dart - AlertDialog without context in Flutter

Question

I want to show an AlertDialog when a http get fails. The function showDialog (https://api.flutter.dev/flutter/material/showDialog.html) has the parameter "@required BuildContext context", but I want to call the AlertDialog from my async function getNews(), which hasn't a context value.

By analogy with Java, where I use null for dialog without an owner, I tried to put context value to null, but it is not accepted.

This is my code:

  Future<dynamic> getNews() async {
    dynamic retVal;
    try {
      var response = await http.get(url));
      if (response.statusCode == HttpStatus.ok) {
        retVal = jsonDecode(response.body);
      }
    } catch (e) {
      alertDlg(?????????, 'Error', e.toString());
  }
    return
    retVal;
  }

  static Future<void> alertDlg(context, String titolo, String messaggio) async {
    return showDialog<void>(
        context: context,
        barrierDismissible: false, // user must tap button!
        builder: (BuildContext context) {
        return AlertDialog(
              title: Text(titolo),
        ...
    );
  }

Answer 1

Solution without third-party libraries or storing BuildContext:

final navigatorKey = GlobalKey<NavigatorState>();

void main() => runApp(
  MaterialApp(
    home: HomePage(),
    navigatorKey: navigatorKey, // Setting a global key for navigator
  ),
);

class HomePage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(),
      body: SafeArea(
        child: Center(
          child: Text('test')
        )
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: showMyDialog, // Calling the function without providing it BuildContext
      ),
    );
  }
}

void showMyDialog() {
  showDialog(
    context: navigatorKey.currentContext,
    builder: (context) => Center(
      child: Material(
        color: Colors.transparent,
        child: Text('Hello'),
      ),
    )
  );
}

After setting a global key for navigator (navigatorKey parameter of MaterialApp class), its current state becomes accessible from any part of the code. We can pass its context to the showDialog function. Make sure not to use it before the first frame is built, otherwise the state will be null.

Basically, dialogs are just another type of routes like MaterialPageRoute or CupertinoPageRoute - they are all derived from the ModalRoute class and their instances are pushed into NavigatorState. Context is only needed to get the current navigator's state. A new route can be pushed without having a context if we have a global navigator key:

navigatorKey.currentState.push(route)

Unfortunately, _DialogRoute class (...flutterlibsrcwidgets outes.dart) used in showDialog function is private and unaccessible, but you make your own dialog route class and push it into the navigator's stack.

UPDATE: Navigator.of method has been updated and it's no longer needed to pass subtree context.