call(
+      ChatScrollObserverCustomAdjustPositionModel(
+        oldPosition: oldPosition,
+        newPosition: newPosition,
+        isScrolling: isScrolling,
+        velocity: velocity,
+        adjustPosition: adjustPosition,
+        observer: observer,
+      ),
+    );
+    if (customAdjustPosition != null) {
+      _handlePositionCallback(ChatScrollObserverHandlePositionResultModel(
+        type: ChatScrollObserverHandlePositionType.keepPosition,
+        mode: observer.innerMode,
+        changeCount: observer.changeCount,
+      ));
+      return customAdjustPosition;
+    }
+
     final model = observer.observeRefItem();
     if (model == null) {
       _handlePositionCallback(ChatScrollObserverHandlePositionResultModel(
@@ -60,6 +81,7 @@ mixin ChatObserverScrollPhysicsMixin on ScrollPhysics {
         newPosition: newPosition,
         isScrolling: isScrolling,
         velocity: velocity,
+        adjustPosition: adjustPosition,
         observer: observer,
         currentItemModel: model,
       ),

lib/src/utils/src/chat/chat_scroll_observer.dart

@@ -74,8 +74,15 @@ class ChatScrollObserver {
   ChatScrollObserverHandleMode innerMode = ChatScrollObserverHandleMode.normal;
 
   /// Customize the delta of the adjustPosition.
+  ///
+  /// If the return value is null, the default processing will be performed.
   ChatScrollObserverCustomAdjustPositionDelta? customAdjustPositionDelta;
 
+  /// Customize the scroll position for new viewport dimensions.
+  ///
+  /// If the return value is null, the default processing will be performed.
+  ChatScrollObserverCustomAdjustPosition? customAdjustPosition;
+
   /// Observation result of reference subparts after ScrollView children update.
   ListViewObserveDisplayingChildModel? observeRefItem() {
     return observerController.observeItem(
@@ -112,6 +119,7 @@ class ChatScrollObserver {
     int refItemRelativeIndexAfterUpdate = 0,
     int refItemIndex = 0,
     int refItemIndexAfterUpdate = 0,
+    ChatScrollObserverCustomAdjustPosition? customAdjustPosition,
     ChatScrollObserverCustomAdjustPositionDelta? customAdjustPositionDelta,
   }) async {
     innerMode = mode;
@@ -200,6 +208,7 @@ class ChatScrollObserver {
     innerRefItemIndex = _innerRefItemIndex;
     innerRefItemIndexAfterUpdate = _innerRefItemIndexAfterUpdate;
     innerRefItemLayoutOffset = _innerRefItemLayoutOffset;
+    this.customAdjustPosition = customAdjustPosition;
     this.customAdjustPositionDelta = customAdjustPositionDelta;
 
     // When the heights of items are similar, the viewport will not call

lib/src/utils/src/chat/chat_scroll_observer_model.dart

@@ -39,6 +39,9 @@ class ChatScrollObserverCustomAdjustPositionDeltaModel {
   /// The current velocity of the scroll position.
   final double velocity;
 
+  /// The scroll position should be given for new viewport dimensions.
+  final double adjustPosition;
+
   /// The [ChatScrollObserver] instance.
   final ChatScrollObserver observer;
 
@@ -50,7 +53,37 @@ class ChatScrollObserverCustomAdjustPositionDeltaModel {
     required this.newPosition,
     required this.isScrolling,
     required this.velocity,
+    required this.adjustPosition,
     required this.observer,
     required this.currentItemModel,
   });
 }
+
+class ChatScrollObserverCustomAdjustPositionModel {
+  /// The old position.
+  final ScrollMetrics oldPosition;
+
+  /// The new position.
+  final ScrollMetrics newPosition;
+
+  /// Whether the ScrollView is scrolling.
+  final bool isScrolling;
+
+  /// The current velocity of the scroll position.
+  final double velocity;
+
+  /// The scroll position should be given for new viewport dimensions.
+  final double adjustPosition;
+
+  /// The [ChatScrollObserver] instance.
+  final ChatScrollObserver observer;
+
+  ChatScrollObserverCustomAdjustPositionModel({
+    required this.oldPosition,
+    required this.newPosition,
+    required this.isScrolling,
+    required this.velocity,
+    required this.adjustPosition,
+    required this.observer,
+  });
+}

lib/src/utils/src/chat/chat_scroll_observer_typedefs.dart

@@ -7,10 +7,15 @@
 import 'package:scrollview_observer/src/utils/src/chat/chat_scroll_observer_model.dart';
 
 /// Customize the delta of the adjustPosition.
-typedef ChatScrollObserverCustomAdjustPositionDelta = double Function(
+typedef ChatScrollObserverCustomAdjustPositionDelta = double? Function(
   ChatScrollObserverCustomAdjustPositionDeltaModel,
 );
 
+/// Customize the scroll position should be given new viewport dimensions.
+typedef ChatScrollObserverCustomAdjustPosition = double? Function(
+  ChatScrollObserverCustomAdjustPositionModel,
+);
+
 enum ChatScrollObserverHandlePositionType {
   /// Nothing will be done.
   none,

test/chat_scroll_observer_test.dart

@@ -167,6 +167,7 @@ void main() {
 
   testWidgets('Keeping position with customAdjustPositionDelta',
       (tester) async {
+    GlobalKey<ChatListViewState> key = GlobalKey();
     final scrollController = ScrollController();
     final observerController = ListObserverController(
       controller: scrollController,
@@ -178,6 +179,7 @@ void main() {
     const double normalItemHeight = 100;
 
