A pointer has contacted the screen at a particular location with a primary button, which might be the start of a tap.
This triggers after the down event, once a short timeout (deadline) has elapsed, or once the gestures has won the arena, whichever comes first.
- kPrimaryButton, the button this callback responds to.
- onSecondaryTapDown, a similar callback but for a secondary button.
- onTertiaryTapDown, a similar callback but for a tertiary button.
- TapDownDetails, which is passed as an argument to this callback.
- GestureDetector.onTapDown, which exposes this callback.