Flutter A value of type 'Future<dynamic>' can't be returned from the method '_onBackPress' because it has a return type of 'Future<bool>'

user_14673929 0
Viewed 5299

i am working on back button in my app but i face this issue in flutter Thank so much in advanced i am doing this before with this function but now it's not working i think because of null safty sorry for my bad english screenshot

here is my flutter doctor

Doctor summary (to see all details, run flutter doctor -v):
[√] Flutter (Channel stable, 2.2.3, on Microsoft Windows [Version 10.0.19043.1110], locale en-US)
[√] Android toolchain - develop for Android devices (Android SDK version 30.0.3)
[√] Chrome - develop for the web
[√] Android Studio (version 4.1.0)
[√] VS Code (version 1.58.2)
[√] Connected device (3 available)

• No issues found!

here is my build method and Future function

Future<bool> _onBackPress() {
    return showDialog(
      context: context,
      builder: (BuildContext context) {
        return AlertDialog(
          content: Text('Exit From The App'),
          actions: [
            TextButton(
              onPressed: () {
                Navigator.of(context).pop(false);
              },
              child: Text('No'),
            ),
            TextButton(
              onPressed: () {
                Navigator.of(context).pop(true);
              },
              child: Text('Yes'),
            ),
          ],
        );
      },
    );
  }

  @override
  Widget build(BuildContext context) {
    return WillPopScope(
      onWillPop: _onBackPress,
      child: Scaffold(
        appBar: AppBar(
          automaticallyImplyLeading: false,
          brightness: Brightness.dark,
          title: Text('Internet Availability Checker'),
        ),
      ),
    );
  }
flutter-test flutter-layout flutter
2 Answers

The showDialog is an async function, which means, it will wait until some result. In this case - until the dialog is closed. Because it is async, you can add .then (meaning "when the value is recieved") right after showDialog closing bracket.

.then((value) => value ?? false);

By the way by default "value" is null, and it will be null when you try to tap outside the dialog area to dispose it. To avoid null returns use '??' check to set the default value (in case of null) to false

 Future<bool> _onBackPress() {
    return showDialog(
      context: context,
      builder: (BuildContext context) {
        return AlertDialog(
          content: Text('Exit From The App'),
          actions: [
            TextButton(
              onPressed: () {
                Navigator.of(context).pop(false);
              },
              child: Text('No'),
            ),
            TextButton(
              onPressed: () {
                Navigator.of(context).pop(true);
              },
              child: Text('Yes'),
            ),
          ],
        );
      },
    ).then((value) => value ?? false);
  }
user_164640740

In your _onBackPress() function you are not returning a bool. It should be rewritten like this:

Future<bool> _onBackPress() async {
  bool goBack = false;
  await showDialog(
    context: context,
    builder: (BuildContext context) {
      return AlertDialog(
        content: Text('Exit From The App'),
        actions: [
          TextButton(
            onPressed: () {
              goBack = false;
              Navigator.pop(context);
            },
            child: Text('No'),
          ),
          TextButton(
            onPressed: () {
              goBack = true;
              Navigator.pop(context);
            },
            child: Text('Yes'),
          ),
        ],
      );
    },
  );
  return goBack;
}
user_126035920
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