Trackpad gestures can trigger GestureRecognizer

Summary

Trackpad gestures on most platforms now send PointerPanZoom sequences and can trigger pan, drag, and scale GestureRecognizer callbacks.

Context

Scrolling on Flutter Desktop prior to version 3.3.0 used PointerScrollEvent messages to represent discrete scroll deltas. This system worked well for mouse scroll wheels, but wasn’t a good fit for trackpad scrolling. Trackpad scrolling is expected to cause momentum, which depends not only on the scroll deltas, but also the timing of when fingers are released from the trackpad. In addition, trackpad pinching-to-zoom could not be represented.

Three new PointerEvents have been introduced: PointerPanZoomStartEvent, PointerPanZoomUpdateEvent, and PointerPanZoomEndEvent. Relevant GestureRecognizers have been updated to register interest in trackpad gesture sequences, and will emit onDrag, onPan, and/or onScale callbacks in response to movements of two or more fingers on the trackpad.

This means both that code designed only for touch interactions might trigger upon trackpad interaction, and that code designed to handle all desktop scrolling might now only trigger upon mouse scrolling, and not trackpad scrolling.

Description of change

The Flutter engine has been updated on all possible platforms to recognize trackpad gestures and send them to the framework as PointerPanZoom events instead of as PointerScrollSignal events. PointerScrollSignal events will still be used to represent scrolling on a mouse wheel.

Depending on the platform and specific trackpad model, the new system might not be used, if not enough data is provided to the Flutter engine by platform APIs. This includes on Windows, where trackpad gesture support is dependent on the trackpad’s driver, and the Web platform, where not enough data is provided by browser APIs, and trackpad scrolling must still use the old `PointerScrollSignal system.

Developers should be prepared to receive both types of events and ensure their apps or packages handle them in the appropriate manner.

Listener now has three new callbacks: onPointerPanZoomStart, onPointerPanZoomUpdate, and onPointerPanZoomEnd which can be used to observe trackpad scrolling and zooming events.

void main() => runApp(Foo());

class Foo extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Listener(
      onPointerSignal: (PointerSignalEvent event) {
        if (event is PointerScrollEvent) {
          debugPrint('mouse scrolled ${event.scrollDelta}');
        }
      },
      onPointerPanZoomStart: (PointerPanZoomStartEvent event) {
        debugPrint('trackpad scroll started');
      },
      onPointerPanZoomUpdate: (PointerPanZoomUpdateEvent event) {
        debugPrint('trackpad scrolled ${event.panDelta}');
      },
      onPointerPanZoomEnd: (PointerPanZoomEndEvent event) {
        debugPrint('trackpad scroll ended');
      },
      child: Container()
    );
  }
}

PointerPanZoomUpdateEvent contains a pan field to represent the cumulative pan of the current gesture, a panDelta field to represent the difference in pan since the last event, a scale event to represent the cumulative zoom of the current gesture, and a rotation event to represent the cumulative rotation (in radians) of the current gesture.

GestureRecognizers now have methods to all the trackpad events from one continuous trackpad gesture. Calling the addPointerPanZoom method on a GestureRecognizer with a PointerPanZoomStartEvent will cause the recognizer to register its interest in that trackpad interaction, and resolve conflicts between multiple GestureRecognizers that could potentially respond to the gesture.

The following example shows the proper use of Listener and GestureRecognizer to respond to trackpad interactions.

void main() => runApp(Foo());

class Foo extends StatefulWidget {
  late final PanGestureRecognizer recognizer;

  @override
  void initState() {
    super.initState();
    recognizer = PanGestureRecognizer()
    ..onStart = _onPanStart
    ..onUpdate = _onPanUpdate
    ..onEnd = _onPanEnd;
  }

  void _onPanStart(DragStartDetails details) {
    debugPrint('onStart');
  }

  void _onPanUpdate(DragUpdateDetails details) {
    debugPrint('onUpdate');
  }

  void _onPanEnd(DragEndDetails details) {
    debugPrint('onEnd');
  }

  @override
  Widget build(BuildContext context) {
    return Listener(
      onPointerDown: recognizer.addPointer,
      onPointerPanZoomStart: recognizer.addPointerPanZoom,
      child: Container()
    );
  }
}

When using GestureDetector, this is done automatically, so code such as the following example will issue its gesture update callbacks in response to both touch and trackpad panning.

void main() => runApp(Foo());

class Foo extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return GestureDetector(
      onPanStart: (details) {
        debugPrint('onStart');
      },
      onPanUpdate: (details) {
        debugPrint('onUpdate');
      },
      onPanEnd: (details) {
        debugPrint('onEnd');
      }
      child: Container()
    );
  }
}

Migration guide

Migration steps depend on whether you want each gesture interaction in your app to be usable via a trackpad, or whether it should be restricted to only touch and mouse usage.

