动态设置AppBar 透明度

const APPBAR_SCROLL_OFFSET = 100;
double appBarAlpha = 0;

_onScroll(offset) {
print(offset);

double alpha = offset / APPBAR_SCROLL_OFFSET;
if (alpha < 0) {
  alpha = 0;
} else if (alpha > 1) {
  alpha = 1;
}

setState(() {
  appBarAlpha = alpha;
});

}