Related DropdownButtons and raising onChange event - flutter

Question

I have two DropdownButton widget. Content of second one depends on first one selection. Second DropdownButton will initiate refresh of third widget. How can I initiate refresh of second DropdownButton when first one is populated? And then how can I refresh third widget when second DropdownButton populated also?
class ParentBloc {
Stream<List<Parent>> get items => _controller.asyncMap(...);
Future<List<Parent>> _callApi() {
// call endpoint /parents
}
}
class ChildBloc {
ChildBloc(this.parentId);
int parentId;
Stream<List<Child>> get items => _controller.asyncMap(...);
Future<List<Child>> _callApi() {
// call endpoint /parents/$parentId/children
}
}
// This bloc created at init state
ParentBloc parentBloc;
// This bloc will be created only after value will
// be selected in the Parent dropdownbutton because
// I need to know `parentId`.
ChildBloc childBloc;
#override
void initState() {
super.initState();
parentBloc = ParentBloc();
}
#override
Widget build(BuildContext context) {
return Row(
children: [
StreamBuilder<List<Parent>>(
stream: parentBloc.items,
builder: (context,snapshot) {
return DropdownButton(
items: snapshot.data.map((item) {
return DropdownButtonItem();
}),
);
}
),
// Content of this widget depends on above one
StreamBuilder<List<Child>>(
stream: childBloc.items,
builder: (context,snapshot) {
return DropdownButton(
items: snapshot.data.map((item) {
return DropdownButtonItem();
}),
);
}
),
// Content of this widget depends on above one
StreamBuilder<List<Grandchild>>(
stream: grandchildBloc.items,
builder: (context,snapshot) {
return ListView(),
);
}
),
]
);
}

Answer 1

Provided you're doing this inside a StatefulWidget, you can use setState inside one of your functions where you update the variables that in turn have to be used to control what is currently displayed in each of your widgets.
It should look something like this (inside your Dropdown):
onChanged: (newValue) {
setState(() {
_currentSelection = newValue;
});
},
Update: after discussion in the comments, here's a working example that I made of how something can be updated based on a value selected inside a dropbox, hope it helps:
import 'package:flutter/material.dart';
class ExampleWidget extends StatefulWidget {
#override
_ExampleWidgetState createState() => _ExampleWidgetState();
}
class _ExampleWidgetState extends State<ExampleWidget> {
List<String> someStringsToSelectFrom = [
'value1',
'value2',
'value3',
];
String selectedValue;
#override
Widget build(BuildContext context) {
return Scaffold(
body: Center(
child: Column(
mainAxisAlignment: MainAxisAlignment.center,
children: <Widget>[
Text('Select something:'),
DropdownButton<String>(
value: selectedValue,
icon: Icon(
Icons.arrow_downward,
color: Colors.deepPurpleAccent,
),
iconSize: 24,
elevation: 16,
style: TextStyle(
fontSize: 30.0,
color: Colors.green,
),
underline: Container(
height: 2,
color: Colors.deepPurpleAccent,
),
onChanged: (String newValue) {
setState(() {
selectedValue = newValue;
});
},
items: someStringsToSelectFrom.map<DropdownMenuItem<String>>((String value) {
return DropdownMenuItem<String>(
value: value,
child: Text(value),
);
}).toList(),
),
Text('This will update after you selected a value:'),
someStringsToSelectFrom.contains(selectedValue) ? Text(selectedValue + ' was selected') :
Text('Still waiting for user action.'),
],
),
),
);
}
}