The pointer has contacted the screen with a primary button, which might be the start of a long-press.
This triggers after the pointer down event.
If the user completes the long-press, and this gesture wins, onLongPressStart will be called after this callback. Otherwise, onLongPressCancel will be called after this callback.
- kPrimaryButton, the button this callback responds to.
- onSecondaryLongPressDown, a similar callback but for a secondary button.
- onTertiaryLongPressDown, a similar callback but for a tertiary button.
- LongPressGestureRecognizer.onLongPressDown, which exposes this callback at the gesture layer.
final GestureLongPressDownCallback? onLongPressDown;