Our app isn't playable yet:

All of this is coded in the mouseDown event handler and we also need a lastCellClicked variable of the Cell type in the Board class. Of course, this is exactly the cell we get in the mouseDown event handler. So, we will set it 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 checked whether the last clicked cell was the twin cell and whether this is still shown. Then, we made sure in (2) that it stays visible. The shown is a new getter in the Cell class to make the code more readable: bool get shown => !hidden;. If at that moment all the cells were shown (the memory is recalled), we again hid them in line (3). If the last clicked cell was not the twin cell, we hid the current cell after one second in line (4).The 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 tested that if every cell is shown, then this variable is set to true (the game is over).The new method named every, is in the Cells List 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;
}

This is it, our game works!