Flutter formfield abnormality - forms

Question

I'm getting an abnormal effect in Flutter and I'm not sure if it's an error in my code or with flutter.
I'm trying to create a FormField object that is a checkbox that can have three values: empty, checked positive, or checked negative. Here is my code so far:
import 'package:flutter/material.dart';
void main() => runApp(new MyApp());
class MyApp extends StatelessWidget {
#override
Widget build(BuildContext context) {
return new MaterialApp(
title: 'Checkbox Test',
theme: new ThemeData(
primarySwatch: Colors.blue,
),
home: new Form(child: MyHomePage(title: 'Checkbox Test')),
);
}
}
class MyHomePage extends StatefulWidget {
MyHomePage({Key key, this.title}) : super(key: key);
final String title;
#override
_MyHomePageState createState() => new _MyHomePageState();
}
class _MyHomePageState extends State<MyHomePage> {
List<String> _savedList = [];
#override
Widget build(BuildContext context) {
ThemeData themeData = Theme.of(context);
return new Scaffold(
appBar: new AppBar(
title: new Text(widget.title),
),
body: new Center(
child: new Column(
mainAxisAlignment: MainAxisAlignment.center,
children: <Widget>[
new CheckboxFormField(
title: "Option 1",
saved: _savedList,
themeData: themeData,
),
new CheckboxFormField(
title: "Option 2",
saved: _savedList,
themeData: themeData,
),
new CheckboxFormField(
title: "Option 3",
saved: _savedList,
themeData: themeData,
),
],
),
),
floatingActionButton: new FloatingActionButton(
onPressed: () {Form.of(context).reset();},
tooltip: 'Reset',
child: new Icon(Icons.refresh),
),
);
}
}
class CheckboxFormField extends FormField<dynamic> {
CheckboxFormField({
Key key,
bool checkState,
bool initialValue = false,
Color activeColor,
String title,
List<String> saved,
ThemeData themeData,
}) : super(
key: key,
initialValue: initialValue,
builder: (FormFieldState<dynamic> field) {
void onChange() {
if (field.value == false) {
activeColor = Colors.green;
print(activeColor);
checkState = true;
field.didChange(true);
saved.add(title);
print(saved);
} else if (field.value == true) {
activeColor = Colors.red;
print(activeColor);
checkState = null;
field.didChange(null);
saved.remove(title);
saved.add("Not " + title);
print(saved);
} else {
activeColor = themeData.textTheme.subhead.color;
checkState = false;
field.didChange(false);
saved.remove("Not " + title);
print(saved);
}
}
return Checkbox(
tristate: true,
value: field.value,
activeColor: activeColor,
onChanged: (value) => onChange());
});
}
Now this works fine when the app is first started, and does exactly what's expected, with each click cycling through the appropriate color and adding the appropriate value to the list.
The problem arises if the Form is reset: function-wise it remains working normally, but the color changing stops working. The checkbox reverts to the theme's base togglableActiveColor, so despite my function's rotation still being preserved with empty box -> positive check -> negative check -> repeat, the checkbox color becomes togglableActiveColor when it is set to true or null, and unselectedWidgetColor when it is false.
I know the code is still working as the checkbox changes appropriately, and the saved list continues to be appended and removed from appropriately, just the color stops behaving. Is this a glitch in the engine or am I missing something in my code?
Thanks
Bry
EDIT: Continuing to look into it further it seems that on the rebuild by Form.of(context).reset the activeColor property somehow gets lost, while everything else is preserved. Maybe this is by design, any ideas?