Question 412790 in Flutter Question posted in
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Rerendering widgets in flutter

I am having trouble understanding state management in flutter I have the folloing build method:

Widget build(BuildContext context) {
    print("calling build with selectedItem: $selectedItem");

    // TODO: implement build
    return SafeArea(
      child: SingleChildScrollView(
          child: Padding(
        padding: const EdgeInsets.all(8.0),
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          mainAxisSize: MainAxisSize.min,
          children: [
            Flexible(
              fit: FlexFit.loose,
              child: DropdownButton<String>(
                value: selectedItem,
                style: const TextStyle(color: Colors.black),
                underline: Container(height: 2, color: Colors.deepPurpleAccent),
                icon: Icon(Icons.arrow_downward),
                iconSize: 20,
                elevation: 16,
                items: widget.streetNamesList
                    .map<DropdownMenuItem<String>>((String value) {
                  return DropdownMenuItem<String>(
                      value: value, child: Text(value));
                }).toList(),
                onChanged: (String? value) {
                  setState(() {
                    selectedItem = value;
                  });
                },
              ),
            ),
            SizedBox(height: 10.0,),
            CarListPerStreetWidgetV2(streetName: selectedItem,)
          ],
        ),
      )),
    );
  }

The CarListPerStreetWidget implements a future provider that uses the streetname to fetch data from the database.
The problem is that when I choose a new value from the drop down menu although the build method gets executed, the build method of CarListPerStreetWidgetV2 also gets executed, but the widget does not get re-initialized with the new selectedItem I am guessing Widgets get initialized once, but how do I pass a state changed from a widget to another?

Tags:

2

Answers


  1. 0

    Can you share the CarListPerStreetWidgetV2 widget? If this widget is a StatefulWidget, you must update that widget in the didUpdateWidget.

    For example:

    @override
void didUpdateWidget(covariant CarListPerStreetWidgetV2 oldWidget) {
  super.didUpdateWidget(oldWidget);
  if(oldWidget.streetName != widget.streetName) {
    setState(() {});
  }
}
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  2. 0

    The code seems to be correct, but there are a few things you haven’t shared so check if these things are correct.

    • selectedItem should be a member of the class, not a local variable in the build method.
    • if it is a class member, selectedItem isn’t reset to default value somewhere in the build method. All initial setup should be done in initState.
    • the dropdown options should be distinct, flutter can optimize equal values that cause no change in the state to not rebuild widgets.

    Also check these things which you might have missed.

    • CarListPerStreetWidgetV2 is using the passed in value not something local to it.
    • Breakpoint is being hit in the build method of both functions, set a breakpoint in the first line of build method.

    If it still doesn’t work, you should share the other parts of the code.

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