1.可悬浮的控件Overlay

加入到屏幕中 Overlay.of(context).insert(overlayEntry);
移除 overlayEntry.remove();

2.可拖动.

Draggable拖动拿到offset,setstate()改变Positioned位置

以下 伪代码

OverlayEntry overlayEntry;
  double overlayWidth = 50;
  Offset offset = Offset.zero;

 @override
  Widget build(BuildContext context) {
    return 可点击控件,点击clickShow;
  }
clickShow(){
      if (overlayEntry != null) {
          return;
        }
        overlayEntry = _showOverlay();
        //往Overlay中插入插入OverlayEntry
        Overlay.of(context).insert(overlayEntry);
}

  OverlayEntry _showOverlay() {
    OverlayEntry overlayEntry = new OverlayEntry(builder: (context) {
      //外层使用Positioned进行定位，控制在Overlay中的位置
      // 控制不可拖出屏幕外
      double dx = 0.0;
      double dy = 0.0;

      if ((offset.dx > 0) &&
          ((offset.dx + overlayWidth) < ScreenUtil().screenWidth)) {
        dx = offset.dx;
      } else if ((offset.dx + overlayWidth) >= ScreenUtil().screenWidth) {
        dx = ScreenUtil().screenWidth - overlayWidth;
      } else {
        dx = 0;
      }

      if ((offset.dy > 0) &&
          ((offset.dy + overlayWidth) < ScreenUtil().screenHeight)) {
        dy = offset.dy;
      } else if ((offset.dy + overlayWidth) >= ScreenUtil().screenHeight) {
        dy = ScreenUtil().screenWidth - overlayWidth;
      } else {
        dy = 0;
      }

      return new Positioned(
        top: dy,
        left: dx,
        child: Draggable(
          feedback: _contentBody(),
          child: _contentBody(),
          childWhenDragging: Container(), //拖动过程回调
          onDraggableCanceled: (Velocity velocity, Offset offset) {
            setState(() {
              //第二个参数为拖动的动态坐标
              this.offset = offset;
            });
          },
        ),
      );
    });
    return overlayEntry;
  }

  Widget _contentBody() {
    return Container(
      color: Colors.red,
      width: overlayWidth,
      height: overlayWidth,
    );
  }

  @override
  void dispose() {
    overlayEntry.remove();//移除
    overlayEntry = null;
    super.dispose();
  }