Level 16

Lifecycle-Aware Collection

lifecycleScope & repeatOnLifecycle

Collecting a `Flow` in a coroutine that keeps running while the screen is in the background wastes resources and can crash. `repeatOnLifecycle` starts/stops collection in sync with the lifecycle.

📚 Explanation

`lifecycleScope` cancels its coroutines when the owner is destroyed, but a long-lived `collect` would still run while the app is backgrounded. `repeatOnLifecycle(STARTED)` restarts the collection each time the screen reaches `STARTED` and cancels it on `STOPPED`, so flows are only collected when visible. In Compose, `collectAsStateWithLifecycle()` wraps this same safe pattern for you.

Note: Code execution is not available for Android/Kotlin in the browser. Use Android Studio to run and test the app.

kotlin

import androidx.activity.ComponentActivity
import androidx.lifecycle.Lifecycle
import androidx.lifecycle.lifecycleScope
import androidx.lifecycle.repeatOnLifecycle
import kotlinx.coroutines.launch

class FeedActivity : ComponentActivity() {

    private val viewModel: FeedViewModel by viewModels()

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)

        // lifecycleScope is cancelled when the Activity is destroyed.
        lifecycleScope.launch {
            // repeatOnLifecycle STARTS collecting when STARTED,
            // and CANCELS the block when the screen goes below STARTED.
            repeatOnLifecycle(Lifecycle.State.STARTED) {
                viewModel.uiState.collect { state ->
                    render(state)
                }
            }
        }
    }
}

// In Compose, the equivalent helper is collectAsStateWithLifecycle():
@Composable
fun FeedScreen(viewModel: FeedViewModel) {
    val state by viewModel.uiState.collectAsStateWithLifecycle()
    FeedContent(state)
}

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Lifecycle-Aware Collection

lifecycleScope & repeatOnLifecycle

Collecting a `Flow` in a coroutine that keeps running while the screen is in the background wastes resources and can crash. `repeatOnLifecycle` starts/stops collection in sync with the lifecycle.

📚 Explanation

Note: Code execution is not available for Android/Kotlin in the browser. Use Android Studio to run and test the app.

import androidx.activity.ComponentActivity

import androidx.lifecycle.Lifecycle

import androidx.lifecycle.lifecycleScope

import androidx.lifecycle.repeatOnLifecycle

import kotlinx.coroutines.launch

class FeedActivity : ComponentActivity() {

private val viewModel: FeedViewModel by viewModels()

override fun onCreate(savedInstanceState: Bundle?) {

super.onCreate(savedInstanceState)

// lifecycleScope is cancelled when the Activity is destroyed.

lifecycleScope.launch {

// repeatOnLifecycle STARTS collecting when STARTED,

// and CANCELS the block when the screen goes below STARTED.

repeatOnLifecycle(Lifecycle.State.STARTED) {

viewModel.uiState.collect { state ->

render(state)

}

// In Compose, the equivalent helper is collectAsStateWithLifecycle():

@Composable

fun FeedScreen(viewModel: FeedViewModel) {

val state by viewModel.uiState.collectAsStateWithLifecycle()

FeedContent(state)

}