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Depends on the use case

• If user is expecting data, then you need to send the errors all the way to your UI to notify user to make sure user won't just see a blank screen or experience unexpected UI. You can sealed classes in Kotlin for this case.
• If you are fetching something which user is not aware of, like syncing data from remote to Local cache in background, In cases like this user is not aware/ do not care about errors, your app should handle. Ideal way is to check in handle error in repository, if something fails, retrying it there or fetching data from alternate source etc.
• Third case would be handling at multiple levels, say you are fetching data from network, it failed, you can handle retry logic in repository level and after you exhaust your retry attempts but still it is failing then you need to acknowledge user as he might be expecting some thing to show up.

As for any best practices, the best answer is "it depends".

If your app is really small and simple then it's okay to pass data from one screen to another in the arguments bundle.

A more complex approach is to store data somewhere outside of these Fragment lifecycles. It's a general rule you can implement as you want. A couple of examples below:

• Data can be stored on Application class level. Application class runs for all application lifecycle. Every fragment can get Application instance from its activity like activity?.getApplication().
• Data can be stored on Activity level if all fragments run in a single activity. Activity can be obtained from Fragment using activity or requireActivity().
• Data can be stored on a parent fragment level if all fragments run in this parent fragment. It can be obtained from child fragments using parentFragment.

All these approaches suppose you to cast some "parent" thing to specific interface or implementation. For example, if your fragments run in the MainActivity which is stores some val data: Data as its property, then you should use it something like this: (requireActivity() as MainActivity).data

Clarification on a couple of popular answers:

• The Shared ViewModel is just a special case of activity-level approach as ViewModel instance is stored in Activity scope.
• The SharedPrefrences like any "database" approach is a special case of application-level approach as SharedPreferences is stored on application-level (in the file storage) no matter where you create its instance. (And SharedPreferences persists data across application starts, so it's not your option)

You are trying to observe a mutable LiveData that is only initialized after the onClickListener so you won't get it to work, also you have a lateinit property that is only initialized if you call the login method which will throw an exception.

To solve your problem you can have a MediatorLiveData that will observe your other live data and pass the result back to your fragment observer.

You can try the following: