I am trying to make a janggi(chess variant) game using flet. I wanted to use drag and drop for the pieces, so I followed the Solitaire tutorial on the flet docs. The problem occured when I tried to do the “move_on_top” part. Before adding the move_on_top code, things worked perfectly as expected. I could move the pieces around freely. However after I added the move_on_top function, It works sometimes, but it was also causing the AssertionError randomly, and the drag is not working anymore.

Future exception was never retrieved
future: <Future finished exception=AssertionError('Control must be added to the page first.')>
Traceback (most recent call last):
  File "C:UsersandypAppDataLocalProgramsPythonPython312Libconcurrentfuturesthread.py", line 58, in run
    result = self.fn(*self.args, **self.kwargs)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:UsersandypAppDataLocalProgramsPythonPython312Libsite-packagesflet_corepage.py", line 950, in wrapper
    handler(*args)
  File "c:limbaksajanggi-v2janggiBoard.py", line 22, in drag
    e.control.update()
  File "C:UsersandypAppDataLocalProgramsPythonPython312Libsite-packagesflet_corecontrol.py", line 314, in update
    assert self.__page, "Control must be added to the page first."
           ^^^^^^^^^^^
AssertionError: Control must be added to the page first.

I thought that this had to do with move_on_top function. I think the error is occuring when I try to update them after the function remove and re-append the piece to the controls. But I do not understand why this is happening since the function adds the piece back into the controls after it removes it. My code is here, I simplified it as much as I could. There are 90 invisible slots, and pieces are placed on the slots according to the board. The janggibase.Board has informations about the board such as the list of pieces on the board. The janggibase.Piece contains informations about the piece such as it’s location. I also uploaded the full code to [my github](github.com/limbaksa/janggi-v2/)

import flet as ft
import janggibase

def move_on_top(piece,controls):
    controls.remove(piece)
    controls.append(piece)
    piece.board.update()

def start_drag(e:ft.DragStartEvent):
    move_on_top(e.control,e.control.board.controls)
    e.control.board.move_start_top=e.control.top
    e.control.board.move_start_left=e.control.left

def bounce_back(board,piece):
    piece.top=board.move_start_top
    piece.left=board.move_start_left
    board.update()

def drag(e: ft.DragUpdateEvent):
   e.control.top = max(0, e.control.top + e.delta_y)
   e.control.left = max(0, e.control.left + e.delta_x)
   e.control.update()

def drop(e:ft.DragEndEvent):
    for slot in e.control.board.slots:
        if (
            abs(e.control.top - slot.top) < 20
            and abs(e.control.left - slot.left) < 20
        ):
            place(e.control, slot)
            break
    else:
        bounce_back(e.control.board,e.control)
    e.control.update()

def place(piece,slot):
        piece.top=slot.top
        piece.left=slot.left
        piece.board.update()


class janggiPiece(ft.GestureDetector):
    def __init__(self, piece: janggibase.Piece, board: "janggiBoard"):
        super().__init__()
        self.piece=piece
        self.board=board
        self.mouse_cursor=ft.MouseCursor.MOVE
        self.drag_interval=5
        self.on_pan_start=start_drag
        self.on_pan_update=drag
        self.on_pan_end=drop
        self.left = 60 * (piece.location // 10)
        self.top = 60 * (9 - piece.location % 10)
        self.content=ft.Container(
            width=60,
            height=60,
            border_radius=ft.border_radius.all(5),
            content=ft.Image(f"img/{str(piece).upper()}{piece.color}.png"),
        )
    



class Slot(ft.Container):
    def __init__(self, top, left):
        super().__init__()
        self.ontop = None
        self.width = 60
        self.height = 60
        self.left = left
        self.top = top


class janggiBoard(ft.Stack):
    def __init__(self, board: janggibase.Board):
        super().__init__()
        self.board = board
        self.slots = []
        self.controls = []
        self.width = 540
        self.height = 600
        for i in range(90):
            self.slots.append(Slot(60 * (9 - i % 10), 60 * (i // 10)))
        self.controls.extend(self.slots)
        self.piecelist = []
        for color in range(2):
            for piece in self.board.pieces[color]:
                self.piecelist.append(janggiPiece(piece, self))
        self.controls.extend(self.piecelist)
        self.move_start_top=None
        self.move_start_left=None


if __name__ == "__main__":
    def main(page):
        page.add(janggiBoard(janggibase.Board(15)))

    ft.app(target=main)

I also tried to switch the move_on_top function as below so that the piece never gets remoed from the controls, but I still got the same error.

def move_on_top(piece,controls:list):
    i=controls.index(piece)
    controls[i],controls[-1]=controls[-1],controls[i]
    piece.board.update()