The issue is with incriment quantity ,decriment quantity and product price

When I pressed one button The other buttons automatically pressed .I don't know the issue . And the second is with product price when i incriment the item the price become 100100 if i have price 100 Please solve my issues.

Here is my code

int _quantity=1; Stream builder

StreamBuilder(
        stream: FirebaseFirestore.instance.collection("Cart").where("id",isEqualTo:FirebaseAuth.instance.currentUser!.uid).snapshots(),
        builder:(BuildContext context,AsyncSnapshot<QuerySnapshot>snapshot){
          if (!snapshot.hasData) {
            return Center(child: LoadingAnimationWidget.staggeredDotsWave(color: Colors.red, size: 100));
          }
         return ListView.builder(
             itemCount: snapshot.data!.docs.length,
             itemBuilder: (itemBuilder,index){
               return Padding(
                 padding: const EdgeInsets.all(8.0),
              

Product price

child: Card(
  child: Container(
    child: Row(
      children: [
            Row(
              children: [
                Text(
           "${ snapshot.data!.docs[index]["productprice"]*_quantity}",
                  style: TextStyle(
                    fontWeight: FontWeight.bold,
                    fontSize: 16,
                  ),
                ),

                SizedBox(width: 16),
                IconButton(
                  icon: Icon(Icons.remove),
                  onPressed: (){
                    setState(() {
                      if (_quantity>1) {
                            _quantity--;
                      }
                    });
                  },
                ),
                SizedBox(width: 8),
                Text(
                  "$_quantity",
                  style: TextStyle(
                    fontWeight: FontWeight.bold,
                    fontSize: 16,
                  ),
                ),
                SizedBox(width: 8),
                IconButton(
                  icon: Icon(Icons.add),
                  onPressed: (){
                    setState(() {
                      _quantity++;
                    });
                  },
                ),
              ],
            ),
          ],
        ),
      ],
    ),
  ),
)));

             });

        } ,

      ),

Looks you have a String value in snapshot.data!.docs[index]["productprice"]. You should convert String to double or int before multiplication.

So you code should look as following

        Text(
           "${ double.parse(snapshot.data!.docs[index]["productprice"].toString()) * _quantity}",
                  style: TextStyle(
                    fontWeight: FontWeight.bold,
                    fontSize: 16,
                  ),
                ),

Edited:

If you want to manage with multiple items, then introduce one field final Map<String, int> quantity = <String, int>{};

StreamBuilder(
        stream: FirebaseFirestore.instance.collection("Cart").where("id",isEqualTo:FirebaseAuth.instance.currentUser!.uid).snapshots(),
        builder:(BuildContext context,AsyncSnapshot<QuerySnapshot>snapshot){
          if (!snapshot.hasData) {
            return Center(child: LoadingAnimationWidget.staggeredDotsWave(color: Colors.red, size: 100));
          }
         return ListView.builder(
             itemCount: snapshot.data!.docs.length,
             itemBuilder: (itemBuilder,index){
               final doc = snapshot.data!.docs[index];
               final _quantity = quantity[doc.id] ?? 0;
               return Padding(
                 padding: const EdgeInsets.all(8.0),
              

child: Card(
  child: Container(
    child: Row(
      children: [
            Row(
              children: [
fina;
                Text(
           "${doc["productprice"]*_quantity}",
                  style: TextStyle(
                    fontWeight: FontWeight.bold,
                    fontSize: 16,
                  ),
                ),

                SizedBox(width: 16),
                IconButton(
                  icon: Icon(Icons.remove),
                  onPressed: (){
                      if (_quantity>1) {
                    setState(() {
                      quantity[doc.id] = _quatity - 1;
                    });
                   }
                  },
                ),
                SizedBox(width: 8),
                Text(
                  "$_quantity",
                  style: TextStyle(
                    fontWeight: FontWeight.bold,
                    fontSize: 16,
                  ),
                ),
                SizedBox(width: 8),
                IconButton(
                  icon: Icon(Icons.add),
                  onPressed: (){
                    setState(() {
                      quantity[doc.id] = _quatity + 1;
                    });
                  },
                ),
              ],
            ),
          ],
        ),
      ],
    ),
  ),
)));