目录
description bug_report

Default multitouch scrolling

目录

Summary

ScrollBehaviors now allow or disallow scrolling speeds to be affected by the number of pointers on the screen. ScrollBehavior.multitouchDragStrategy, by default, prevents multiple pointers interacting wih the scrollable at the same time from affecting the speed of scrolling.

Context

Prior to this change, for each pointer dragging a Scrollable widget, the scroll speed would increase. This did not match platform expectations when interacting with Flutter applications.

Now, the inherited ScrollBehavior manages how multiple pointers affect scrolling widgets as specified by ScrollBehavior.multitouchDragStrategy. This enum, MultitouchDragStrategy, can also be configured for the prior behavior.

Description of change

This change fixed the unexpected ability to increase scroll speeds by dragging with more than one finger.

If you have relied on the previous behavior in your application, there are several ways to control and configure this feature.

  • Extend ScrollBehavior, MaterialScrollBehavior, or CupertinoScrollBehavior to modify the default behavior, overriding ScrollBehavior.multitouchDragStrategy.

    • With your own ScrollBehavior, you can apply it app-wide by setting MaterialApp.scrollBehavior or CupertinoApp.scrollBehavior.
    • Or, if you wish to only apply it to specific widgets, add a ScrollConfiguration above the widget in question with your custom ScrollBehavior.

Your scrollable widgets then inherit and reflect this behavior.

  • Instead of creating your own ScrollBehavior, another option for changing the default behavior is to copy the existing ScrollBehavior, and set different multitouchDragStrategy.
    • Create a ScrollConfiguration in your widget tree, and provide a modified copy of the existing ScrollBehavior in the current context using copyWith.

To accommodate the new configuration DragGestureRecognizer was updated to support MultitouchDragStrategy as well in other dragging contexts.

Migration guide

Setting a custom ScrollBehavior for your application

Code before migration:

MaterialApp(
  // ...
);

Code after migration:

class MyCustomScrollBehavior extends MaterialScrollBehavior {
  // Override behavior methods and getters like multitouchDragStrategy
  @override
  MultitouchDragStrategy get multitouchDragStrategy => MultitouchDragStrategy.sumAllPointers;
}

// Set ScrollBehavior for an entire application.
MaterialApp(
  scrollBehavior: MyCustomScrollBehavior(),
  // ...
);

Setting a custom ScrollBehavior for a specific widget

Code before migration:

final ScrollController controller = ScrollController();
ListView.builder(
  controller: controller,
  itemBuilder: (BuildContext context, int index) {
    return Text('Item $index');
  },
);

Code after migration:

class MyCustomScrollBehavior extends MaterialScrollBehavior {
  // Override behavior methods and getters like multitouchDragStrategy
  @override
  MultitouchDragStrategy get multitouchDragStrategy => MultitouchDragStrategy.sumAllPointers;
}

// ScrollBehavior can be set for a specific widget.
final ScrollController controller = ScrollController();
ScrollConfiguration(
  behavior: MyCustomScrollBehavior(),
  child: ListView.builder(
    controller: controller,
    itemBuilder: (BuildContext context, int index) {
      return Text('Item $index');
    },
  ),
);

Copy and modify existing ScrollBehavior

Code before migration:

final ScrollController controller = ScrollController();
ListView.builder(
  controller: controller,
  itemBuilder: (BuildContext context, int index) {
    return Text('Item $index');
  },
);

Code after migration:

// ScrollBehavior can be copied and adjusted.
final ScrollController controller = ScrollController();
ScrollConfiguration(
  behavior: ScrollConfiguration.of(context).copyWith(
    multitouchDragStrategy: MultitouchDragStrategy.sumAllPointers,
  ),
  child: ListView.builder(
    controller: controller,
    itemBuilder: (BuildContext context, int index) {
      return Text('Item $index');
    },
  ),
);

Timeline

Landed in version: 3.18.0-4.0.pre
In stable release: 3.19.0

References

API documentation:

Relevant issue:

Relevant PRs: