docs(dynamic-links): update Firebase documentation on how to receive link from terminated state

[russellwheatley]

Description

As reported by this user, the iOS app could receive the link from a terminated state from either the getInitialLink method call or the event listener onLink. I believe the user can call getInitialLink before it has been passed to the app and therefore I have updated the documentation to reflect this.

If you wish to test this yourself, you can:

1. Run the app in release mode.
2. Open the app from a terminated state using a Dynamic Link you're created in the Firebase console and configured for your app.
3. Use the below noted, crude code snippet in your main.dart file. Open the app and note the success rate of getInitialLink. It doesn't always successfully retrieve the Dynamic Link upon app opening. It is more successful when you uncomment the Future.delayed. The onLink will always receive the opening Dynamic Link.

import 'dart:async';

import 'package:firebase_core/firebase_core.dart';
import 'package:firebase_dynamic_links/firebase_dynamic_links.dart';
import 'package:flutter/material.dart';

import 'firebase_options.dart';

Future<void> main() async {
  WidgetsFlutterBinding.ensureInitialized();
  await Firebase.initializeApp(
    options: DefaultFirebaseOptions.currentPlatform,
  );
  runApp(MyApp());
}

class MyApp extends StatefulWidget {
  MyApp({Key? key}) : super(key: key);

  @override
  _MyAppState createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {
  String _linkMessage = 'no-state';

  @override
  void initState() {
    super.initState();
    initDL();
  }

  Future initDL() async {
    // FirebaseDynamicLinks.instance.onLink.listen(
    //   (event) {
    //     setState(() {
    //       _linkMessage = event.link.toString() + ' onLink';
    //     });
    //   },
    // );
    // await Future.delayed(const Duration(milliseconds: 3000));
    final initialLink = await FirebaseDynamicLinks.instance.getInitialLink();

    if(initialLink != null){
      setState(() {
        _linkMessage = initialLink.link.toString() + ' getInitialLink';
      });
    } else {
      setState(() {
        _linkMessage = 'nothing' + ' getInitialLink';
      });
    }
  }

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
          appBar: AppBar(
            title: const Text('Testing Something'),
          ),
          body: Column(
            children: [
              ElevatedButton(
                  onPressed: () {
                    setState(() {
                      _linkMessage = 'reset';
                    });
                  },
                  child: const Text('reset link')),
              Text(_linkMessage),
            ],
          )),
    );
  }
}

Related Issues

closes #7546

Checklist

Before you create this PR confirm that it meets all requirements listed below by checking the relevant checkboxes ([x]).
This will ensure a smooth and quick review process. Updating the pubspec.yaml and changelogs is not required.

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
• All existing and new tests are passing.
• I updated/added relevant documentation (doc comments with ///).
• The analyzer (melos run analyze) does not report any problems on my PR.
• I read and followed the Flutter Style Guide.
• I signed the CLA.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

Does your PR require plugin users to manually update their apps to accommodate your change?

• Yes, this is a breaking change.
• No, this is not a breaking change.

[russellwheatley]

Hey @kevinthecheung, do you mind taking a look and seeing what you think? Thanks.