What's the difference between "viewModel: MyViewModel = viewModel()" and "viewModel: MyViewMo

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Here my composable

@Composable
fun CounterScreen(viewModel: MyViewModel = viewModel()) {
    val uiState by viewModel.uiState.collectAsState()

    Column(
        modifier = Modifier.fillMaxSize(),
        verticalArrangement = Arrangement.Center,
        horizontalAlignment = Alignment.CenterHorizontally
    ) {
        Text(text = "Counter: ${uiState.count}")
        Spacer(modifier = Modifier.height(16.dp))
        FloatingActionButton(
            onClick = {
                viewModel.increment()
            },
        ) {
            Icon(imageVector = Icons.Default.Add, contentDescription = "Increment")
        }
    }
}

Why the state is preserved with recomposition when I wrote fun CounterScreen(viewModel: MyViewModel = viewModel()) but not with fun CounterScreen(viewModel: MyViewModel = MyViewModel()). However they both create a new instance of MyViewModel. Does someone can explain me? Thanks

Here my composable

@Composable
fun CounterScreen(viewModel: MyViewModel = viewModel()) {
    val uiState by viewModel.uiState.collectAsState()

    Column(
        modifier = Modifier.fillMaxSize(),
        verticalArrangement = Arrangement.Center,
        horizontalAlignment = Alignment.CenterHorizontally
    ) {
        Text(text = "Counter: ${uiState.count}")
        Spacer(modifier = Modifier.height(16.dp))
        FloatingActionButton(
            onClick = {
                viewModel.increment()
            },
        ) {
            Icon(imageVector = Icons.Default.Add, contentDescription = "Increment")
        }
    }
}

Why the state is preserved with recomposition when I wrote fun CounterScreen(viewModel: MyViewModel = viewModel()) but not with fun CounterScreen(viewModel: MyViewModel = MyViewModel()). However they both create a new instance of MyViewModel. Does someone can explain me? Thanks

• android
• android-jetpack-compose
• viewmodel

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2 Answers 2

Sorted by: Reset to default 1

With MyViewModel() you always create a new instance.

The viewModel() function, on the other hand, uses a view model factory behind the scenes to obtain an instance. And that factory provides the same instance for the same ViewModelStoreOwner. The latter defaults to LocalViewModelStoreOwner.current.

Until that changes, you will always get the same instance. From the documentation:

Returns an existing ViewModel or creates a new one in the given owner (usually, a fragment or an activity), defaulting to the owner provided by LocalViewModelStoreOwner.

The created ViewModel is associated with the given viewModelStoreOwner and will be retained as long as the owner is alive (e. g. if it is an activity, until it is finished or process is killed).

Please have a look at the official documentation for the viewModel() function:

viewModel() returns an existing ViewModel or creates a new one. By default, the returned ViewModel is scoped to the enclosing activity, fragment or navigation destination, and is retained as long as the scope is alive.

So when you use viewModel() in any Composable in the same Activity / Fragment / NavGraph destination, it will only create a new instance at the first time, and all other invocations will return the existing ViewModel instance.

You should be able to see the difference once you extend your code sample as follows:

@Composable
fun CounterScreens() {
    Column {
        CounterScreenA()
        CounterScreenB()
    }
}

@Composable
fun CounterScreenA(viewModel: MyViewModel = viewModel()) {
    val uiState by viewModel.uiState.collectAsState()

    Column(
        modifier = Modifier.fillMaxWidth(),
        verticalArrangement = Arrangement.Center,
        horizontalAlignment = Alignment.CenterHorizontally
    ) {
        Text(text = "Counter: ${uiState.count}")
        Spacer(modifier = Modifier.height(16.dp))
        FloatingActionButton(
            onClick = {
                viewModel.increment()
            },
        ) {
            Icon(imageVector = Icons.Default.Add, contentDescription = "Increment")
        }
    }
}

@Composable
fun CounterScreenB(viewModel: MyViewModel = viewModel()) {
    val uiState by viewModel.uiState.collectAsState()

    Column(
        modifier = Modifier.fillMaxWidth(),
        verticalArrangement = Arrangement.Center,
        horizontalAlignment = Alignment.CenterHorizontally
    ) {
        Text(text = "Counter: ${uiState.count}")
        Spacer(modifier = Modifier.height(16.dp))
        FloatingActionButton(
            onClick = {
                viewModel.increment()
            },
        ) {
            Icon(imageVector = Icons.Default.Add, contentDescription = "Increment")
        }
    }
}

CounterScreenA and CounterScreenB will now share the same ViewModel instance and will use the same counter. If you replace viewModel() with MyViewModel(), each of them will have their own counter.

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