Flutter: Checkbox Tristate won't work on Null Safety - flutter

Question

How can I archive tristate for checkbox with nullsafety.... i have to pass null into that value but nullsafety has to perform null check on a variable, this is just contradicting .....
Putting null check as usual:
bool? parentvalue;
void update() {
parentvalue = null;
}
Checkbox(
....
value: parentvalue!,
onChanged: update(),
....
),
ERROR: Null check operator used on a null value
if I remove null check, code cannot compile at all
bool parentvalue;
void update() {
parentvalue = null;
}
Checkbox(
....
value: parentvalue,
onChanged: update(),
....
),
ERROR: A value of type 'Null' can't be assigned to a variable of type 'bool'.
=================================================================
UPDATE:
Checkbox(
tristate: true,
checkColor: Colors.white,
fillColor: MaterialStateProperty.resolveWith(getColor),
value: isChecked,
onChanged: (bool? value) {
setState(() {
isChecked = value!;
});
},
),
ERROR: Null check operator used on a null value
if i remove null check
onChanged: (bool value) {
setState(() {
isChecked = value;
});
},
ERROR: The argument type 'void Function(bool)' can't be assigned to the parameter type 'void Function(bool?)?'.

Answer 1

You need to add tristate: true in order to accept null. Also, the update() method needed to be changed because it provides nullable bool on callback.
Checkbox(
tristate: true,
value: parentvalue,
onChanged: update,
// onChanged:(value) { update(); },
),
And update method will be
void update(value) {
parentvalue = value;
setState(() {});
}
More about Checkbox on flutter.dev.

Answer 2

Here's an example with setting tristate to true:
import 'package:flutter/material.dart';
void main() {
runApp(MyApp());
}
class MyApp extends StatelessWidget {
#override
Widget build(BuildContext context) {
return MaterialApp(
home: Scaffold(
body: Row(
children: [
MyCheckbox(),
],
),
),
);
}
}
class MyCheckbox extends StatefulWidget {
const MyCheckbox({Key? key}) : super(key: key);
#override
State<MyCheckbox> createState() => _MyCheckboxState();
}
class _MyCheckboxState extends State<MyCheckbox> {
bool? parentValue;
void update() {
setState(() {
parentValue = null;
});
}
#override
Widget build(BuildContext context) {
return Checkbox(
tristate: true,
value: parentValue,
onChanged: (_) {
update();
});
}
}

Answer 3

tristate property used for pass null value to checkbox as place of boolean
so if you want to pass null value to checkbox without showing any error then you have to set true value to the property of checkbox named tristate
like this
CheckBox(
....
tristate:true,
value:null,
...),

Answer 4

Let's discuss how Null safety works.
If you expecting a value(variable/object) that shouldn't be null then simply make that value null safety. In your case
bool? parentvalue .
& if you read Checkbox property
/// This property must not be null.
final bool? value;
/// If tristate is false (the default), [value] must not be null.
final bool tristate;
here value is null safety & you cannot assign null value on it.
so if you assign your parentvalue null this will be not accepted by, Checkbox value property. it only except Boolean(true/false).
void update() {
//Wrong
parentvalue = null;
}
So either you should make your parentvalue by default true or false as per requirements. or you can also use value: parentvalue??false/true to maintain null safety.

Answer 5

the value of Checkbox must be nut null
if you want to Checkbox be unChecked when value is null you can use this instead of null check
value: parentvalue??false