View Model can be heavy if we use single activity based architecture

As google suggests single activity based application. I have a situation where I need clarification.

I have one activity containing 3 fragments and each fragment is linked to different fragments having other screens.

1. should I use only one viewmodel for each tabs or should I use one view model having different screens.

2. As I have only one activity and view Model resides till activity is destroyed. Do all viewModel that I will create for each screen will contain data until activity is destroyed. If this is the case will it make my app heavy.

> 1. should I use only one viewmodel for each tabs or should I use one view model having different screens.

You should use the smallest scope for each ViewModel possible. Generally, this means that data associated with only one fragment should use a ViewModel associated with just that one fragment.

> 2. As I have only one activity and view Model resides till activity is destroyed. Do all viewModel that I will create for each screen will contain data until activity is destroyed. If this is the case will it make my app heavy.

ViewModels live only as long as the ViewModelStore they're attached to is around. Therefore if you have a ViewModel associated with a fragment, it'll survive only as long as that fragment exists. For example, if that fragment gets popped off the back or you call remove(), then the ViewModel is destroyed. ViewModels only live as long as your activity if you specifically create them using the activity as the ViewModelStoreOwner (for example, by using ViewModelProvider(activity)).

> should I use only one viewmodel for each tabs or should I use one view
> model having different screens?

In single activity architecture, you can use ViewModel with either Activity Scope or with Fragment scope. As @ianhanniballake suggested in his answer, use ViewModel with Fragment which means that each tab in your case will have its own ViewModel attached to fragment. This will provide separation of concerns for functionality in each tab.

> As I have only one activity and view Model resides till activity is
> destroyed. Do all viewModel that I will create for each screen will
> contain data until activity is destroyed. If this is the case will it
> make my app heavy.

Again answer to this question is connected to your first question and explanation provided to it.

While working with single activity architecture narrow down scope as much as possible. Use ViewModel associated with activity (lets call it Global ViewModel) only to keep data which is used across multiple fragments. This way viewmodel will contain data till activity is in backstack and fragments can use it when required.
This will provide you 2 advantages

• ViewModel will not contain additional data which you mentioned in your 2nd question
• And most importantly there will not be any data discrepancies while accessing fragment multiple times. For e.g. if you open any fragment, fetch data from api and keep it in Global ViewModel and fail to delete it when remove fragment from backstack then your fragment will show old/obsolete data when you will open fragment next time. To avoid this scenario it is better to keep this data in viewmodel with fragment scope.

I hope this will be useful.

1. Each Fragment usually should have a ViewModel itself but in some cases where you want to share the same ViewModel instance, you can achieve it with having a ViewModel scoped to an activity.

2. ViewModel objects are scoped to the Lifecycle passed to the ViewModelProvider when getting the ViewModel it is found on the docs and you might have to read other details there but technically you can scope a ViewModel to a Fragment or to an Activity.