DropdownButton 未显示在 Flutter 应用程序的第二页中

Question

My DropdownButton will not show up on the second page of flutter app.

This is the DropdownButton code implemented in ConversionRates class (page2):

Widget buildDropDownButton(String currencyCategory) {
return DropdownButton(
    value: currencyCategory,
    items: converter.currencies.keys
        .map((String value) => DropdownMenuItem(
            value: value,
            child: Row(children: <Widget>[
              Text(value),
            ])))
        .toList(),
    onChanged: (String value) {
      if (currencyCategory == converter.fromCurrency) {
        setState(() {
          converter.onFromChanged(value);
        });
      } else {
        setState(() {
          converter.onToChanged(value);
        });
      }
    });

This is the code of the rest of the class ConversionRates (page2):

class ConversionRates extends StatefulWidget {
Map<String, double> currencies;

ConversionRates({Key key, @required this.currencies}) : super(key: key);

@override
ConversionRatesState createState() => ConversionRatesState(currencies);
}

class ConversionRatesState extends State<ConversionRates> {
final Converter converter = new Converter.defaultConstructor();

String _currentItemSelected = "SEK";

Map<string, double> currencies;

ConversionRatesState(this.currencies);

@override
void initState() {
  super.initState();

  setState(() {});
}

@override
Widget build(BuildContext context) {
  return Scaffold(
      backgroundColor: Colors.blueAccent,
      appBar: AppBar(
        backgroundColor: Colors.black87,
        title: Center(
            child: Text(
          "Conversion Rates",
          style: TextStyle(
              color: Colors.white,
              fontWeight: FontWeight.bold,
              fontStyle: FontStyle.italic,
              fontSize: 35.0),
        )),
      ),
      body: converter.currencies.keys == null
          ? Center(child: CircularProgressIndicator())
          : Stack(children: [
              Container(
                  color: Colors.black26,
                  alignment: Alignment.center,
                  height: MediaQuery.of(context).size.height,
                  width: MediaQuery.of(context).size.width,
                  child: Center(
                      child: Stack(children: [
                    Positioned(
                        top: 40,
                        left: 70,
                        child: Container(
                            color: Colors.transparent,
                            child: Column(
                                mainAxisAlignment:
                                    MainAxisAlignment.spaceEvenly,
                                children: [
                                  ListTile(
                                      title: TextField(),
                                      trailing: buildDropDownButton(
                                          converter.fromCurrency)),
                                  RichText(
                                      text: TextSpan(
                                    children: <TextSpan>[
                                      TextSpan(
                                          text: "SEK: " +
                                              converter.currencies["SEK"]
                                                  .toString(),
                                          style: TextStyle(
                                              fontWeight: FontWeight.bold,
                                              fontSize: 30.0)),
                                      TextSpan(
                                          text: "\n\nUSD: " +
                                              converter.currencies["USD"]
                                                  .toString(),
                                          style: TextStyle(
                                              fontWeight: FontWeight.bold,
                                              fontSize: 30.0)),
                                      TextSpan(
                                          text: "\n\nEUR: " +
                                              converter.currencies["EUR"]
                                                  .toString(),
                                          style: TextStyle(
                                              fontWeight: FontWeight.bold,
                                              fontSize: 30.0)),
                                      TextSpan(
                                          text: "\n\nGBP: " +
                                              converter.currencies["GBP"]
                                                  .toString(),
                                          style: TextStyle(
                                              fontWeight: FontWeight.bold,
                                              fontSize: 30.0)),
                                      TextSpan(
                                          text: "\n\nCNY: " +
                                              converter.currencies["CNY"]
                                                  .toString(),
                                          style: TextStyle(
                                              fontWeight: FontWeight.bold,
                                              fontSize: 30.0)),
                                      TextSpan(
                                          text: "\n\nJPY: " +
                                              converter.currencies["JPY"]
                                                  .toString(),
                                          style: TextStyle(
                                              fontWeight: FontWeight.bold,
                                              fontSize: 30.0)),
                                      TextSpan(
                                          text: "\n\nKRW: " +
                                              converter.currencies["KRW"]
                                                  .toString(),
                                          style: TextStyle(
                                              fontWeight: FontWeight.bold,
                                              fontSize: 30.0)),
                                    ],
                                  ))
                                ])))
                  ])))
            ]));
}

The data I am receiveng from page1 is a map<string,double> currencies. The thing is that when I dont have a dropdownbutton on the second page I can access the data in the currencies map. So the data transfer is not the problem here. But as soon as I try to make a DropdownButton I cant access the data in the currencies anymore. I also dont get any dropdownbutton. I have tried solving this problem in different ways but I am ending up with either not have anything showing in the second page (except the bar and scaffold), like the Textspan or DropdownButton or I am ending upp with emulator error saying:

NoSuchMethodError: The getter 'keys' was called on null.

So I think that the problem may be with the state managment but I dont really know how to manage it correctly. I am also creating an instance converter so I can access some data from Converter class. I also tried changing currencies map to map<dynamic,dynamic> currencies but that didnt solve the problem.

Any ideas?

Answer 1

Your currency list of the converter is null too. You need to check it. It will look like this:

     converter.currencies?.keys == null