gui construction - Modify Dynamic value within EventHandler

I'm trying to build a tic tac toe program using a 3x3 board list running inside of a DynamicModule with a separate window displaying the current value and accepting clicks from the user. For now I'm just trying to use EventHandler to set any empty boxes to X's when clicked.

When I click on a box, I get the error

Part::pkspec1: The expression x cannot be used as a part specification.

Is this an issue with assigning a new value for board inside of the EventHandler?

How else should I structure this?

createBox[elem_] := Show[
  Graphics[{White, Rectangle[]}, Frame -> True, 
   FrameStyle -> Thickness[.02], FrameTicks -> None],
  Graphics@Text[Style[elem, FontSize -> 100], {.5, .5}],
  ImageSize -> 150
  ]  


DynamicModule[{board = Table[Table[" ", 3], 3]},
  CreateWindow[
   DocumentNotebook[
    (*Displaying grid*)
    Dynamic@Grid[
      Table[Table[
        EventHandler[
         createBox[board[[y]][[x]]]
         , {"MouseClicked" :> (
            If[board[[y]][[x]] == " ", board[[y]][[x]] = "X";];

            )}]
        , {x, 3}], {y, 3}],
      Spacings -> {0, 0}]
    ],
   WindowElements -> {}, WindowTitle -> "Tic Tac Toe", 
   WindowSize -> All, Editable -> False, Selectable -> False, 
   ShowCellBracket -> False, WindowFrameElements -> {"CloseBox"}];
  ];

Answer

Main issues:

• 
  

  set part

  
  

  board[[y]][[x]] = val will not work, you need board[[y, x]] = "X"

  
  

  See: Part not behaving the way I expect it to

  
  


• 
  

  x and y can't get inside :>

  
  

  RuleDelayed holds and so after Table is done you will have board[[y, x]] in each item, instead of expected e.g. board[[2,1]] etc.

  
  

  The way to go is to use With[{i=i},... idiom, see: Function in Table

  
  
  


• 
  

  DynamicModule is 'a scoped rectangle in your screen'

  
  

  So instead of DynamicModule[.. CreateWindow.. it should rather be CreateWindow[.. DynamicModule..

  
  

  See e.g. IntroductionToDynamic

  
  

Secondary issues

• 
  
  

  you don't need Dynamic around Grid, only content of board changes so you can put it inside createBox[Dynamic[...]]

  
  


• 
  

  it is better to use === instead of == to compare non numerical values

  
  


• 
  

  Show[Graphics...] can be combined in one Graphics[...]

  
  


• 
  

  Unless you need it we can replace CreateWindow[NotebookDocument with CreateDocument (omg, I just realized the origin of this function name, thanks)

  
  
  


• 
  

  but your notebook's options suggest it may be enough to use CreateDialog

  
  


• 
  

  you can combine nested Table into one

  
  

Summing up:

createBox[elem_] := Graphics[

  { { White, Rectangle[] }
  , Text[Style[elem, FontSize -> 100], {.5, .5}]
  }
, ImageSize -> 150
, Frame -> True
, FrameStyle -> Thickness[.02]
, FrameTicks -> None
];

CreateDialog[

  DynamicModule[
    {board = ConstantArray[" ", {3, 3}]}
  , Grid[
      Table[ With[{x = x, y = y}
      , EventHandler[
          createBox[ Dynamic @ board[[y, x]]]
        , {"MouseClicked" :> If[board[[y, x]] === " ", board[[y, x]] = "X"]
          }
        ]]
      , {x, 3}, {y, 3}

      ]
    , Spacings -> {0, 0}
    ]
  ]
, WindowTitle -> "Tic Tac Toe"
, WindowSize -> All
];