     Widget widget = ChatListView(
+      key: key,
       scrollController: scrollController,
       observerController: observerController,
       chatScrollObserver: chatScrollObserver,
@@ -207,6 +209,35 @@ void main() {
     final displayingChildModelList =
         observeResult?.displayingChildModelList ?? [];
     expect(displayingChildModelList, isNotEmpty);
+    expect(observeResult?.firstChild?.index, 0);
+    expect(observeResult?.firstChild?.leadingMarginToViewport, 0);
+
+    // Check adjustPosition.
+    key.currentState?.updateData(
+      index: 1,
+      needSetState: true,
+    );
+    double? adjustPosition;
+    await chatScrollObserver.standby(
+      mode: ChatScrollObserverHandleMode.specified,
+      refIndexType: ChatScrollObserverRefIndexType.itemIndex,
+      refItemIndex: 0,
+      refItemIndexAfterUpdate: 0,
+      customAdjustPositionDelta: (model) {
+        adjustPosition = model.adjustPosition;
+        return null;
+      },
+    );
+    await tester.pumpAndSettle();
+    expect(adjustPosition, 0);
+
+    result = await observerController.dispatchOnceObserve(
+      isForce: true,
+      isDependObserveCallback: false,
+    );
+    observeResult = result.observeResult;
+    expect(observeResult?.firstChild?.index, 0);
+    expect(observeResult?.firstChild?.leadingMarginToViewport, 0);
 
     final targetIndex = displayingChildModelList.last.index + 1;
     // Jump to targetIndex and align its bottom with the viewport bottom.
@@ -261,6 +292,7 @@ void main() {
       refItemIndex: refItemIndex,
       refItemIndexAfterUpdate: refItemIndex,
       customAdjustPositionDelta: (model) {
+        adjustPosition = model.adjustPosition;
         return normalItemHeight - expandedItemHeight;
       },
     );
@@ -273,6 +305,76 @@ void main() {
     lastDisplayingChildModel = observeResult?.displayingChildModelList.last;
     expect(lastDisplayingChildModel?.index, targetIndex);
     expect(lastDisplayingChildModel?.trailingMarginToViewport, 0);
+    expect(adjustPosition, greaterThan(0));
+
+    scrollController.dispose();
+  });
+
+  testWidgets('Keeping position with customAdjustPosition', (tester) async {
+    GlobalKey<ChatListViewState> key = GlobalKey();
+    final scrollController = ScrollController();
+    final observerController = ListObserverController(
+      controller: scrollController,
+    );
+    ChatScrollObserverHandlePositionResultModel? onHandlePositionResultModel;
+    final chatScrollObserver = ChatScrollObserver(observerController)
+      ..onHandlePositionResultCallback = (model) {
+        onHandlePositionResultModel = model;
+      };
+
+    Widget widget = ChatListView(
+      key: key,
+      scrollController: scrollController,
+      observerController: observerController,
+      chatScrollObserver: chatScrollObserver,
+      itemBuilder: (context, index) {
+        final dataList = key.currentState?.dataList ?? [];
+        return Text(dataList[index]);
+      },
+    );
+    await tester.pumpWidget(widget);
+
+    final chatListViewState = key.currentState;
+    expect(chatListViewState, isNotNull);
+
+    final dataListLength = chatListViewState?.dataList.length ?? 0;
+    final lastIndex = dataListLength - 1;
+    observerController.jumpTo(index: lastIndex);
+    await tester.pumpAndSettle();
+
+    var result = await observerController.dispatchOnceObserve(
+      isForce: true,
+      isDependObserveCallback: false,
+    );
+    var lastModel = result.observeResult?.displayingChildModelList.last;
+    expect(lastModel?.index, lastIndex);
+    expect(lastModel?.trailingMarginToViewport, 0);
+    expect(onHandlePositionResultModel, isNull);
+
+    chatListViewState?.updateData(
+      index: lastIndex,
+      needSetState: true,
+    );
+    await chatScrollObserver.standby(
+      customAdjustPosition: (model) {
+        final delta =
+            model.newPosition.extentAfter - model.oldPosition.extentAfter;
+        return model.adjustPosition + delta;
+      },
+    );
+    await tester.pumpAndSettle();
+
+    result = await observerController.dispatchOnceObserve(
+      isForce: true,
+      isDependObserveCallback: false,
+    );
+    lastModel = result.observeResult?.displayingChildModelList.last;
+    expect(lastModel?.index, lastIndex);
+    expect(lastModel?.trailingMarginToViewport, 0);
+    expect(
+      onHandlePositionResultModel?.type,
+      ChatScrollObserverHandlePositionType.keepPosition,
+    );
 
     scrollController.dispose();
   });
@@ -302,6 +404,17 @@ class ChatListViewState extends State<ChatListView> {
   List<String> dataList =
       List.generate(100, (index) => index.toString()).toList();
 
+  void updateData({
+    int index = 0,
+    String? appendStr,
+    bool needSetState = true,
+  }) {
+    dataList[index] += appendStr ?? 'updateData' * 10;
+    if (needSetState) {
+      setState(() {});
+    }
+  }
+
   @override
   Widget build(BuildContext context) {
     return MaterialApp(