Flutter 向列表中添加新项目替换列表中的旧项目

当我向列表中添加新项时，它将覆盖先前的值。我想附加列表，但它没有发生

Container(
  // color: Colors.red,
  child: Row(
    mainAxisAlignment: MainAxisAlignment.end,
    children: <Widget>[
      Container(
        width: 50,
        height: 50,
        // color: Colors.blue,
        child: FlatButton(onPressed: (){
          if(_itemCount > 0){
            setState(() {
              _itemCount--;
            });

          }
        }, child: Image(image: AssetImage("images/minus.png"),width: 20,height: 20,),),
      ),
      Container(
        child: Text("$_itemCount"),
      ),
      Container(
        width: 50,
        height: 50,
        //  color: Colors.green,
        child: FlatButton(onPressed: () async{
          setState(() {
            _itemCount++;
          });
          
          cartItemList = [{"index":widget.intex,"itemObj":widget.items,"quantity":_itemCount}];
          print(addedItems(cartItemList));
          
          final prefs = await SharedPreferences.getInstance();
          await prefs.setStringList('itemList', addedItems(cartItemList));
        }, child: Image(image: AssetImage("images/plus.png"),width: 20,height: 20,),),
      ),
    ],
  ),
),

输出为

当我添加第一项时

 [{"itemObj":{"item_price":22.0,"item_name":"DINE SPECIAL BREAKFAST","item_img":" ","item_code":"001","item_discount":0.0,"item_id":552,"category_id":12},"quantity":1}]

当我再次添加相同的项目时

 [{"itemObj":{"item_price":22.0,"item_name":"DINE SPECIAL BREAKFAST","item_img":" ","item_code":"001","item_discount":0.0,"item_id":552,"category_id":12},"quantity":2}]

（数量增加）

当我添加新项目时

 [{"itemObj":{"item_price":20.0,"item_name":"English Breakfast","item_img":" ","item_code":"002","item_discount":0.0,"item_id":71,"category_id":12},"quantity":1}]

（餐厅的特色菜在哪里？）

---

但我想要像这样的输出

[{"itemObj":{"item_price":22.0,"item_name":"DINE SPECIAL BREAKFAST","item_img":" ","item_code":"001","item_discount":0.0,"item_id":552,"category_id":12},"quantity":2},{"item_price":20.0,"item_name":"English Breakfast","item_img":" ","item_code":"002","item_discount":0.0,"item_id":71,"category_id":12},"quantity":1}]

---

您每次单击按钮时都会设置列表，如下所示

cartItemList = [{"index":widget.intex,"itemObj":widget.items,"quantity":_itemCount}];

也许你需要做的是

cartItemList.add({"index":widget.intex,"itemObj":widget.items,"quantity":_itemCount});

---

您每次单击按钮时都会设置列表，如下所示

cartItemList = [{"index":widget.intex,"itemObj":widget.items,"quantity":_itemCount}];

也许你需要做的是

cartItemList.add({"index":widget.intex,"itemObj":widget.items,"quantity":_itemCount});