startListening method

void startListening()

Attach listeners for each cell - the grid is now listening for changes to any cell, and will broadcast them through _onChange

Implementation

void startListening() {
  _cellStreamSubs = [];
  _onChange = new StreamController.broadcast();

  for (int r = 0; r < 9; r++) {
    for (int c = 0; c < 9; c++) {
      _cellStreamSubs
          .add(_matrix![r][c].change.listen((cell) => _onChange.add(cell)));
    }
  }
}