read / write
A pointer that is in contact with the screen with a primary button and moving has moved again.
The distance traveled by the pointer since the last update is provided in
details argument, which is a DragUpdateDetails object.
If this gesture recognizer recognizes movement on a single axis (a
VerticalDragGestureRecognizer or HorizontalDragGestureRecognizer),
details will reflect movement only on that axis and its
DragUpdateDetails.primaryDelta will be non-null.
If this gesture recognizer recognizes movement in all directions
(a PanGestureRecognizer), then
details will reflect movement on
both axes and its DragUpdateDetails.primaryDelta will be null.
allowedButtonsFilter, which decides which button will be allowed.
- DragUpdateDetails, which is passed as an argument to this callback.