Flutter：动态添加拖放堆栈小部件中的任意位置小部件

Question

How can we dynamically add drag and drop anywhere widget in flutter? Here is what I tried to implement. Suggest changes to solve the problem or suggest the proper method to implement it.

The problem I need to solve is that the canvasItemObj.x and canvasItemObj.y variables are updating according to t the dragUpdate. But the position of the widget is not updating. ( top and left parameters of the Positioned() widget )

Here is the CanvasItemProps class for holding a widget and its properties.

class CanvasItemProps {
  Widget canvasItem;
  double x = 0, y = 0, dx, dy;
  double scale = 1;
  Color color;
  CanvasItemProps();
  CanvasItemProps.fromItem(this.canvasItem);
}

canvasItemProps holds the list of CanvasItemProps

List<CanvasItemProps> canvasItemProps = [];

I have a flatbutton which onPressed execute the following:

var canvasItemObj = CanvasItemProps();
canvasItemProps.add(canvasItemObj);

canvasItemObj.canvasItem = Positioned(
                  left: canvasItemObj.x,
                  top: canvasItemObj.y,
                  child: GestureDetector(
                    child: Text(
                      addTextController.text,
                      textScaleFactor: 5,
                    ),
                    onVerticalDragUpdate: (dragDetails) {
                      setState(() {
                        canvasItemObj.y = dragDetails.localPosition.dy;
                        canvasItemObj.x = dragDetails.localPosition.dx;
                      });
                    },

                  ));
              addTextController.text = "";
              setState(() {});
            },

In build function there is a Stack:

Stack(children:canvasItemProps.isEmpty()?
               Text("No Item"):
                getCanvasItem)

The following getCanvasItem function returns the list of widget to pass to the Stack widget

List<Widget> getCanvasItem() {
    List<Widget> list = [];

    for (int i = 0; i < canvasItemProps.length; i++) {
      list.add(canvasItemProps[i].canvasItem);
    }
    return list;
  }

On the other hand, this works fine when I am dealing with only one widget that is not dynamically added(x and y are two variables declared in the scope and are initialized to 0.)

double x=0,y=0;

widget is as follows:

Container(
            margin: EdgeInsets.fromLTRB(50, 100, 90, 100),
            color: Colors.yellow,
            child: Stack(
            children: <Widget>[
              Positioned(
            top: y,
            left: x,
            child: GestureDetector(
              onVerticalDragUpdate: (v){
                setState(() {
                  x = v.localPosition.dx;
                  y = v.localPosition.dy;
                });
              },
              child:Text("Text"),
            )
          ),
            ],
          ),
          )

Answer 1

The issue is the Top and Left parameters are final so you cant change the values. But instead, you should reinitialize the positioned widget each time with the updated values.