Add method to instance.

Author: ditman

lib/web_ui/lib/pointer.dart

@@ -153,7 +153,23 @@ class PointerData {
   }
 }
 
+/// A function used to notify the engine about how a [PointerDataPacket] was used in the framework.
+///
+/// For example, can be used to [preventDefault] the source events on the web.
+typedef _PointerDataPacketAcknowledgeFn = void Function({
+  bool? preventDefault,
+});
+
+// The default _PointerDataPacketAcknowledgeFn, which is a noop.
+void _defaultPointerDataPacketAcknowledgeFn ({
+  bool? preventDefault,
+}) {}
+
 class PointerDataPacket {
-  const PointerDataPacket({this.data = const <PointerData>[]});
+  const PointerDataPacket({
+    this.data = const <PointerData>[],
+    this.acknowledge = _defaultPointerDataPacketAcknowledgeFn,
+  });
   final List<PointerData> data;
+  final _PointerDataPacketAcknowledgeFn acknowledge;
 }

lib/web_ui/lib/src/engine/pointer_binding.dart

@@ -403,7 +403,14 @@ class ClickDebouncer {
   }
 
   void _sendToFramework(DomEvent? event, List<ui.PointerData> data) {
-    final ui.PointerDataPacket packet = ui.PointerDataPacket(data: data.toList());
+    final ui.PointerDataPacket packet = ui.PointerDataPacket(
+      data: data.toList(),
+      acknowledge: ({bool? preventDefault}) {
+        if (preventDefault ?? false) {
+          event?.preventDefault();
+        }
+      },
+    );
     if (_debugLogFlutterEvents) {
       for(final ui.PointerData datum in data) {
         print('fw:${datum.change}    ${datum.physicalX},${datum.physicalY}');
@@ -728,11 +735,9 @@ mixin _WheelEventListenerMixin on _BaseAdapter {
     if (_debugLogPointerEvents) {
       print(event.type);
     }
+    // PointerData has a binding to the native `event` so users can choose to
+    // prevent default (or not).
     _callback(e, _convertWheelEventToPointerData(event));
-    // Prevent default so mouse wheel event doesn't get converted to
-    // a scroll event that semantic nodes would process.
-    //
-    event.preventDefault();
   }
 
   /// For browsers that report delta line instead of pixels such as FireFox