A simple drag behaviour added to bouncing "Swipe to Start" widget.

verticaldragbehavior.dart

//A simple container with a SingleChildScrollView. The trick is to set the child of SingleChildScrollView height
//exceed the height of parent scroll widget so it can be scrolled. The BouncingScrollPhysics helps the scroll retain its
//original position if it doesn't cross the threshold to play reveal animation.
//This widget is added by _getInitScreenWidgets() method 
Positioned(
        right: 0,
        left: 0,
        bottom: 0,
        child: Container(
          height: widget.height * .5,
          child: SingleChildScrollView(
            controller: _scrollController,
            physics: BouncingScrollPhysics(),
            child: Container(
              height: widget.height * .5 + .1,
              // color:Colors.yellow,
            ),
          ),
        ),
      ),

 //Intercepts the bounce animation and start dragg animation
  void _handleSwipe() {
    _swipeUpBounceAnimationController.stop(canceled: true);
    double dy = _scrollController.position.pixels;

    double scrollRatio =
        math.min(1.0, _scrollController.position.pixels / _swipeDistance);

    //If user scroll 70% of the scrolling region we proceed towards reveal animation
    if (scrollRatio > .7)
      _playRevealAnimation();
    else
      setState(() {
        _swipeUpDy = dy * -1;
      });
  }