ViewModel LifeCycle Over Navigations

When is a ViewModel Created?

Suppose you have:

class ProfileViewModel : ViewModel()

and inside your screen:

@Composable
fun ProfileScreen(
    viewModel: ProfileViewModel = viewModel()
) {

}

The ViewModel is not created when the app starts.

It is created when the screen first requests it.

User opens Profile Screen
          ↓
viewModel() called
          ↓
Android checks:
"Do I already have a ProfileViewModel?"
          ↓
No
          ↓
Create ProfileViewModel

What happens during recomposition?

Suppose state changes:

Text(name)

changes from:

Manoj

to:

Manoj Sai

Compose recomposes.

Many beginners think:

Screen recomposed
↓
ViewModel recreated

❌ Wrong.

The same ViewModel instance is reused.

Screen Recompose #1
      ↓
Same ViewModel

Screen Recompose #2
      ↓
Same ViewModel

Screen Recompose #100
      ↓
Same ViewModel

This is why ViewModels survive recompositions.

What happens during screen rotation?

Imagine:

Portrait
↓
Rotate
↓
Landscape

Activity gets recreated.

Without ViewModel:

plaintext
Data lost

With ViewModel:

Activity destroyed
↓
New Activity created
↓
Same ViewModel reused

That's one of the biggest reasons ViewModels exist.

## When is ViewModel Destroyed?

The ViewModel lives as long as its owner lives.

For example:

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Profile Screen opened
↓
ViewModel created
↓
User stays on screen
↓
ViewModel alive

Then:

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User presses Back
↓
Profile screen removed
↓
ViewModel no longer needed
↓
onCleared()
↓
ViewModel destroyed

## What is onCleared()?

Before Android destroys the ViewModel it calls:

kotlin
override fun onCleared() {
super.onCleared()
}

Think of it as:

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"Hey ViewModel,
I'm about to destroy you.
Clean up anything you're using."

At this moment:

kotlin
viewModelScope.cancel()

is automatically triggered.

Real Example with Navigation

Suppose:

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HomeScreen
↓
ProfileScreen

User opens Profile:

ProfileScreen created
↓
ProfileViewModel created

User stays there:

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ProfileViewModel alive

User returns to Home:

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ProfileScreen removed from back stack
↓
ProfileViewModel.onCleared()
↓
ProfileViewModel destroyed

## Navigation Compose Case

Suppose:

plaintext
Home
↓
Profile
↓
Settings

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Current back stack:

Home
Profile
Settings

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When you're on Settings:

ProfileViewModel

plaintext

may still be alive because Profile is still in the navigation back stack.

Only when Profile is removed from the back stack:

Home
Settings

plaintext

then:

ProfileViewModel destroyed
Complete Lifecycle

User opens screen
        ↓
Android creates ViewModel
        ↓
ViewModelScope created
        ↓
Coroutines launched
        ↓
Screen recomposes many times
        ↓
Same ViewModel reused
        ↓
User leaves screen permanently
        ↓
onCleared()
        ↓
viewModelScope cancelled
        ↓
All coroutines cancelled
        ↓
ViewModel destroyed

This is why we say:

A ViewModel survives recompositions and configuration changes (like rotation), but it is destroyed when its owner is permanently removed and Android calls onCleared().