feat(ChatScrollObserver): add customAdjustPositionDelta

Author: LinXunFeng

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

@@ -52,7 +52,27 @@ mixin ChatObserverScrollPhysicsMixin on ScrollPhysics {
       mode: observer.innerMode,
       changeCount: observer.changeCount,
     ));
-    final delta = model.layoutOffset - observer.innerRefItemLayoutOffset;
+
+    // Customize the delta of the adjustPosition.
+    double? customDelta = observer.customAdjustPositionDelta?.call(
+      ChatScrollObserverCustomAdjustPositionDeltaModel(
+        oldPosition: oldPosition,
+        newPosition: newPosition,
+        isScrolling: isScrolling,
+        velocity: velocity,
+        observer: observer,
+        currentItemModel: model,
+      ),
+    );
+
+    // Calculate the final delta.
+    //
+    // If the customDelta is not null, use the customDelta.
+    // Otherwise, use the layoutOffset minus innerRefItemLayoutOffset to get
+    // the difference in the leading offset of the item.
+    final delta =
+        customDelta ?? (model.layoutOffset - observer.innerRefItemLayoutOffset);
+
     return adjustPosition + delta;
   }
 

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

@@ -73,6 +73,9 @@ class ChatScrollObserver {
   /// The mode of processing.
   ChatScrollObserverHandleMode innerMode = ChatScrollObserverHandleMode.normal;
 
+  /// Customize the delta of the adjustPosition.
+  ChatScrollObserverCustomAdjustPositionDelta? customAdjustPositionDelta;
+
   /// Observation result of reference subparts after ScrollView children update.
   ListViewObserveDisplayingChildModel? observeRefItem() {
     return observerController.observeItem(
@@ -109,6 +112,7 @@ class ChatScrollObserver {
     int refItemRelativeIndexAfterUpdate = 0,
     int refItemIndex = 0,
     int refItemIndexAfterUpdate = 0,
+    ChatScrollObserverCustomAdjustPositionDelta? customAdjustPositionDelta,
   }) async {
     innerMode = mode;
     this.isRemove = isRemove;
@@ -196,6 +200,7 @@ class ChatScrollObserver {
     innerRefItemIndex = _innerRefItemIndex;
     innerRefItemIndexAfterUpdate = _innerRefItemIndexAfterUpdate;
     innerRefItemLayoutOffset = _innerRefItemLayoutOffset;
+    this.customAdjustPositionDelta = customAdjustPositionDelta;
 
     // When the heights of items are similar, the viewport will not call
     // [performLayout], In this case, the [adjustPositionForNewDimensions] of

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

@@ -3,7 +3,11 @@
  * @Repo: https://github.com/LinXunFeng/flutter_scrollview_observer
  * @Date: 2023-05-13 10:33:00
  */
-import 'chat_scroll_observer_typedefs.dart';
+import 'package:flutter/material.dart';
+
+import 'package:scrollview_observer/src/common/models/observe_displaying_child_model_mixin.dart';
+import 'package:scrollview_observer/src/utils/src/chat/chat_scroll_observer.dart';
+import 'package:scrollview_observer/src/utils/src/chat/chat_scroll_observer_typedefs.dart';
 
 class ChatScrollObserverHandlePositionResultModel {
   /// The type of processing location.
@@ -21,3 +25,32 @@ class ChatScrollObserverHandlePositionResultModel {
     required this.changeCount,
   });
 }
+
+class ChatScrollObserverCustomAdjustPositionDeltaModel {
+  /// 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 [ChatScrollObserver] instance.
+  final ChatScrollObserver observer;
+
+  /// The observation result of the current item.
+  final ObserveDisplayingChildModelMixin currentItemModel;
+
+  ChatScrollObserverCustomAdjustPositionDeltaModel({
+    required this.oldPosition,
+    required this.newPosition,
+    required this.isScrolling,
+    required this.velocity,
+    required this.observer,
+    required this.currentItemModel,
+  });
+}

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

@@ -4,6 +4,13 @@
  * @Date: 2022-10-31 14:57:45
  */
 
+import 'package:scrollview_observer/src/utils/src/chat/chat_scroll_observer_model.dart';
+
+/// Customize the delta of the adjustPosition.
+typedef ChatScrollObserverCustomAdjustPositionDelta = double Function(
+  ChatScrollObserverCustomAdjustPositionDeltaModel,
+);
+
 enum ChatScrollObserverHandlePositionType {
   /// Nothing will be done.
   none,