Question

How would you adhere to the “Tell, don’t ask” principle (henceforth “the principle”) in the following simple scenario? In a Tetris game, I have Board, BlockGrid and Piece classes relevant to the following example:

public class Board
{
    private var fallingPiece:Piece;
    private var blockGrid:BlockGrid;
    ...
    public function moveFallingPiece(xDirection:int, yDirection:int):void
    {
        blockGrid.movePiece(fallingPiece, xDirection, yDirection);
    }
} 

Once fallingPiece is placed in the bottom row of BlockGrid, it should no longer be the “fallingPiece”. Am I right in that I’m not violating the principle with the following?

if(blockGrid.getPiecePosition(piece).y == 0)
{
    fallingPiece = null;
}

But is that really different from this, which I think clearly violates the principle?

public function moveFallingPiece(xDirection:int, yDirection:int):void
{
    if(blockGrid.getPiecePosition(piece).y > 0)
    {
        blockGrid.movePiece(fallingPiece, xDirection, yDirection);
    }
    else
    {
        fallingPiece = null;
    }
}

I’m not assuming that I’ve designed these class relationships in the proper way to work with the principle. Please advice on an alternate design if that’s what I’m missing.

---

EDIT, Proposed solution:

I went with the answers proposing “command feedback” via events. Board tells BlockGrid to move a piece. BlockGrid’s movePiece method dispatches MOVED_TO or MOVE_FAILED events depending on the result, which Board can listen to and use to determine whether a piece has stopped falling. Please don’t hesitate to provide feedback on this solution.

public class Board
{
    ...
    public function Board()
    {
        ...
        blockGrid.addEventListener(PieceMoveEvent.MOVE_FAILED, onPieceMoveFailed);
        ...
    }

    public function moveFallingPiece(xDirection:int, yDirection:int):void
    {
            blockGrid.movePiece(fallingPiece, xDirection, yDirection);
    }

    public function onPieceMoveFailed(event:MovePieceEvent):void
    {
        if(event.instance == currentlyFallingPiece && event.fromPosition.y != event.toPosition.y)
        {
             currentlyFallingPiece = null;
        }
    }

Answer 1

I think, to better follow the Tell, Don’t Ask principle, you should have blockGrid notifying your Board class when fallingPiece has reaches it’s resting point. In both scenarios above, you are asking blockGrid if the piece’s position.y == 0 in order to determine whether or not fallingPiece should be null. Instead, you want blockGrid to tell the Board class that fallingPiece.y has hit 0.