ListViewController+ToggleCompleted.swift

func toggleCompleted(forItem listItem: ListItem, on collectionView: UICollectionView) {
  let foundIndex = items.index { (given) -> Bool in
    return given.id == listItem.id
  }
  if(listItem.isCompleted) {
    if let index = foundIndex {
      let item = items.remove(at: index)
      items.insert(item, at: 0)
      item.isCompleted = false
      collectionView.moveItem(at: IndexPath(item: index, section: 0), to: IndexPath(item: 0, section: 0))
    }
  } else {
    if let index = foundIndex {
      let item = items.remove(at: index)
      items.append(item)
      item.isCompleted = true
      collectionView.moveItem(at: IndexPath(item: index, section: 0), to: IndexPath(item: items.count - 1, section: 0))
    }
  }
}