在 Flutter 中为 DropdownButton 创建项目列表

Question

I'm back with another issue with the DropdownButton.

The DropdownButton is not enabled. I found this in api.flutter.dev

If the onChanged callback is null or the list of items is null then the dropdown button will be disabled, ie its arrow will be displayed in grey and it will not respond to input.

Here is my code now:

return new DropdownButton<String>(
                            hint: new Text("Select Agency"),
                            value: _currentAgency,
                            onChanged: changedDropDownAgency,
                            items: snapshot.data.docs.forEach((document) {
                              return new DropdownMenuItem<String>(
                                value: document.data()['name'],
                                child: new Text(document.data()['name']),
                              );
                            }),
                          );

void changedDropDownAgency(String selected_agency) {
    setState(() {
      _currentAgency = selected_agency;
    });
    globals.selectedAgency = selected_agency;
  }

The forEach loop runs fine and in debug mode I can see that there is data in the document object. I don't know how to debug the DropdownButton code to see why the button is not active. Any suggestions would be helpful. Thanks.

Answer 1

forEach() on Iterables does not return any value (see: https://api.dart.dev/stable/2.10.5/dart-core/Iterable/forEach.html ), and thus items is null and the DropdownButton is disabled. Use map instead ( https://api.dart.dev/stable/2.10.5/dart-core/Iterable/map.html ). Example:

snapshot.data.docs.map<DropdownMenuItem<String>>((document) {
    return new DropdownMenuItem<String>(
        value: document.data()['name'],
        child: new Text(document.data()['name']),
    );
}).toList(),