如何创建可以从 ViewModel 和 Activity 访问的 function？

Question

I am building an android app that has a MainActivity and several fragments. I am using a ViewModel and ViewModelFactory for each of the fragments. I need to create a function that will perform network requests and would like to have it available in one of the fragments so I can display a progress bar to the user. I would also like to access this function from MainActivity in the override function onOptionsItemSelected when a user clicks on the button in the menu in the top right. When called from this button everything should be in the background and there is no need to show the user anything until the task is completed.

I have tried two approaches.

1. Place the function inside the ViewModel. This works perfect for when called from the fragment and everything works. But when calling from the Main activity I cannot seem to reference the ViewModel correctly.

I placed this in MainActivity before onCreate

private lateinit var syncViewModel: SyncViewModel

And this in onCreate

syncViewModel = ViewModelProvider(this).get(SyncViewModel::class.java)

And I get the below error message.

java.lang.RuntimeException: Unable to start activity ComponentInfo java.lang.RuntimeException: Cannot create an instance of class com. . ._.SyncViewModel

How can I access the already created ViewModel from the activity instead of creating one?

2. So my second try was to place this function in a separate file. I could then access this from the ViewModel and the MainActivity. But I could not find a way to update the progress bar in the ViewModel.

In the ViewModel I have the below, and then the xml in this fragment references this syncProgress value.

private var _syncProgress = MutableLiveData<Int>()

val syncProgress: LiveData<Int> = _syncProgress

What am I missing to connect this separate function to be able to update the values in my ViewModel?

It seems like option #1 is the right way to do this, but maybe there is a third way that I dont know about that would be better! Any help is appreciated!

Update 1

My ViewModel is defined as

class SyncViewModel(
val database: RoomDatabase,
application: Application) : AndroidViewModel(application) {

And in the fragment as

// Create an Instance of the ViewModel Factory
val dataSource = RoomDatabase.getDatabase(application)
val viewModelFactory = SyncViewModelFactory(dataSource, application)

private lateinit var syncViewModel: SyncViewModel
syncViewModel = ViewModelProvider(this,viewModelFactory).get(SyncViewModel::class.java)

But I am not sure how I need to modify the SyncViewModel in MainActivity?

Answer 1

I would follow your option 1, and use view model provider delegates to simplify getting your reference. You should specify an Activity-scoped ViewModel in the Fragment so it gets the same instance that the Activity gets.

In your Activity:

private val syncViewModel: SyncViewModel by viewModels()

In your Fragment:

private val syncViewModel: SyncViewModel by activityViewModels()

Since your database can be acquired using the Application instance, you can avoid creating a factory by moving it out of the constructor like this:

class SyncViewModel(application: Application) : AndroidViewModel(application) {

    val database = RoomDatabase.getDatabase(application)

If you do want to continue using your factory, you can pass it as a parameter to the delegate functions, for example:

private val dataSource = RoomDatabase.getDatabase(application)
private val syncViewModel: SyncViewModel by activityViewModels(SyncViewModelFactory(dataSource, application))

Answer 2

If I understand correctly the function should be outside of the ViewModel because it has to be accessed in other places too.

The LiveData in the ViewModel won't get updated on the state of the function but one way to fix this would be to just make the ViewModel immediately "listen" to whatever the function's current state is.

Let's say you would normally update the value/progress from 0 to 100 in a Repository, update a LiveData object with that progress and make the ViewModel or/and Activity listen to it. LiveData in Repositories isn't something that's perfect but it seems like a solution based on what I read.