Our app isn't playable yet:

All of this is coded in the mouseDown event handler, and we also need a variable lastCellClicked of type Cell in the Board class. Of course, this is exactly the cell which we get in the mouseDown event handler. So, we set this in line (5) in the following code snippet:

void onMouseDown(MouseEvent e) {
  // same code as in Spiral 4 - 
 if (cell.twin == lastCellClicked && lastCellClicked.shown) { (1)
   lastCellClicked.hidden = false;                            (2)
     if (memory.recalled)   memory.hide();                    (3)
   } else {
     new Timer(const Duration(milliseconds: 1000), () =>     cell.hidden = true);                  (4)
   }
   lastCellClicked = cell;                                    (5)
 }

In line (1), we check whether the last clicked cell was the twin cell, and if this is still shown. Then we make sure in (2) that it stays visible. shown is a new getter in the Cell class to make the code more readable: bool get shown => !hidden;. If at that moment all cells are shown (the memory is recalled), we again hide them all in line (3). If the last clicked cell was not the twin cell, we hide the current cell after one second in line (4). recalled is a simple getter (read-only property) in the Memory class, and it makes use of a Boolean variable in Memory that is initialized to false (_recalled = false;):

bool get recalled {
    if (!_recalled) {
      if (cells.every((c) => c.shown)) {                   (6)
        _recalled = true;
      }
    }
    return _recalled;
}

In line (6), we test that if every cell is shown then this variable is set to true (the game is over). every is a new method in Cells and a nice functional way to write this is given as follows:

bool every(Function f) => list.every(f);

The hide method is straightforward: hide every cell and reset the _recalled variable to false:

hide() {
    for (final cell in cells) cell.hidden = true;
    _recalled = false;
}

That's it, our game works!