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Flutter: How To Change Variables Through TextFormField In A AlertDialog?

I want to implement a function as following: There is a button named 自定义. In this page, there is a variable named money. When I click the button, an AlertDialog with a TextFormField inside will occur. I hope that after inputting a number X into the TextFormField and clicking button ok to exit the AlertDialog, money would be changed to X. I have used onSaved to save the variable, and used _formkey.currentState.save(), but money didn't change. What's wrong with my codes? Here are my codes:

void _showMessageDialog() {
    //int addMoney = 0;
    showDialog(
      context: context,
      builder: (BuildContext context) {
        // return object of type Dialog
        return AlertDialog(
          key: _formKey,
          title: new Text('INPUT'),
          content: TextFormField(
            maxLines: 1,
            keyboardType: TextInputType.emailAddress,
            autofocus: false,
            style: TextStyle(fontSize: 15),
            decoration: new InputDecoration(
                border: InputBorder.none,
                hintText: 'input',
            ),
            onSaved: (value) {
              money = int.parse(value?.trim() ?? '0') as double;
              print(money);
            }
          ),
          actions: <Widget>[
            new TextButton(
              key: _formKey,
              child: new Text("ok"),
              onPressed: () {
                _formKey.currentState?.save();
                Navigator.of(context).pop();
              },
            ),
          ],
        );
      },
    );
  }

Here are the codes relative to the button 自定义

OutlinedButton(
                                style: OutlinedButton.styleFrom(
                                  side: BorderSide(
                                    width: 1,
                                    color: Colors.blueAccent
                                  )
                                ),
                                onPressed: () {
                                  // Navigator.of(context).push(
                                  //   _showMessageDialog()
                                  // );
                                  _showMessageDialog();
                                },
                                child: Text(
                                  "自定义",
                                  style: TextStyle(
                                    fontSize: 20,
                                    fontWeight: FontWeight.w700,
                                    color: Colors.blueAccent
                                  ),
                                ),
                              ),

I know maybe I have made a big mistake, but it is my first Flutter project. Thanks for your advices.

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Answer

I would use ValueNotifier for this. But first you need to add a controller to your TextFormField so you can get the text user typed in.

//initialize it
final myController = TextEditingController();

TextFormField(
  controller: myController, //pass it to the TextFormField
),

TextButton(
  child: new Text("ok"),
  onPressed: () {
    String input = myController.text; //this is how you get the text input
    _formKey.currentState?.save();
    Navigator.of(context).pop();
  },
),

As I said you also need to initialize ValueNotifier and pass it to _showMessageDialog

ValueNotifier<int> money = ValueNotifier(0);
_showMessageDialog(money); //and pass it to your function

void _showMessageDialog(ValueNotifier<int> money) {
  TextButton(
    child: new Text("ok"),
    onPressed: () {
      String input = myController.text; //this is how you get the text input
      money.value = int.parse(input); //and then update your money variable
      _formKey.currentState?.save();
      Navigator.of(context).pop();
    },
  ),
}

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source: stackoverflow.com
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