CategoryCellModel.swift

struct CategoryCellModel { //CellModel儲存category跟isSelected參數
    let category: Category
    var isSelected: Bool = false
}

var categories = [CategoryCellModel]() {
    didSet {
        DispatchQueue.main.async {
            self.categoryCollectionView.reloadData()
        }
    }
}

func setProductType() {
    ProductService.shared.loadProductType(id: user.member_id, pwd: user.member_pwd) { responseCategories in
        //新增isFirstCategory，map後將responseCategories的第一個項目isSelected設成true
        var isFirstCategory = true
        self.categories = responseCategories.map {
            if isFirstCategory {
              //第一個項目會跑到這裡，之後因為isFirstcategory == false，所以不會設到isSelected狀態(false)
                isFirstCategory = false
                return CategoryCellModel(category: $0, isSelected: true)
            } else {
                return CategoryCellModel(category: $0)
            }
        }
    }
}

func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
    guard let item = categoryCollectionView.dequeueReusableCell(withReuseIdentifier: CategoryCollectionViewCell.reuseIdentifier, for: indexPath) as? CategoryCollectionViewCell else {
        return UICollectionViewCell()
    }
    let category = categories[indexPath.row]
    item.configure(with: category)
    item.isItemSelected = category.isSelected //isItemSelected是cell對應isSelected true/false進行UI變動
  //從這裡因為category[0].isSelected == true，所以會變更UI外觀
        
    return item
}

func collectionView(_ collectionView: UICollectionView, didSelectItemAt indexPath: IndexPath) {
    for (index, _) in categories.enumerated() {
      //當index與indexPath.item有對應到，isSelected = true，狀態改變
        categories[index].isSelected = index == indexPath.item
    }
}
//Cell的property
var isItemSelected: Bool = false {
    didSet {
        indicatorView.alpha = isItemSelected ? 1 : 0
        categoryLabel.textColor = isItemSelected ? UIColor(hex: "#5F9EA0") : .systemGray2
    }
}