onSerialTapDown property Null safety
A pointer has contacted the screen at a particular location, which might be the start of a serial tap.
If this recognizer loses the arena before the serial tap is completed (either because the gesture does not end up being a tap or because another recognizer wins the arena), onSerialTapCancel is called next. Otherwise, onSerialTapUp is called next.
The SerialTapDownDetails.count that is passed to this callback specifies the series tap count.