数据未更新到另一个小部件颤动

Question

I am trying to show the price of items in the cart but the total value should be shown in TextField. I am saving data to SQLite and retrieving then show to a widget, but when I try to access total_price to another widget it's not updating, but When I press hot reload again the data shows but not first time when I am opening the page

  return FutureBuilder<List<CartModel>>(
      future: fetchCartFromDatabase(),
      builder: (context, snapshot) {

        if (snapshot.hasData && snapshot.data.length > 0) {
           cartCount = snapshot.data.length;
           for(int i = 0;i<snapshot.data.length;i++){
            var price = snapshot.data[i].product_price.split("₹");
            total_price+=double.parse(price[1]);
          }
  } else if (snapshot.hasData && snapshot.data.length == 0) {
          return new Text("No Data found");
        }
        else
          {
            return new Container(
              alignment: AlignmentDirectional.center,
              child: new CircularProgressIndicator(),
            );
          }
);

value initialized

  int cartCount = 0;
  double total_price=0.0;

Answer 1

The FutureBuilder updates only its children. To update the value of another widget you must use setState.

The best way would be putting FutureBuilder in an upper level or using some sort of state manager, like provider.

To use setState you need to initialize you fetch from an initState of a stetefullWidget (or to call it from a function). This way you will not need a FutureBuilder and must refactor your code:

class YourWidget extends StatefulWidget {
  @override
  _YourWidgetState createState() => _YourWidgetState();
}

class _YourWidgetState extends State<YourWidget> {
  double total_price = 0;

  @override
  void initState() {
    super.initState();
    fetchCartFromDatabase().then((value){
      setState((){
        for(int i = 0;i<value.length;i++){
          var price = value[i].product_price.split("₹");
          total_price+=double.parse(price[1]);
        } 
      });
    });
  }
}

The addPostFrameCallback is not a good solution, since it updates the value only in the next frame. When the app grows it leads to lags.

To continue using the FutureBuilder, move your widget tree that needs to be updated to be inside of the FutureBuilder.