For gesture interactions suitable for trackpad usage

Using GestureDetector

No change is needed, GestureDetector will automatically process trackpad gesture events and trigger callbacks if recognized.

Using GestureRecognizer and Listener

Ensure that onPointerPanZoomStart is passed through to each recognizer from the Listener. The addPointerPanZoom method of `GestureRecognizer must be called for it to show interest and start tracking each trackpad gesture.

Code before migration:

void main() => runApp(Foo());

class Foo extends StatefulWidget {
  late final PanGestureRecognizer recognizer;

  @override
  void initState() {
    super.initState();
    recognizer = PanGestureRecognizer()
    ..onStart = _onPanStart
    ..onUpdate = _onPanUpdate
    ..onEnd = _onPanEnd;
  }

  void _onPanStart(DragStartDetails details) {
    debugPrint('onStart');
  }

  void _onPanUpdate(DragUpdateDetails details) {
    debugPrint('onUpdate');
  }

  void _onPanEnd(DragEndDetails details) {
    debugPrint('onEnd');
  }

  @override
  Widget build(BuildContext context) {
    return Listener(
      onPointerDown: recognizer.addPointer,
      child: Container()
    );
  }
}

Code after migration:

void main() => runApp(Foo());

class Foo extends StatefulWidget {
  late final PanGestureRecognizer recognizer;

  @override
  void initState() {
    super.initState();
    recognizer = PanGestureRecognizer()
    ..onStart = _onPanStart
    ..onUpdate = _onPanUpdate
    ..onEnd = _onPanEnd;
  }

  void _onPanStart(DragStartDetails details) {
    debugPrint('onStart');
  }

  void _onPanUpdate(DragUpdateDetails details) {
    debugPrint('onUpdate');
  }

  void _onPanEnd(DragEndDetails details) {
    debugPrint('onEnd');
  }

  @override
  Widget build(BuildContext context) {
    return Listener(
      onPointerDown: recognizer.addPointer,
      onPointerPanZoomStart: recognizer.addPointerPanZoom,
      child: Container()
    );
  }
}

Using raw Listener

The following code using PointerScrollSignal will no longer be called upon all desktop scrolling. PointerPanZoomUpdate events should be captured to receive trackpad gesture data.

Code before migration:

void main() => runApp(Foo());

class Foo extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Listener(
      onPointerSignal: (PointerSignalEvent event) {
        if (event is PointerScrollEvent) {
          debugPrint('scroll wheel event');
        }
      }
      child: Container()
    );
  }
}

Code after

void main() => runApp(Foo());

class Foo extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Listener(
      onPointerSignal: (PointerSignalEvent event) {
        if (event is PointerScrollEvent) {
          debugPrint('scroll wheel event');
        }
      },
      onPointerPanZoomUpdate: (PointerPanZoomUpdateEvent event) {
        debugPrint('trackpad scroll event');
      }
      child: Container()
    );
  }
}

Please note: Use of raw Listener in this way could cause conflicts with other gesture interactions as it does not participate in the gesture disambiguation arena.

For gesture interactions not suitable for trackpad usage

Using GestureDetector

If using Flutter 3.3.0, RawGestureDetector can be used instead of GestureDetector to ensure each GestureRecognizer created by the GestureDetector has supportedDevices set to exclude PointerDeviceKind.trackpad. Starting in version 3.4.0, there will be a supportedDevices parameter directly on GestureDetector.

