Error DropdownMenuItems detected with same value Flutter

Question

I get this error from the DropdownButton Widget.

`There should be exactly one item with [DropdownButton]'s value: Seleccionar. 
Either zero or 2 or more [DropdownMenuItem]s were detected with the same value
'package:flutter/src/material/dropdown.dart':
package:flutter/…/material/dropdown.dart:1
Failed assertion: line 890 pos 15: 'items == null || items.isEmpty || value == null ||
              items.where((DropdownMenuItem<T> item) {
                return item.value == value;
              }).length == 1'`

Here's the code

class _NewCauseMenuState extends State<NewCauseMenu> {
  final db = FirebaseDatabase.instance;
  var dropdownValue;
  var setDefaultSelection = true;

  @override
  Widget build(BuildContext context) {
    final myRef = db.ref();
    return WillPopScope(
      onWillPop: () {
        return _onWillPopScope();
      },
      child: SafeArea(
          child: Scaffold(
        appBar: AppBar(
          title: Text('Menú Nueva Causa ${widget.area}'),
        ),
        body: Center(
          child: Column(
            children: [
              const SizedBox(
                height: 25,
              ),
              StreamBuilder(
                stream: myRef.child(widget.area.toString()).onValue,
                builder: (BuildContext context, AsyncSnapshot snapshot) {
                  if (snapshot.hasData) {
                    final lista = Map<String, dynamic>.from(
                        (snapshot.data)!.snapshot.value as Map);
                    return DropdownButton(
                      value: dropdownValue,
                      icon: const Icon(Icons.arrow_downward),
                      isExpanded: false,
                      elevation: 16,
                      underline: Container(
                        height: 2,
                        color: Colors.blueAccent,
                      ),
                      items: lista.keys.map(((e) {
                        return DropdownMenuItem(
                            value: e.toString(), child: Text(e.toString()));
                      })).toList(),
                      onChanged: (value) {
                        setState(() {
                          dropdownValue = value;
                          setDefaultSelection = false;
                        });
                      },
                    );
                  }
                  if (setDefaultSelection) {
                    dropdownValue = 'Seleccionar';
                  }
                  return Container();
                },
              ),
            ],
          ),
        ),
      )),
    );
  }

I was searching for a solution and I found that the value from the DropdownButton must be unique or that the value doesn't pass to the DropdownMenuItem, but I don't see clearly where I'm wrong. Help me, please.

I tried use a if sentence where a variable defined with boolean true pass to false where the value changed on the DropdownButton, so the value 'Seleccionar' doesn't pass to all the values from the MenuItem. But it didn't work.

When I remove the value property from the DropdownButton, occurs this:

I guess either lists is empty or you have the same key twice in lista — w461, Jan 06 '23 at 16:16
I edited the post. The snapshot gets me the keys showed in the gif. So the keys are not duplicated nor the list is empty — Juan Peralta Torres, Jan 06 '23 at 16:38

rrttrr · Accepted Answer · 2023-01-06T16:57:51.503

1

This might be because the stream is returning duplicate values.

Change this part of the code,

 items: lista.keys.map(((e) {
   return DropdownMenuItem(
      value: e.toString(), child: Text(e.toString()));
   })).toList().toSet().toList(),
      // notice toSet()

This will remove duplicate items from the list.

Or, the error might be because of below reason.

The DropdownButton Widget's value should be present in the DropdownMenu items. Otherwise you will get this error.

In your case, Seleccionar is the value of dropdown. But Seleccionar is not there among the dropdown items.

if (setDefaultSelection) {
  dropdownValue = 'Seleccionar';
}

The default value is not there in the DropdownMenu items.

If you change default value to Obsolecencia, Producto caducado or Stock out the error will go away.

edited Jan 06 '23 at 16:57

answered Jan 06 '23 at 16:21

rrttrr

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Didn't work. I guess that the var `dropdownValue` is repeating for each value in the list. – Juan Peralta Torres Jan 06 '23 at 16:27
can you print "lista" and post the result? – rrttrr Jan 06 '23 at 16:42
lista prints this: `{Obsolecencia: {Costo de operación: {Producción: {Costo de materias primas: {Precios de combustible: {Solución: Realizar acuerdos con distribuidoras de combustible para obtener precios bajos para las unidades de la empresa.}, Distancia: {Solución: Encontrar canteras que se encuentren cerca de las plantas de producción.}}}, Mantenimiento: {Procedimiento de mantenimiento:` and more... – Juan Peralta Torres Jan 06 '23 at 16:45
lista.keys prints this: `(Obsolecencia, Producto caducado, Stock out)` That's what I need and it shows in the dropdownbutton but the value doesn't change – Juan Peralta Torres Jan 06 '23 at 16:46

Error DropdownMenuItems detected with same value Flutter

1 Answers1