docs(firebase_messaging): Update example app & documentation for initializing Firebase app from top-level.

[russellwheatley]

Description

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Related Issues

fixes #6087

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[github-actions]

Visit the preview URL for this PR (updated for commit f7a5e9d):

https://flutter-firebase-docs--pr8061-russell-messaging-f-v7uc3sjd.web.app

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[russellwheatley]

Decided to solve this at the root. I believe there are too many connections/writes at the same time to the SQL instance hence the error. Possibly need to cache Firebase or Firestore instance if initializing app from separate isolate.

[russellwheatley]

Closing as the issue appears to be resolved.

[deepak786]

@russellwheatley Is it resolved from the flutter firebase package itself?

[russellwheatley]

@deepak786 The issue is closed so I presumed it was resolved. Are you still experiencing the issue?

[deepak786]

@russellwheatley we are using the approach defined in this PR. So I want to know if we should continue with that or not.

[russellwheatley]

@deepak786 I'm not sure, I presumed the lack of responses to the issue, and that it was closed meant it had been resolved at the level of the SDK. I think I'd prefer to reopen the issue if it is still a problem.

[Mayb3Nots]

@deepak786 I'm not sure, I presumed the lack of responses to the issue, and that it was closed meant it had been resolved at the level of the SDK. I think I'd prefer to reopen the issue if it is still a problem.

Hi @russellwheatley, Im getting same error message in Crashlytics today. I saw that you mentioned fixing it at the root where the SQL instance seemed to be overwhelm, is that still planned or do I have to implement the fix suggested by this PR?

@Lyokone since you closed the issue could you give some information on the reason?

With further investigation I found another crash log in Crashlytics.

java.lang.RuntimeException: Failed to gain exclusive lock to the Cloud Firestore client's offline persistence. This generally means you are using Cloud Firestore from multiple processes in your app. Keep in mind that multi-process Android apps execute the code in your Application class in all processes, so you may need to avoid initializing Cloud Firestore in your Application class. If you are intentionally using Cloud Firestore from multiple processes, you can only enable offline persistence (that is, call setPersistenceEnabled(true)) in one of them.

From this error message it seems like we shouldn't be initializing two instances of Firebase (Assuming its creating a firestore instance under the hood). If we do so I think we need to set FirebaseFirestore.instance.enablePersistence(PersistenceSettings(synchronizeTabs: false)) after initialising in our isolate (Havent test it so).

[giorgio79]

Hope making initapp a global will solve the originating issue:
#6087

I reworked my app based on the Firebase Messaging Flutter example app, and with two initilizeApp calls I started seeing the referenced github issue...