/**
    const BottomSheet({
    Key key,
    this.animationController,
    this.enableDrag = true,
    this.elevation = 0.0,
    @required this.onClosing,
    @required this.builder
    })
 */

floatingActionButton: Builder(builder: (BuildContext context) {
  return FloatingActionButton(
    onPressed: () =>
        Scaffold
            .of(context)
            .showBottomSheet<Null>(
                (BuildContext context) {
              return Container(
                height: 300.0,
                color: Color(0xffff0000),
              );
            })

        //closed：當此對象控制的元素不可見時完成
        // whenComplete：完成時調(diào)用的函數(shù)
            .closed
            .then((value) => print(value))
            .whenComplete(() {
          // mounted：bool值，這個State對象當前是否在結(jié)構(gòu)樹中
          if (!mounted) {
            print("已經(jīng)不在結(jié)構(gòu)樹中了");
          } else {
            print("在結(jié)構(gòu)樹中");
          }
        }),
    child: Text("click"),
  );
})

碼云地址：https://gitee.com/xgljh/Flutter.git