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Add extension function to retrieve a Flow of NavBackStackEntries #89

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Add extension function to retrieve a Flow of NavBackStackEntries #89

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c58ad20
Add extension function to retrieve a flow of NavBackStackEntries
simonschiller Oct 14, 2020
5950e78
Use Truth in tests
simonschiller Oct 15, 2020
9fe60e6
Change to extension property
simonschiller Oct 21, 2020
a04c781
Rename method to be more consistent with existing API
simonschiller Oct 24, 2020
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1 change: 1 addition & 0 deletions navigation/navigation-runtime-ktx/api/current.txt
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Expand Up @@ -39,6 +39,7 @@ package androidx.navigation {

public final class NavControllerKt {
method public static inline androidx.navigation.NavGraph createGraph(androidx.navigation.NavController, optional @IdRes int id, @IdRes int startDestination, kotlin.jvm.functions.Function1<? super androidx.navigation.NavGraphBuilder,kotlin.Unit> builder);
method public static kotlinx.coroutines.flow.Flow<androidx.navigation.NavBackStackEntry> getCurrentBackStackEntryFlow(androidx.navigation.NavController);
}

public final class NavHostKt {
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1 change: 1 addition & 0 deletions navigation/navigation-runtime-ktx/api/public_plus_experimental_current.txt
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Expand Up @@ -39,6 +39,7 @@ package androidx.navigation {

public final class NavControllerKt {
method public static inline androidx.navigation.NavGraph createGraph(androidx.navigation.NavController, optional @IdRes int id, @IdRes int startDestination, kotlin.jvm.functions.Function1<? super androidx.navigation.NavGraphBuilder,kotlin.Unit> builder);
method public static kotlinx.coroutines.flow.Flow<androidx.navigation.NavBackStackEntry> getCurrentBackStackEntryFlow(androidx.navigation.NavController);
}

public final class NavHostKt {
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1 change: 1 addition & 0 deletions navigation/navigation-runtime-ktx/api/restricted_current.txt
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Expand Up @@ -39,6 +39,7 @@ package androidx.navigation {

public final class NavControllerKt {
method public static inline androidx.navigation.NavGraph createGraph(androidx.navigation.NavController, optional @IdRes int id, @IdRes int startDestination, kotlin.jvm.functions.Function1<? super androidx.navigation.NavGraphBuilder,kotlin.Unit> builder);
method public static kotlinx.coroutines.flow.Flow<androidx.navigation.NavBackStackEntry> getCurrentBackStackEntryFlow(androidx.navigation.NavController);
}

public final class NavHostKt {
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31 changes: 31 additions & 0 deletions ...tion/navigation-runtime-ktx/src/androidTest/java/androidx/navigation/NavControllerTest.kt
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Expand Up @@ -21,6 +21,12 @@ import androidx.test.core.app.ApplicationProvider
import androidx.test.filters.SmallTest
import androidx.testutils.TestNavigator
import androidx.testutils.test
import com.google.common.truth.Truth.assertWithMessage
import kotlinx.coroutines.flow.collect
import kotlinx.coroutines.flow.onEach
import kotlinx.coroutines.flow.take
import kotlinx.coroutines.flow.withIndex
import kotlinx.coroutines.runBlocking
import org.junit.Assert.assertTrue
import org.junit.Test

Expand All @@ -41,6 +47,31 @@ class NavControllerTest {
DESTINATION_ID in graph
)
}

@Test
@Suppress("EXPERIMENTAL_API_USAGE")
fun currentBackStackEntryFlow() = runBlocking {
navController.graph = navController.createGraph(startDestination = 1) {
test(1)
test(2)
test(3)
}

navController.currentBackStackEntryFlow
.take(navController.graph.count())
.withIndex()
.onEach { (index, backStackEntry) ->
val expectedDestination = index + 1
assertWithMessage("Flow emitted unexpected back stack entry (wrong destination)")
.that(backStackEntry.destination.id)
.isEqualTo(expectedDestination)

if (expectedDestination < navController.graph.count()) {
navController.navigate(expectedDestination + 1)
}
}
.collect()
}
}

private const val DESTINATION_ID = 1
21 changes: 21 additions & 0 deletions navigation/navigation-runtime-ktx/src/main/java/androidx/navigation/NavController.kt
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Expand Up @@ -17,6 +17,11 @@
package androidx.navigation

import androidx.annotation.IdRes
import kotlinx.coroutines.ExperimentalCoroutinesApi
import kotlinx.coroutines.channels.awaitClose
import kotlinx.coroutines.channels.sendBlocking
import kotlinx.coroutines.flow.Flow
import kotlinx.coroutines.flow.callbackFlow

/**
* Construct a new [NavGraph]
Expand All @@ -26,3 +31,19 @@ public inline fun NavController.createGraph(
@IdRes startDestination: Int,
builder: NavGraphBuilder.() -> Unit
): NavGraph = navigatorProvider.navigation(id, startDestination, builder)

/**
* Creates and returns a [Flow] that will emit the currently active [NavBackStackEntry] whenever
* it changes. If there is no active [NavBackStackEntry], no item will be emitted.
*/
@ExperimentalCoroutinesApi
public val NavController.currentBackStackEntryFlow: Flow<NavBackStackEntry> get() = callbackFlow {
val listener = NavController.OnDestinationChangedListener { controller, _, _ ->
controller.currentBackStackEntry?.let(::sendBlocking)
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would it be better to use second argument instead? given that it is non null it will make things as easy as sendBlocking(destination)

another corner case question: sendBlocking can theoretically block on the main thread, when there is slow consumer and a lot of actions here. Though it seems like fairly theoretical concern

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Would be possible, but then it would be a Flow<NavDestination> instead of a Flow<NavBackStackEntry>, which is not what we want I guess (as NavBackStackEntry is the class that gives access to things like ViewModelStore and the SavedStateRegistry). But maybe it would make sense to have both, a backStackEntryFlow and a destinationFlow?

Valid point, I also wasn't sure what's best to use here. The other option would be to use offer, which could lead to lost items it the consumer is slow.

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We are specifically looking for the NavBackStackEntry here, not NavDestination.

}

addOnDestinationChangedListener(listener)
awaitClose {
removeOnDestinationChangedListener(listener)
}
}