Checkmark does not show on flutter checkbox even after setState

Question

    Widget build(BuildContext context) {
    // TODO: implement build
    var TaskTextField;
    var _newValue = false;
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(
          title: Text("To-Do List V2 R1"),
        ),
        body: Container(
          child: Column(
            children: <Widget>[
              Container(
                child: TextField(
                  decoration:
                      InputDecoration(hintText: "Enter task to be added"),
                  onChanged: (taskTextField) {
                    TaskTextField = taskTextField;
                    print(TaskTextField);
                  },
                ),
                margin: EdgeInsets.all(16.0),
              ),
              Text("Click Me!"),
              Checkbox(
                activeColor: Colors.blue,
                value: _newValue,
                onChanged: (val) {
                  setState(() {
                    _newValue = !_newValue;
                  });
                },
              ),
            ],
          ),
        ),
      ),
    );
  }
}

The checkmark does not show up even though the debugger shows that _newValue is true when clicked. I tried different methods, such as using _newValue = !_newValue but it still does not work. I am running on Flutter 1.2.1.

Answer 1

Variables declared inside a function/method are recreated every time the function is executed. setState() causes build() to rerun, so _newValue is initialized to false every time.

Move the field out of the build() method to retain the value

var _newValue = false;

Widget build(BuildContext context) {
  // TODO: implement build
  var TaskTextField;
  ...