Load the graph for a nested NavigatorHost that replaces another instance

navigation-fragments/src/main/java/dev/hotwire/navigation/activities/HotwireActivityDelegate.kt

@@ -86,7 +86,7 @@ class HotwireActivityDelegate(val activity: HotwireActivity) {
     internal fun registerNavigatorHost(host: NavigatorHost) {
         logDebug("navigatorRegistered", listOf("navigator" to host.navigator.configuration.name))
 
-        if (navigatorHosts[host.id] == null) {
+        if (navigatorHosts[host.id] !== host) {
             navigatorHosts[host.id] = host
             listenToDestinationChanges(host)
 
@@ -105,7 +105,11 @@ class HotwireActivityDelegate(val activity: HotwireActivity) {
 
     internal fun unregisterNavigatorHost(host: NavigatorHost) {
         logDebug("navigatorUnregistered", listOf("navigator" to host.navigator.configuration.name))
-        navigatorHosts.remove(host.id)
+
+        // A newer host instance can take over the same ID before this one is destroyed.
+        if (navigatorHosts[host.id] === host) {
+            navigatorHosts.remove(host.id)
+        }
     }
 
     internal fun onNavigatorHostReady(host: NavigatorHost) {

navigation-fragments/src/test/kotlin/dev/hotwire/navigation/activities/HotwireActivityDelegateTest.kt

@@ -0,0 +1,93 @@
+package dev.hotwire.navigation.activities
+
+import android.os.Bundle
+import android.view.LayoutInflater
+import android.view.View
+import android.view.ViewGroup
+import android.widget.FrameLayout
+import android.widget.LinearLayout
+import androidx.fragment.app.Fragment
+import dev.hotwire.navigation.navigator.NavigatorConfiguration
+import dev.hotwire.navigation.navigator.NavigatorHost
+import org.assertj.core.api.Assertions.assertThat
+import org.junit.Before
+import org.junit.Test
+import org.junit.runner.RunWith
+import org.robolectric.Robolectric
+import org.robolectric.RobolectricTestRunner
+
+// A screen that hosts a nested NavigatorHost can be replaced by a new instance of itself. The new
+// nested host is created before the old one is destroyed, and both use the same view ID.
+@RunWith(RobolectricTestRunner::class)
+class HotwireActivityDelegateTest {
+
+    private lateinit var activity: TestActivity
+
+    @Before
+    fun setUp() {
+        activity = Robolectric.buildActivity(TestActivity::class.java).setup().get()
+    }
+
+    @Test
+    fun `new nested host loads its graph while the old host with the same id is still registered`() {
+        addScreen(CONTAINER_A_ID)
+
+        val newScreen = addScreen(CONTAINER_B_ID)
+
+        assertThat(newScreen.navigatorHost.isGraphInitialized).isTrue()
+    }
+
+    @Test
+    fun `destroying the old nested host keeps the new host with the same id registered`() {
+        val oldScreen = addScreen(CONTAINER_A_ID)
+        val newScreen = addScreen(CONTAINER_B_ID)
+
+        activity.supportFragmentManager.beginTransaction().remove(oldScreen).commitNow()
+
+        assertThat(activity.delegate.findNavigatorHost(NESTED_HOST_ID))
+            .isSameAs(newScreen.navigatorHost)
+    }
+
+    private fun addScreen(containerId: Int): NestedHostScreen {
+        val screen = NestedHostScreen()
+        activity.supportFragmentManager.beginTransaction().add(containerId, screen).commitNow()
+        return screen
+    }
+
+    class NestedHostScreen : Fragment() {
+        val navigatorHost get() = childFragmentManager.findFragmentById(NESTED_HOST_ID) as NavigatorHost
+
+        override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View {
+            return FrameLayout(requireContext()).apply { id = NESTED_HOST_ID }
+        }
+
+        override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
+            childFragmentManager.beginTransaction().add(NESTED_HOST_ID, NavigatorHost()).commitNow()
+        }
+    }
+
+    class TestActivity : HotwireActivity() {
+        override fun navigatorConfigurations() = listOf(
+            NavigatorConfiguration(
+                name = "nested",
+                startLocation = "https://example.com/start",
+                navigatorHostId = NESTED_HOST_ID
+            )
+        )
+
+        override fun onCreate(savedInstanceState: Bundle?) {
+            setTheme(androidx.appcompat.R.style.Theme_AppCompat_Light_NoActionBar)
+            super.onCreate(savedInstanceState)
+            setContentView(LinearLayout(this).apply {
+                addView(FrameLayout(context).apply { id = CONTAINER_A_ID })
+                addView(FrameLayout(context).apply { id = CONTAINER_B_ID })
+            })
+        }
+    }
+
+    companion object {
+        private val NESTED_HOST_ID = View.generateViewId()
+        private val CONTAINER_A_ID = View.generateViewId()
+        private val CONTAINER_B_ID = View.generateViewId()
+    }
+}