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

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'.

CodePudding user response：

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

CodePudding user response：

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(() {});
  }

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