Code before migration:

void main() => runApp(Foo());

class Foo extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return GestureDetector(
      onPanStart: (details) {
        debugPrint('onStart');
      },
      onPanUpdate: (details) {
        debugPrint('onUpdate');
      },
      onPanEnd: (details) {
        debugPrint('onEnd');
      }
      child: Container()
    );
  }
}

Code after migration (Flutter 3.3.0):

// Example of code after the change.
void main() => runApp(Foo());

class Foo extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return RawGestureDetector(
      gestures: {
        PanGestureRecognizer:
            GestureRecognizerFactoryWithHandlers<PanGestureRecognizer>(
          () => PanGestureRecognizer(
            supportedDevices: {
              PointerDeviceKind.touch,
              PointerDeviceKind.mouse,
              PointerDeviceKind.stylus,
              PointerDeviceKind.invertedStylus,
              // Do not include PointerDeviceKind.trackpad
            }
          ),
          (recognizer) {
            recognizer
              ..onStart = (details) {
                debugPrint('onStart');
              }
              ..onUpdate = (details) {
                debugPrint('onUpdate');
              }
              ..onEnd = (details) {
                debugPrint('onEnd');
              };
          },
        ),
      },
      child: Container()
    );
  }
}

Code after migration: (Flutter 3.4.0):

void main() => runApp(Foo());

class Foo extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return GestureDetector(
      supportedDevices: {
        PointerDeviceKind.touch,
        PointerDeviceKind.mouse,
        PointerDeviceKind.stylus,
        PointerDeviceKind.invertedStylus,
        // Do not include PointerDeviceKind.trackpad
      },
      onPanStart: (details) {
        debugPrint('onStart');
      },
      onPanUpdate: (details) {
        debugPrint('onUpdate');
      },
      onPanEnd: (details) {
        debugPrint('onEnd');
      }
      child: Container()
    );
  }
}

Using RawGestureRecognizer

Explicitly ensure that supportedDevices does not include PointerDeviceKind.trackpad.

Code before migration:

void main() => runApp(Foo());

class Foo extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return RawGestureDetector(
      gestures: {
        PanGestureRecognizer:
            GestureRecognizerFactoryWithHandlers<PanGestureRecognizer>(
          () => PanGestureRecognizer(),
          (recognizer) {
            recognizer
              ..onStart = (details) {
                debugPrint('onStart');
              }
              ..onUpdate = (details) {
                debugPrint('onUpdate');
              }
              ..onEnd = (details) {
                debugPrint('onEnd');
              };
          },
        ),
      },
      child: Container()
    );
  }
}

Code after migration:

// Example of code after the change.
void main() => runApp(Foo());

class Foo extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return RawGestureDetector(
      gestures: {
        PanGestureRecognizer:
            GestureRecognizerFactoryWithHandlers<PanGestureRecognizer>(
          () => PanGestureRecognizer(
            supportedDevices: {
              PointerDeviceKind.touch,
              PointerDeviceKind.mouse,
              PointerDeviceKind.stylus,
              PointerDeviceKind.invertedStylus,
              // Do not include PointerDeviceKind.trackpad
            }
          ),
          (recognizer) {
            recognizer
              ..onStart = (details) {
                debugPrint('onStart');
              }
              ..onUpdate = (details) {
                debugPrint('onUpdate');
              }
              ..onEnd = (details) {
                debugPrint('onEnd');
              };
          },
        ),
      },
      child: Container()
    );
  }
}

Using GestureRecognizer and Listener

After upgrading to Flutter 3.3.0, there won’t be a change in behavior, as addPointerPanZoom needs to be called on each GestureRecognizer to allow it to track gestures. The following code will not receive pan gesture callbacks when the trackpad is scrolled.

void main() => runApp(Foo());

class Foo extends StatefulWidget {
  late final PanGestureRecognizer recognizer;

  @override
  void initState() {
    super.initState();
    recognizer = PanGestureRecognizer()
    ..onStart = _onPanStart
    ..onUpdate = _onPanUpdate
    ..onEnd = _onPanEnd;
  }

  void _onPanStart(DragStartDetails details) {
    debugPrint('onStart');
  }

  void _onPanUpdate(DragUpdateDetails details) {
    debugPrint('onUpdate');
  }

  void _onPanEnd(DragEndDetails details) {
    debugPrint('onEnd');
  }

  @override
  Widget build(BuildContext context) {
    return Listener(
      onPointerDown: recognizer.addPointer,
      // recognizer.addPointerPanZoom is not called
      child: Container()
    );
  }
}

Timeline

Landed in version: 3.3.0-0.0.pre In stable release: 3.3.0

References

API documentation:

Design Document

Relevant issues:

Relevant PRs: