Use OnBackPressedCallback in NavController

Replace the brittle OnBackStackChangedListener
with an OnBackPressedCallback registered at the
NavController level which works on all Navigators.

It also helps fix a timing issue where the state
of the FragmentNavigator isn't updated after a Fragment
is popped off the stack, leading to a mismatch in
expected actions available if the newly visible Fragment
calls navigate() in a lifecycle method.

Test: updated tests, new OnBackPressedTest
BUG: 111598096
Change-Id: Id679ea6ac9b238240649656fc0796552d6191757

17 files changed, 254 insertions, 406 deletions

diff --git a/navigation/common/api/restricted_2.1.0-alpha03.txt b/navigation/common/api/restricted_2.1.0-alpha03.txt
index 37d70c254db..da4f6cc18fe 100644
--- a/navigation/common/api/restricted_2.1.0-alpha03.txt
+++ b/navigation/common/api/restricted_2.1.0-alpha03.txt
@@ -1,17 +1 @@
 // Signature format: 3.0
-package androidx.navigation {
-
-  public abstract class Navigator<D extends androidx.navigation.NavDestination> {
-    method @RestrictTo(androidx.annotation.RestrictTo.Scope.LIBRARY_GROUP) public final void addOnNavigatorBackPressListener(androidx.navigation.Navigator.OnNavigatorBackPressListener);
-    method @RestrictTo(androidx.annotation.RestrictTo.Scope.LIBRARY_GROUP) public final void dispatchOnNavigatorBackPress();
-    method @RestrictTo(androidx.annotation.RestrictTo.Scope.LIBRARY_GROUP) protected void onBackPressAdded();
-    method @RestrictTo(androidx.annotation.RestrictTo.Scope.LIBRARY_GROUP) protected void onBackPressRemoved();
-    method @RestrictTo(androidx.annotation.RestrictTo.Scope.LIBRARY_GROUP) public final void removeOnNavigatorBackPressListener(androidx.navigation.Navigator.OnNavigatorBackPressListener);
-  }
-
-  @RestrictTo(androidx.annotation.RestrictTo.Scope.LIBRARY_GROUP) public static interface Navigator.OnNavigatorBackPressListener {
-    method public void onPopBackStack(androidx.navigation.Navigator);
-  }
-
-}
-
diff --git a/navigation/common/api/restricted_current.txt b/navigation/common/api/restricted_current.txt
index 37d70c254db..da4f6cc18fe 100644
--- a/navigation/common/api/restricted_current.txt
+++ b/navigation/common/api/restricted_current.txt
@@ -1,17 +1 @@
 // Signature format: 3.0
-package androidx.navigation {
-
-  public abstract class Navigator<D extends androidx.navigation.NavDestination> {
-    method @RestrictTo(androidx.annotation.RestrictTo.Scope.LIBRARY_GROUP) public final void addOnNavigatorBackPressListener(androidx.navigation.Navigator.OnNavigatorBackPressListener);
-    method @RestrictTo(androidx.annotation.RestrictTo.Scope.LIBRARY_GROUP) public final void dispatchOnNavigatorBackPress();
-    method @RestrictTo(androidx.annotation.RestrictTo.Scope.LIBRARY_GROUP) protected void onBackPressAdded();
-    method @RestrictTo(androidx.annotation.RestrictTo.Scope.LIBRARY_GROUP) protected void onBackPressRemoved();
-    method @RestrictTo(androidx.annotation.RestrictTo.Scope.LIBRARY_GROUP) public final void removeOnNavigatorBackPressListener(androidx.navigation.Navigator.OnNavigatorBackPressListener);
-  }
-
-  @RestrictTo(androidx.annotation.RestrictTo.Scope.LIBRARY_GROUP) public static interface Navigator.OnNavigatorBackPressListener {
-    method public void onPopBackStack(androidx.navigation.Navigator);
-  }
-
-}
-
diff --git a/navigation/common/src/main/java/androidx/navigation/Navigator.java b/navigation/common/src/main/java/androidx/navigation/Navigator.java
index 979e8d6d0d9..46627005005 100644
--- a/navigation/common/src/main/java/androidx/navigation/Navigator.java
+++ b/navigation/common/src/main/java/androidx/navigation/Navigator.java
@@ -25,11 +25,9 @@ import android.os.Bundle;
 
 import androidx.annotation.NonNull;
 import androidx.annotation.Nullable;
-import androidx.annotation.RestrictTo;
 
 import java.lang.annotation.Retention;
 import java.lang.annotation.Target;
-import java.util.concurrent.CopyOnWriteArrayList;
 
 /**
  * Navigator defines a mechanism for navigating within an app.
@@ -67,9 +65,6 @@ public abstract class Navigator<D extends NavDestination> {
         String value();
     }
 
-    private final CopyOnWriteArrayList<OnNavigatorBackPressListener> mOnBackPressListeners =
-            new CopyOnWriteArrayList<>();
-
     /**
      * Construct a new NavDestination associated with this Navigator.
      *
@@ -133,67 +128,6 @@ public abstract class Navigator<D extends NavDestination> {
     }
 
     /**
-     * @hide
-     */
-    @RestrictTo(RestrictTo.Scope.LIBRARY_GROUP)
-    protected void onBackPressAdded() {
-    }
-
-    /**
-     * @hide
-     */
-    @RestrictTo(RestrictTo.Scope.LIBRARY_GROUP)
-    protected void onBackPressRemoved() {
-    }
-
-    /**
-     * @hide
-     */
-    @RestrictTo(RestrictTo.Scope.LIBRARY_GROUP)
-    public final void addOnNavigatorBackPressListener(
-            @NonNull OnNavigatorBackPressListener listener) {
-        boolean added = mOnBackPressListeners.add(listener);
-        if (added && mOnBackPressListeners.size() == 1) {
-            onBackPressAdded();
-        }
-    }
-
-    /**
-     * @hide
-     */
-    @RestrictTo(RestrictTo.Scope.LIBRARY_GROUP)
-    public final void removeOnNavigatorBackPressListener(
-            @NonNull OnNavigatorBackPressListener listener) {
-        boolean removed = mOnBackPressListeners.remove(listener);
-        if (removed && mOnBackPressListeners.isEmpty()) {
-            onBackPressRemoved();
-        }
-    }
-
-    /**
-     * @hide
-     */
-    @RestrictTo(RestrictTo.Scope.LIBRARY_GROUP)
-    public final void dispatchOnNavigatorBackPress() {
-        for (OnNavigatorBackPressListener listener : mOnBackPressListeners) {
-            listener.onPopBackStack(this);
-        }
-    }
-
-    /**
-     * @hide
-     */
-    @RestrictTo(RestrictTo.Scope.LIBRARY_GROUP)
-    public interface OnNavigatorBackPressListener {
-        /**
-         * This method is called after the Navigator navigates to a new destination.
-         *
-         * @param navigator
-         */
-        void onPopBackStack(@NonNull Navigator navigator);
-    }
-
-    /**
      * Interface indicating that this class should be passed to its respective
      * {@link Navigator} to enable Navigator specific behavior.
      */
diff --git a/navigation/fragment/src/androidTest/AndroidManifest.xml b/navigation/fragment/src/androidTest/AndroidManifest.xml
index 5e6b71dc9e7..03b4dc38715 100644
--- a/navigation/fragment/src/androidTest/AndroidManifest.xml
+++ b/navigation/fragment/src/androidTest/AndroidManifest.xml
@@ -19,6 +19,7 @@
           package="androidx.navigation.fragment">
     <uses-sdk android:targetSdkVersion="${target-sdk-version}"/>
     <application>
+        <activity android:name="androidx.navigation.fragment.test.NavigationActivity"/>
         <activity android:name="androidx.navigation.fragment.XmlNavigationActivity" />
         <activity android:name="androidx.navigation.fragment.DynamicNavigationActivity" />
         <activity android:name="androidx.navigation.fragment.EmbeddedXmlActivity" />
diff --git a/navigation/fragment/src/androidTest/java/androidx/navigation/fragment/FragmentNavigatorTest.kt b/navigation/fragment/src/androidTest/java/androidx/navigation/fragment/FragmentNavigatorTest.kt
index b65f205d6f8..9bad25b147e 100644
--- a/navigation/fragment/src/androidTest/java/androidx/navigation/fragment/FragmentNavigatorTest.kt
+++ b/navigation/fragment/src/androidTest/java/androidx/navigation/fragment/FragmentNavigatorTest.kt
@@ -23,7 +23,6 @@ import androidx.fragment.app.FragmentFactory
 import androidx.fragment.app.FragmentManager
 import androidx.lifecycle.Lifecycle
 import androidx.navigation.NavOptions
-import androidx.navigation.Navigator
 import androidx.navigation.fragment.test.EmptyFragment
 import androidx.navigation.fragment.test.R
 import androidx.test.annotation.UiThreadTest
@@ -40,9 +39,6 @@ import org.junit.Before
 import org.junit.Rule
 import org.junit.Test
 import org.junit.runner.RunWith
-import org.mockito.Mockito.mock
-import org.mockito.Mockito.verify
-import org.mockito.Mockito.verifyNoMoreInteractions
 
 @LargeTest
 @RunWith(AndroidJUnit4::class)
@@ -179,45 +175,6 @@ class FragmentNavigatorTest {
 
     @UiThreadTest
     @Test
-    fun testNavigateWithPopUpToThenPopWithFragmentManager() {
-        val fragmentNavigator = FragmentNavigator(emptyActivity, fragmentManager, R.id.container)
-        val backPressListener = mock(Navigator.OnNavigatorBackPressListener::class.java)
-        fragmentNavigator.addOnNavigatorBackPressListener(backPressListener)
-        val destination = fragmentNavigator.createDestination()
-        destination.id = INITIAL_FRAGMENT
-        destination.className = EmptyFragment::class.java.name
-
-        // Push initial fragment
-        assertThat(fragmentNavigator.navigate(destination, null, null, null))
-            .isEqualTo(destination)
-        fragmentManager.executePendingTransactions()
-
-        // Push a second fragment
-        destination.id = SECOND_FRAGMENT
-        assertThat(fragmentNavigator.navigate(destination, null, null, null))
-            .isEqualTo(destination)
-        fragmentManager.executePendingTransactions()
-
-        // Pop and then push third fragment, simulating popUpTo to initial.
-        val success = fragmentNavigator.popBackStack()
-        assertWithMessage("FragmentNavigator should popBackStack successfully")
-                .that(success)
-                .isTrue()
-        destination.id = THIRD_FRAGMENT
-        assertThat(fragmentNavigator.navigate(destination, null,
-                NavOptions.Builder().setPopUpTo(INITIAL_FRAGMENT, false).build(), null))
-            .isEqualTo(destination)
-        fragmentManager.executePendingTransactions()
-
-        // Now pop the Fragment
-        val popped = fragmentManager.popBackStackImmediate()
-        assertTrue("FragmentNavigator should return true when popping the third fragment", popped)
-        verify(backPressListener).onPopBackStack(fragmentNavigator)
-        verifyNoMoreInteractions(backPressListener)
-    }
-
-    @UiThreadTest
-    @Test
     fun testSingleTopInitial() {
         val fragmentNavigator = FragmentNavigator(emptyActivity, fragmentManager, R.id.container)
         val destination = fragmentNavigator.createDestination()
@@ -249,8 +206,6 @@ class FragmentNavigatorTest {
     @Test
     fun testSingleTop() {
         val fragmentNavigator = FragmentNavigator(emptyActivity, fragmentManager, R.id.container)
-        val backPressListener = mock(Navigator.OnNavigatorBackPressListener::class.java)
-        fragmentNavigator.addOnNavigatorBackPressListener(backPressListener)
         val destination = fragmentNavigator.createDestination()
         destination.className = EmptyFragment::class.java.name
 
@@ -463,47 +418,8 @@ class FragmentNavigatorTest {
 
     @UiThreadTest
     @Test
-    fun testPopWithFragmentManager() {
-        val fragmentNavigator = FragmentNavigator(emptyActivity, fragmentManager, R.id.container)
-        val backPressListener = mock(Navigator.OnNavigatorBackPressListener::class.java)
-        fragmentNavigator.addOnNavigatorBackPressListener(backPressListener)
-        val destination = fragmentNavigator.createDestination()
-        destination.id = INITIAL_FRAGMENT
-        destination.className = EmptyFragment::class.java.name
-
-        // First push an initial Fragment
-        assertThat(fragmentNavigator.navigate(destination, null, null, null))
-            .isEqualTo(destination)
-        fragmentManager.executePendingTransactions()
-        val fragment = fragmentManager.findFragmentById(R.id.container)
-        assertNotNull("Fragment should be added", fragment)
-
-        // Now push the Fragment that we want to pop
-        destination.id = SECOND_FRAGMENT
-        assertThat(fragmentNavigator.navigate(destination, null, null, null))
-            .isEqualTo(destination)
-        fragmentManager.executePendingTransactions()
-        val replacementFragment = fragmentManager.findFragmentById(R.id.container)
-        assertNotNull("Replacement Fragment should be added", replacementFragment)
-        assertTrue("Replacement Fragment should be the correct type",
-                replacementFragment is EmptyFragment)
-        assertEquals("Replacement Fragment should be the primary navigation Fragment",
-                replacementFragment, fragmentManager.primaryNavigationFragment)
-
-        // Now pop the Fragment
-        fragmentManager.popBackStackImmediate()
-        verify(backPressListener).onPopBackStack(fragmentNavigator)
-        assertEquals("Fragment should be the primary navigation Fragment after pop",
-                fragment, fragmentManager.primaryNavigationFragment)
-        verifyNoMoreInteractions(backPressListener)
-    }
-
-    @UiThreadTest
-    @Test
-    fun testDeepLinkPopWithFragmentManager() {
+    fun testDeepLinkPop() {
         val fragmentNavigator = FragmentNavigator(emptyActivity, fragmentManager, R.id.container)
-        val backPressListener = mock(Navigator.OnNavigatorBackPressListener::class.java)
-        fragmentNavigator.addOnNavigatorBackPressListener(backPressListener)
         val destination = fragmentNavigator.createDestination()
         destination.id = INITIAL_FRAGMENT
         destination.className = EmptyFragment::class.java.name
@@ -528,21 +444,18 @@ class FragmentNavigatorTest {
                 replacementFragment, fragmentManager.primaryNavigationFragment)
 
         // Now pop the Fragment
-        fragmentManager.popBackStackImmediate()
-        verify(backPressListener).onPopBackStack(fragmentNavigator)
+        fragmentNavigator.popBackStack()
+        fragmentManager.executePendingTransactions()
         val fragment = fragmentManager.findFragmentById(R.id.container)
         assertEquals("Fragment should be the primary navigation Fragment after pop",
                 fragment, fragmentManager.primaryNavigationFragment)
-        verifyNoMoreInteractions(backPressListener)
     }
 
     @UiThreadTest
     @Test
-    fun testDeepLinkPopWithFragmentManagerWithSaveState() {
+    fun testDeepLinkPopWithSaveState() {
         var fragmentNavigator = FragmentNavigator(emptyActivity,
                 fragmentManager, R.id.container)
-        val backPressListener = mock(Navigator.OnNavigatorBackPressListener::class.java)
-        fragmentNavigator.addOnNavigatorBackPressListener(backPressListener)
         val destination = fragmentNavigator.createDestination()
         destination.id = INITIAL_FRAGMENT
         destination.className = EmptyFragment::class.java.name
@@ -568,19 +481,16 @@ class FragmentNavigatorTest {
 
         // Create a new FragmentNavigator, replacing the previous one
         val savedState = fragmentNavigator.onSaveState()
-        fragmentNavigator.removeOnNavigatorBackPressListener(backPressListener)
         fragmentNavigator = FragmentNavigator(emptyActivity,
                 fragmentManager, R.id.container)
         fragmentNavigator.onRestoreState(savedState)
-        fragmentNavigator.addOnNavigatorBackPressListener(backPressListener)
 
         // Now pop the Fragment
-        fragmentManager.popBackStackImmediate()
-        verify(backPressListener).onPopBackStack(fragmentNavigator)
+        fragmentNavigator.popBackStack()
+        fragmentManager.executePendingTransactions()
         val fragment = fragmentManager.findFragmentById(R.id.container)
         assertEquals("Fragment should be the primary navigation Fragment after pop",
                 fragment, fragmentManager.primaryNavigationFragment)
-        verifyNoMoreInteractions(backPressListener)
     }
 
     @UiThreadTest
@@ -588,8 +498,6 @@ class FragmentNavigatorTest {
     fun testNavigateThenPopAfterSaveState() {
         var fragmentNavigator = FragmentNavigator(emptyActivity,
                 fragmentManager, R.id.container)
-        val backPressListener = mock(Navigator.OnNavigatorBackPressListener::class.java)
-        fragmentNavigator.addOnNavigatorBackPressListener(backPressListener)
         val destination = fragmentNavigator.createDestination()
         destination.id = INITIAL_FRAGMENT
         destination.className = EmptyFragment::class.java.name
@@ -618,11 +526,9 @@ class FragmentNavigatorTest {
 
         // Create a new FragmentNavigator, replacing the previous one
         val savedState = fragmentNavigator.onSaveState()
-        fragmentNavigator.removeOnNavigatorBackPressListener(backPressListener)
         fragmentNavigator = FragmentNavigator(emptyActivity,
                 fragmentManager, R.id.container)
         fragmentNavigator.onRestoreState(savedState)
-        fragmentNavigator.addOnNavigatorBackPressListener(backPressListener)
 
         // Now push a third fragment after the state save
         destination.id = THIRD_FRAGMENT
@@ -637,22 +543,18 @@ class FragmentNavigatorTest {
                 replacementFragment, fragmentManager.primaryNavigationFragment)
 
         // Now pop the Fragment
-        fragmentManager.popBackStackImmediate()
-        verify(backPressListener).onPopBackStack(fragmentNavigator)
+        fragmentNavigator.popBackStack()
+        fragmentManager.executePendingTransactions()
         fragment = fragmentManager.findFragmentById(R.id.container)
         assertEquals("Fragment should be the primary navigation Fragment after pop",
                 fragment, fragmentManager.primaryNavigationFragment)
-
-        verifyNoMoreInteractions(backPressListener)
     }
 
     @UiThreadTest
     @Test
-    fun testMultipleNavigateFragmentTransactionsThenPopWithFragmentManager() {
+    fun testMultipleNavigateFragmentTransactionsThenPop() {
         val fragmentNavigator = FragmentNavigator(emptyActivity,
                 fragmentManager, R.id.container)
-        val backPressListener = mock(Navigator.OnNavigatorBackPressListener::class.java)
-        fragmentNavigator.addOnNavigatorBackPressListener(backPressListener)
         val destination = fragmentNavigator.createDestination()
         destination.className = EmptyFragment::class.java.name
 
@@ -665,19 +567,16 @@ class FragmentNavigatorTest {
         fragmentNavigator.navigate(destination, null, null, null)
 
         // Now pop the Fragment
-        val popped = fragmentManager.popBackStackImmediate()
+        val popped = fragmentNavigator.popBackStack()
+        fragmentManager.executePendingTransactions()
         assertTrue("FragmentNavigator should return true when popping the third fragment", popped)
-        verify(backPressListener).onPopBackStack(fragmentNavigator)
-        verifyNoMoreInteractions(backPressListener)
     }
 
     @UiThreadTest
     @Test
-    fun testMultiplePopFragmentTransactionsThenPopWithFragmentManager() {
+    fun testMultiplePopFragmentTransactionsThenPop() {
         val fragmentNavigator = FragmentNavigator(emptyActivity,
                 fragmentManager, R.id.container)
-        val backPressListener = mock(Navigator.OnNavigatorBackPressListener::class.java)
-        fragmentNavigator.addOnNavigatorBackPressListener(backPressListener)
         val destination = fragmentNavigator.createDestination()
         destination.className = EmptyFragment::class.java.name
 
@@ -696,10 +595,9 @@ class FragmentNavigatorTest {
         fragmentNavigator.popBackStack()
         fragmentNavigator.popBackStack()
 
-        val popped = fragmentManager.popBackStackImmediate()
+        val popped = fragmentNavigator.popBackStack()
+        fragmentManager.executePendingTransactions()
         assertTrue("FragmentNavigator should return true when popping the third fragment", popped)
-        verify(backPressListener).onPopBackStack(fragmentNavigator)
-        verifyNoMoreInteractions(backPressListener)
     }
 }
 
diff --git a/navigation/fragment/src/androidTest/java/androidx/navigation/fragment/OnBackPressedTest.kt b/navigation/fragment/src/androidTest/java/androidx/navigation/fragment/OnBackPressedTest.kt
new file mode 100644
index 00000000000..e3dc4c6e39a
--- /dev/null
+++ b/navigation/fragment/src/androidTest/java/androidx/navigation/fragment/OnBackPressedTest.kt
@@ -0,0 +1,105 @@
+/*
+ * Copyright 2019 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package androidx.navigation.fragment
+
+import android.os.Bundle
+import androidx.fragment.app.Fragment
+import androidx.navigation.fragment.test.EmptyFragment
+import androidx.navigation.fragment.test.NavigationActivity
+import androidx.navigation.fragment.test.R
+import androidx.test.annotation.UiThreadTest
+import androidx.test.ext.junit.runners.AndroidJUnit4
+import androidx.test.filters.MediumTest
+import androidx.test.rule.ActivityTestRule
+import com.google.common.truth.Truth.assertWithMessage
+import org.junit.Rule
+import org.junit.Test
+import org.junit.runner.RunWith
+
+@MediumTest
+@RunWith(AndroidJUnit4::class)
+class OnBackPressedTest {
+
+    @get:Rule
+    var activityRule = ActivityTestRule(NavigationActivity::class.java)
+
+    @UiThreadTest
+    @Test
+    fun testOnBackPressedOnRoot() {
+        val activity = activityRule.activity
+        val navController = activity.navController
+        navController.setGraph(R.navigation.nav_simple)
+        activity.onBackPressed()
+        assertWithMessage("onBackPressed() should finish the activity on the root")
+            .that(activity.isFinishing)
+            .isTrue()
+    }
+
+    @UiThreadTest
+    @Test
+    fun testOnBackPressedAfterNavigate() {
+        val activity = activityRule.activity
+        val navController = activity.navController
+        navController.setGraph(R.navigation.nav_simple)
+        navController.navigate(R.id.empty_fragment)
+
+        activity.onBackPressed()
+        assertWithMessage("onBackPressed() should trigger NavController.popBackStack()")
+            .that(navController.currentDestination?.id)
+            .isEqualTo(R.id.start_fragment)
+    }
+
+    @UiThreadTest
+    @Test
+    fun testOnBackPressedWithChildBackStack() {
+        val activity = activityRule.activity
+        val navHostFragment = activity.supportFragmentManager.primaryNavigationFragment
+                as NavHostFragment
+        val navHostFragmentManager = navHostFragment.childFragmentManager
+        val navController = navHostFragment.navController
+        navController.setGraph(R.navigation.nav_simple)
+        navController.navigate(R.id.child_back_stack_fragment)
+        navHostFragmentManager.executePendingTransactions()
+
+        val currentFragment = navHostFragmentManager.primaryNavigationFragment
+                as ChildBackStackFragment
+        assertWithMessage("Current Fragment should have a child Fragment by default")
+            .that(currentFragment.childFragment)
+            .isNotNull()
+
+        activity.onBackPressed()
+        assertWithMessage("onBackPressed() should not trigger NavController when there is a " +
+                "child back stack")
+            .that(navController.currentDestination?.id)
+            .isEqualTo(R.id.child_back_stack_fragment)
+        assertWithMessage("Child Fragment should be popped")
+            .that(currentFragment.childFragment)
+            .isNull()
+    }
+}
+
+class ChildBackStackFragment : EmptyFragment() {
+    val childFragment get() = childFragmentManager.findFragmentByTag("child")
+
+    override fun onCreate(savedInstanceState: Bundle?) {
+        super.onCreate(savedInstanceState)
+        childFragmentManager.beginTransaction()
+            .add(Fragment(), "child")
+            .addToBackStack(null)
+            .commit()
+    }
+}
\ No newline at end of file
diff --git a/navigation/fragment/src/androidTest/java/androidx/navigation/fragment/test/EmptyFragment.kt b/navigation/fragment/src/androidTest/java/androidx/navigation/fragment/test/EmptyFragment.kt
index 8ab7d3f97fe..094edf20872 100644
--- a/navigation/fragment/src/androidTest/java/androidx/navigation/fragment/test/EmptyFragment.kt
+++ b/navigation/fragment/src/androidTest/java/androidx/navigation/fragment/test/EmptyFragment.kt
@@ -23,7 +23,7 @@ import android.view.ViewGroup
 import android.widget.FrameLayout
 import androidx.fragment.app.Fragment
 
-class EmptyFragment : Fragment() {
+open class EmptyFragment : Fragment() {
     override fun onCreateView(
         inflater: LayoutInflater,
         container: ViewGroup?,
diff --git a/navigation/fragment/src/androidTest/java/androidx/navigation/fragment/test/NavigationActivity.kt b/navigation/fragment/src/androidTest/java/androidx/navigation/fragment/test/NavigationActivity.kt
new file mode 100644
index 00000000000..1669f10c585
--- /dev/null
+++ b/navigation/fragment/src/androidTest/java/androidx/navigation/fragment/test/NavigationActivity.kt
@@ -0,0 +1,24 @@
+/*
+ * Copyright 2019 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package androidx.navigation.fragment.test
+
+import androidx.fragment.app.FragmentActivity
+import androidx.navigation.findNavController
+
+class NavigationActivity : FragmentActivity(R.layout.navigation_activity) {
+    val navController get() = findNavController(R.id.nav_host)
+}
\ No newline at end of file
diff --git a/navigation/fragment/src/androidTest/res/navigation/nav_simple.xml b/navigation/fragment/src/androidTest/res/navigation/nav_simple.xml
new file mode 100644
index 00000000000..95f2c5790e3
--- /dev/null
+++ b/navigation/fragment/src/androidTest/res/navigation/nav_simple.xml
@@ -0,0 +1,31 @@
+<?xml version="1.0" encoding="utf-8"?>
+<!--
+  Copyright 2019 The Android Open Source Project
+
+  Licensed under the Apache License, Version 2.0 (the "License");
+  you may not use this file except in compliance with the License.
+  You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+  Unless required by applicable law or agreed to in writing, software
+  distributed under the License is distributed on an "AS IS" BASIS,
+  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+  See the License for the specific language governing permissions and
+  limitations under the License.
+  -->
+
+<navigation xmlns:android="http://schemas.android.com/apk/res/android"
+    xmlns:app="http://schemas.android.com/apk/res-auto"
+    android:id="@+id/nav_simple"
+    app:startDestination="@+id/start_fragment">
+    <fragment
+        android:id="@+id/start_fragment"
+        android:name="androidx.navigation.fragment.test.EmptyFragment"/>
+    <fragment
+        android:id="@+id/empty_fragment"
+        android:name="androidx.navigation.fragment.test.EmptyFragment"/>
+    <fragment
+        android:id="@+id/child_back_stack_fragment"
+        android:name="androidx.navigation.fragment.ChildBackStackFragment"/>
+</navigation>
\ No newline at end of file
diff --git a/navigation/fragment/src/main/java/androidx/navigation/fragment/FragmentNavigator.java b/navigation/fragment/src/main/java/androidx/navigation/fragment/FragmentNavigator.java
index 9c125546562..da02a88e23e 100644
--- a/navigation/fragment/src/main/java/androidx/navigation/fragment/FragmentNavigator.java
+++ b/navigation/fragment/src/main/java/androidx/navigation/fragment/FragmentNavigator.java
@@ -39,7 +39,6 @@ import androidx.navigation.NavigatorProvider;
 
 import java.util.ArrayDeque;
 import java.util.Collections;
-import java.util.Iterator;
 import java.util.LinkedHashMap;
 import java.util.Map;
 
@@ -62,54 +61,9 @@ public class FragmentNavigator extends Navigator<FragmentNavigator.Destination>
     private static final String KEY_BACK_STACK_IDS = "androidx-nav-fragment:navigator:backStackIds";
 
     private final Context mContext;
-    @SuppressWarnings("WeakerAccess") /* synthetic access */
-    final FragmentManager mFragmentManager;
+    private final FragmentManager mFragmentManager;
     private final int mContainerId;
-    @SuppressWarnings("WeakerAccess") /* synthetic access */
-    ArrayDeque<Integer> mBackStack = new ArrayDeque<>();
-    @SuppressWarnings("WeakerAccess") /* synthetic access */
-    boolean mIsPendingBackStackOperation = false;
-
-    /**
-     * A fragment manager back stack change listener used to detect a fragment being popped due to
-     * onBackPressed().
-     *
-     * Since a back press is a pop in the FragmentManager not caused by this navigator a flag is
-     * used to identify operations by this navigator. If the flag is ON, this listener doesn't do
-     * anything since the change in the fragment manager's back stack was caused by the navigator.
-     * The flag is reset once this navigator's back stack matches the fragment manager's back stack.
-     * If the flag is OFF then a change in the back stack was not caused by this navigator, it is
-     * then appropriate to check if a fragment was popped to dispatch navigator events.
-     *
-     * Note that onBackPressed() invokes popBackStackImmediate(), meaning pending transactions - if
-     * any - before the pop will be executed causing this listener to be called one or more times
-     * until the flag is reset. Finally, the pop due to pressing back occurs, at which it is
-     * appropriate to dispatch a navigator popped event.
-     */
-    private final FragmentManager.OnBackStackChangedListener mOnBackStackChangedListener =
-            new FragmentManager.OnBackStackChangedListener() {
-
-                @Override
-                public void onBackStackChanged() {
-                    // If we have pending operations made by us then consume this change, otherwise
-                    // detect a pop in the back stack to dispatch callback.
-                    if (mIsPendingBackStackOperation) {
-                        mIsPendingBackStackOperation = !isBackStackEqual();
-                        return;
-                    }
-
-                    // The initial Fragment won't be on the back stack, so the
-                    // real count of destinations is the back stack entry count + 1
-                    int newCount = mFragmentManager.getBackStackEntryCount() + 1;
-                    if (newCount < mBackStack.size()) {
-                        // Handle cases where the user hit the system back button
-                        while (mBackStack.size() > newCount) {
-                            mBackStack.removeLast();
-                        }
-                        dispatchOnNavigatorBackPress();
-                    }
-                }
-            };
+    private ArrayDeque<Integer> mBackStack = new ArrayDeque<>();
 
     public FragmentNavigator(@NonNull Context context, @NonNull FragmentManager manager,
             int containerId) {
@@ -118,16 +72,6 @@ public class FragmentNavigator extends Navigator<FragmentNavigator.Destination>
         mContainerId = containerId;
     }
 
-    @Override
-    protected void onBackPressAdded() {
-        mFragmentManager.addOnBackStackChangedListener(mOnBackStackChangedListener);
-    }
-
-    @Override
-    protected void onBackPressRemoved() {
-        mFragmentManager.removeOnBackStackChangedListener(mOnBackStackChangedListener);
-    }
-
     /**
      * {@inheritDoc}
      * <p>
@@ -154,7 +98,6 @@ public class FragmentNavigator extends Navigator<FragmentNavigator.Destination>
             mFragmentManager.popBackStack(
                     generateBackStackName(mBackStack.size(), mBackStack.peekLast()),
                     FragmentManager.POP_BACK_STACK_INCLUSIVE);
-            mIsPendingBackStackOperation = true;
         } // else, we're on the first Fragment, so there's nothing to pop from FragmentManager
         mBackStack.removeLast();
         return true;
@@ -204,6 +147,7 @@ public class FragmentNavigator extends Navigator<FragmentNavigator.Destination>
      * asynchronously, so the new Fragment is not instantly available
      * after this call completes.
      */
+    @SuppressWarnings("deprecation") /* Using instantiateFragment for forward compatibility */
     @Nullable
     @Override
     public NavDestination navigate(@NonNull Destination destination, @Nullable Bundle args,
@@ -217,7 +161,6 @@ public class FragmentNavigator extends Navigator<FragmentNavigator.Destination>
         if (className.charAt(0) == '.') {
             className = mContext.getPackageName() + className;
         }
-        //noinspection deprecation needed to maintain forward compatibility
         final Fragment frag = instantiateFragment(mContext, mFragmentManager,
                 className, args);
         frag.setArguments(args);
@@ -259,12 +202,10 @@ public class FragmentNavigator extends Navigator<FragmentNavigator.Destination>
                         generateBackStackName(mBackStack.size(), mBackStack.peekLast()),
                         FragmentManager.POP_BACK_STACK_INCLUSIVE);
                 ft.addToBackStack(generateBackStackName(mBackStack.size(), destId));
-                mIsPendingBackStackOperation = true;
             }
             isAdded = false;
         } else {
             ft.addToBackStack(generateBackStackName(mBackStack.size() + 1, destId));
-            mIsPendingBackStackOperation = true;
             isAdded = true;
         }
         if (navigatorExtras instanceof Extras) {
@@ -336,40 +277,6 @@ public class FragmentNavigator extends Navigator<FragmentNavigator.Destination>
     }
 
     /**
-     * Checks if this FragmentNavigator's back stack is equal to the FragmentManager's back stack.
-     */
-    @SuppressWarnings("WeakerAccess") /* synthetic access */
-    boolean isBackStackEqual() {
-        int fragmentBackStackCount = mFragmentManager.getBackStackEntryCount();
-        // Initial fragment won't be on the FragmentManager's back stack so +1 its count.
-        if (mBackStack.size() != fragmentBackStackCount + 1) {
-            return false;
-        }
-
-        // From top to bottom verify destination ids match in both back stacks/
-        Iterator<Integer> backStackIterator = mBackStack.descendingIterator();
-        int fragmentBackStackIndex = fragmentBackStackCount - 1;
-        while (backStackIterator.hasNext() && fragmentBackStackIndex >= 0) {
-            int destId = backStackIterator.next();
-            try {
-                int fragmentDestId = getDestId(mFragmentManager
-                        .getBackStackEntryAt(fragmentBackStackIndex--)
-                        .getName());
-                if (destId != fragmentDestId) {
-                    return false;
-                }
-            } catch (NumberFormatException e) {
-                throw new IllegalStateException("Invalid back stack entry on the "
-                        + "NavHostFragment's back stack - use getChildFragmentManager() "
-                        + "if you need to do custom FragmentTransactions from within "
-                        + "Fragments created via your navigation graph.");
-            }
-        }
-
-        return true;
-    }
-
-    /**
      * NavDestination specific to {@link FragmentNavigator}
      */
     @NavDestination.ClassType(Fragment.class)
diff --git a/navigation/fragment/src/main/java/androidx/navigation/fragment/NavHostFragment.java b/navigation/fragment/src/main/java/androidx/navigation/fragment/NavHostFragment.java
index c289b8a9169..8a344e96a18 100644
--- a/navigation/fragment/src/main/java/androidx/navigation/fragment/NavHostFragment.java
+++ b/navigation/fragment/src/main/java/androidx/navigation/fragment/NavHostFragment.java
@@ -202,8 +202,12 @@ public class NavHostFragment extends Fragment implements NavHost {
     public void onCreate(@Nullable Bundle savedInstanceState) {
         super.onCreate(savedInstanceState);
         final Context context = requireContext();
+        // TODO Fix Fragments to register their OnBackPressedCallback eagerly
+        getChildFragmentManager();
 
         mNavController = new NavController(context);
+        mNavController.setHostLifecycleOwner(this);
+        mNavController.setHostOnBackPressedDispatcherOwner(requireActivity());
         mNavController.setHostViewModelStore(getViewModelStore());
         onCreateNavController(mNavController);
 
diff --git a/navigation/runtime/api/2.1.0-alpha03.txt b/navigation/runtime/api/2.1.0-alpha03.txt
index 2fd3e0f1c35..8fe3dc47333 100644
--- a/navigation/runtime/api/2.1.0-alpha03.txt
+++ b/navigation/runtime/api/2.1.0-alpha03.txt
@@ -68,6 +68,8 @@ package androidx.navigation {
     method @CallSuper public void setGraph(@NavigationRes int, android.os.Bundle?);
     method @CallSuper public void setGraph(androidx.navigation.NavGraph);
     method @CallSuper public void setGraph(androidx.navigation.NavGraph, android.os.Bundle?);
+    method public void setHostLifecycleOwner(androidx.lifecycle.LifecycleOwner);
+    method public void setHostOnBackPressedDispatcherOwner(androidx.activity.OnBackPressedDispatcherOwner);
     method public void setHostViewModelStore(androidx.lifecycle.ViewModelStore);
     field public static final String KEY_DEEP_LINK_INTENT = "android-support-nav:controller:deepLinkIntent";
   }
diff --git a/navigation/runtime/api/current.txt b/navigation/runtime/api/current.txt
index 2fd3e0f1c35..8fe3dc47333 100644
--- a/navigation/runtime/api/current.txt
+++ b/navigation/runtime/api/current.txt
@@ -68,6 +68,8 @@ package androidx.navigation {
     method @CallSuper public void setGraph(@NavigationRes int, android.os.Bundle?);
     method @CallSuper public void setGraph(androidx.navigation.NavGraph);
     method @CallSuper public void setGraph(androidx.navigation.NavGraph, android.os.Bundle?);
+    method public void setHostLifecycleOwner(androidx.lifecycle.LifecycleOwner);
+    method public void setHostOnBackPressedDispatcherOwner(androidx.activity.OnBackPressedDispatcherOwner);
     method public void setHostViewModelStore(androidx.lifecycle.ViewModelStore);
     field public static final String KEY_DEEP_LINK_INTENT = "android-support-nav:controller:deepLinkIntent";
   }
diff --git a/navigation/runtime/build.gradle b/navigation/runtime/build.gradle
index 3a02683f2a4..d21d9e69e7c 100644
--- a/navigation/runtime/build.gradle
+++ b/navigation/runtime/build.gradle
@@ -33,6 +33,7 @@ android {
 
 dependencies {
     api(project(":navigation:navigation-common"))
+    api(project(":activity"))
     api(project(":lifecycle:lifecycle-viewmodel"))
 
     androidTestImplementation(project(":navigation:navigation-testing"))
diff --git a/navigation/runtime/src/androidTest/java/androidx/navigation/NavControllerTest.kt b/navigation/runtime/src/androidTest/java/androidx/navigation/NavControllerTest.kt
index bfbf4ef9b66..bab44e2e330 100644
--- a/navigation/runtime/src/androidTest/java/androidx/navigation/NavControllerTest.kt
+++ b/navigation/runtime/src/androidTest/java/androidx/navigation/NavControllerTest.kt
@@ -505,52 +505,6 @@ class NavControllerTest {
     }
 
     @Test
-    fun testNavigateFromNestedThenNavigatorInstigatedPop() {
-        val navController = createNavController()
-        navController.setGraph(R.navigation.nav_nested_start_destination)
-        val navigator = navController.navigatorProvider.getNavigator(TestNavigator::class.java)
-        assertEquals(R.id.nested_test, navController.currentDestination?.id ?: 0)
-        assertEquals(1, navigator.backStack.size)
-
-        navController.navigate(R.id.second_test)
-        assertEquals(R.id.second_test, navController.currentDestination?.id ?: 0)
-        assertEquals(2, navigator.backStack.size)
-
-        // A Navigator can pop a destination off its own back stack
-        // then inform the NavController via dispatchOnNavigatorNavigated
-        navigator.backStack.removeLast()
-        val newDestination = navigator.current.first
-        assertNotNull(newDestination)
-        navigator.dispatchOnNavigatorBackPress()
-        assertEquals(R.id.nested_test, navController.currentDestination?.id ?: 0)
-        assertEquals(1, navigator.backStack.size)
-    }
-
-    @Test
-    fun testNavigateNestedPopUpToThenNavigatorInstigatedPop() {
-        val navController = createNavController()
-        navController.setGraph(R.navigation.nav_nested_start_destination)
-        val navigator = navController.navigatorProvider.getNavigator(TestNavigator::class.java)
-        assertEquals(R.id.nested_test, navController.currentDestination?.id ?: 0)
-        assertEquals(1, navigator.backStack.size)
-
-        navController.navigate(R.id.pop_forward)
-        assertEquals(R.id.nested_second_test, navController.currentDestination?.id ?: 0)
-        assertEquals(1, navigator.backStack.size)
-
-        // A Navigator can pop a destination off its own back stack
-        // then inform the NavController via dispatchOnNavigatorNavigated
-        navigator.backStack.removeLast()
-        navigator.dispatchOnNavigatorBackPress()
-        assertWithMessage("The last destination should be popped off the stack")
-            .that(navController.currentDestination)
-            .isNull()
-        assertWithMessage("TestNavigator should not nothing on its back stack")
-            .that(navigator.backStack.size)
-            .isEqualTo(0)
-    }
-
-    @Test
     fun testNavigateThenNavigateWithPop() {
         val navController = createNavController()
         navController.setGraph(R.navigation.nav_simple)
diff --git a/navigation/runtime/src/main/java/androidx/navigation/NavController.java b/navigation/runtime/src/main/java/androidx/navigation/NavController.java
index 976374e4177..b5e664c7074 100644
--- a/navigation/runtime/src/main/java/androidx/navigation/NavController.java
+++ b/navigation/runtime/src/main/java/androidx/navigation/NavController.java
@@ -25,12 +25,16 @@ import android.os.Bundle;
 import android.os.Parcelable;
 import android.util.Log;
 
+import androidx.activity.OnBackPressedCallback;
+import androidx.activity.OnBackPressedDispatcher;
+import androidx.activity.OnBackPressedDispatcherOwner;
 import androidx.annotation.CallSuper;
 import androidx.annotation.IdRes;
 import androidx.annotation.NavigationRes;
 import androidx.annotation.NonNull;
 import androidx.annotation.Nullable;
 import androidx.core.app.TaskStackBuilder;
+import androidx.lifecycle.LifecycleOwner;
 import androidx.lifecycle.ViewModelStore;
 
 import java.util.ArrayDeque;
@@ -87,57 +91,10 @@ public class NavController {
     @SuppressWarnings("WeakerAccess") /* synthetic access */
     final Deque<NavBackStackEntry> mBackStack = new ArrayDeque<>();
 
+    private LifecycleOwner mLifecycleOwner;
     private NavControllerViewModel mViewModel;
 
-    private final NavigatorProvider mNavigatorProvider = new NavigatorProvider() {
-        @Nullable
-        @Override
-        public Navigator<? extends NavDestination> addNavigator(@NonNull String name,
-                @NonNull Navigator<? extends NavDestination> navigator) {
-            Navigator<? extends NavDestination> previousNavigator =
-                    super.addNavigator(name, navigator);
-            if (previousNavigator != navigator) {
-                if (previousNavigator != null) {
-                    previousNavigator.removeOnNavigatorBackPressListener(mOnBackPressListener);
-                }
-                navigator.addOnNavigatorBackPressListener(mOnBackPressListener);
-            }
-            return previousNavigator;
-        }
-    };
-
-    @SuppressWarnings("WeakerAccess") /* synthetic access */
-    final Navigator.OnNavigatorBackPressListener mOnBackPressListener =
-            new Navigator.OnNavigatorBackPressListener() {
-                @Override
-                public void onPopBackStack(@NonNull Navigator navigator) {
-                    // Find what destination just got popped
-                    NavDestination lastFromNavigator = null;
-                    Iterator<NavBackStackEntry> iterator = mBackStack.descendingIterator();
-                    while (iterator.hasNext()) {
-                        NavDestination destination = iterator.next().getDestination();
-                        Navigator currentNavigator = getNavigatorProvider().getNavigator(
-                                destination.getNavigatorName());
-                        if (currentNavigator == navigator) {
-                            lastFromNavigator = destination;
-                            break;
-                        }
-                    }
-                    if (lastFromNavigator == null) {
-                        throw new IllegalArgumentException("Navigator " + navigator
-                                + " reported pop but did not have any destinations"
-                                + " on the NavController back stack");
-                    }
-                    // Pop all intervening destinations from other Navigators off the
-                    // back stack
-                    popBackStackInternal(lastFromNavigator.getId(), false);
-                    // Now record the pop operation that we were sent
-                    if (!mBackStack.isEmpty()) {
-                        mBackStack.removeLast();
-                    }
-                    dispatchOnDestinationChanged();
-                }
-            };
+    private final NavigatorProvider mNavigatorProvider = new NavigatorProvider();
 
     private final CopyOnWriteArrayList<OnDestinationChangedListener>
             mOnDestinationChangedListeners = new CopyOnWriteArrayList<>();
@@ -1032,6 +989,52 @@ public class NavController {
     }
 
     /**
+     * Sets the host's {@link LifecycleOwner}.
+     *
+     * @param owner The {@link LifecycleOwner} associated with the containing {@link NavHost}.
+     * @see #setHostOnBackPressedDispatcherOwner(OnBackPressedDispatcherOwner)
+     */
+    public void setHostLifecycleOwner(@NonNull LifecycleOwner owner) {
+        mLifecycleOwner = owner;
+    }
+
+    /**
+     * Sets the host's {@link OnBackPressedDispatcherOwner}. If set, NavController will
+     * register a {@link OnBackPressedCallback} to handle system Back button events.
+     * <p>
+     * If you have not explicitly called {@link #setHostLifecycleOwner(LifecycleOwner)},
+     * the owner you pass here will be used as the {@link LifecycleOwner} for registering
+     * the {@link OnBackPressedCallback}.
+     *
+     * @param owner The {@link OnBackPressedDispatcherOwner} associated with the containing
+     * {@link NavHost}.
+     * @see #setHostLifecycleOwner(LifecycleOwner)
+     */
+    public void setHostOnBackPressedDispatcherOwner(@NonNull OnBackPressedDispatcherOwner owner) {
+        if (mLifecycleOwner == null) {
+            mLifecycleOwner = owner;
+        }
+        OnBackPressedDispatcher dispatcher = owner.getOnBackPressedDispatcher();
+        dispatcher.addCallback(mLifecycleOwner, new OnBackPressedCallback(true) {
+            @Override
+            public boolean isEnabled() {
+                int destinationCount = 0;
+                for (NavBackStackEntry entry : mBackStack) {
+                    if (!(entry.getDestination() instanceof NavGraph)) {
+                        destinationCount++;
+                    }
+                }
+                return destinationCount > 1;
+            }
+
+            @Override
+            public void handleOnBackPressed() {
+                popBackStack();
+            }
+        });
+    }
+
+    /**
      * Sets the host's ViewModelStore used by the NavController to store ViewModels at the
      * navigation graph level. This is required to call {@link #getViewModelStore} and
      * should generally be called for you by your {@link NavHost}.
diff --git a/navigation/runtime/src/main/java/androidx/navigation/NavHost.java b/navigation/runtime/src/main/java/androidx/navigation/NavHost.java
index a6b59d5ed9c..c8bdc194c32 100644
--- a/navigation/runtime/src/main/java/androidx/navigation/NavHost.java
+++ b/navigation/runtime/src/main/java/androidx/navigation/NavHost.java
@@ -19,7 +19,10 @@ package androidx.navigation;
 import android.os.Bundle;
 import android.view.View;
 
+import androidx.activity.OnBackPressedDispatcherOwner;
 import androidx.annotation.NonNull;
+import androidx.lifecycle.LifecycleOwner;
+import androidx.lifecycle.ViewModelStore;
 
 /**
  * A host is a single context or container for navigation via a {@link NavController}.
@@ -29,6 +32,17 @@ import androidx.annotation.NonNull;
  * <li>Handle {@link NavController#saveState() saving} and
  * {@link NavController#restoreState(Bundle) restoring} their controller's state</li>
  * <li>Call {@link Navigation#setViewNavController(View, NavController)} on their root view</li>
+ * <li>Route system Back button events to the NavController either by manually calling
+ * {@link NavController#popBackStack()} or by calling
+ * {@link NavController#setHostOnBackPressedDispatcherOwner(OnBackPressedDispatcherOwner)}
+ * when constructing the NavController.</li>
+ * </ul>
+ * Optionally, a navigation host should consider calling:
+ * <ul>
+ * <li>Call {@link NavController#setHostLifecycleOwner(LifecycleOwner)} to associate the
+ * NavController with a specific Lifecycle.</li>
+ * <li>Call {@link NavController#setHostViewModelStore(ViewModelStore)} to enable usage of
+ * {@link NavController#getViewModelStore(int)} and navigation graph scoped ViewModels.</li>
  * </ul>
  */
 public interface NavHost {