Ensure manually adding a fragment to the back stack fails

If you are using Navigation and do a manual fragment transaction that
attempts to add a fragment to the fragment manager's back stack, the
fragment manager and Navigation back stacks will fall out of sync and
cause unexpected behavior.

Instead of allowing this, we should always throw whenever a fragment
transaction attempts to add a fragment to the fragment manager back
stack.

RelNote: "When using Navigation with Fragments, attempting to manually
do a `FragmentTransaction` that adds a fragment to the
`FragmentManager`'s back stack will now throw an
`IllegalArgumentException`. You should always add fragments via the
`navigate()` API."
Test: added FragmentNavigatorTest

Change-Id: I6d38e3fac4b7552881655d18304d5efec6168d03

Author: Jeremy Woods

navigation/navigation-fragment/src/androidTest/java/androidx/navigation/fragment/FragmentNavigatorTest.kt

@@ -53,6 +53,7 @@ import androidx.testutils.withActivity
 import androidx.testutils.withUse
 import com.google.common.truth.Truth.assertThat
 import com.google.common.truth.Truth.assertWithMessage
+import java.lang.IllegalArgumentException
 import java.util.concurrent.CountDownLatch
 import java.util.concurrent.TimeUnit
 import kotlin.reflect.KClass
@@ -555,7 +556,7 @@ class FragmentNavigatorTest {
 
     @UiThreadTest
     @Test
-    fun testNavigateAndAddIndependentFragment() {
+    fun testNavigationAndAddIndependentFragmentWithoutBackStack() {
         val entry = createBackStackEntry()
 
         fragmentNavigator.navigate(listOf(entry), null, null)
@@ -584,6 +585,42 @@ class FragmentNavigatorTest {
             .isEqualTo(independentFragment)
     }
 
+    @UiThreadTest
+    @Test
+    fun testNavigateAndAddIndependentFragmentWithBackStack() {
+        val entry = createBackStackEntry()
+
+        fragmentNavigator.navigate(listOf(entry), null, null)
+        assertThat(navigatorState.backStack.value)
+            .containsExactly(entry)
+        fragmentManager.executePendingTransactions()
+        val fragment = fragmentManager.findFragmentById(R.id.container)
+        assertWithMessage("Fragment should be added")
+            .that(fragment)
+            .isNotNull()
+        assertWithMessage("Fragment should be the correct type")
+            .that(fragment)
+            .isInstanceOf(EmptyFragment::class.java)
+        assertWithMessage("Fragment should be the primary navigation Fragment")
+            .that(fragmentManager.primaryNavigationFragment)
+            .isSameInstanceAs(fragment)
+
+        val independentFragment = EmptyFragment()
+        fragmentManager.beginTransaction()
+            .replace(R.id.container, independentFragment)
+            .addToBackStack(null)
+            .commit()
+        try {
+            fragmentManager.executePendingTransactions()
+        } catch (e: IllegalArgumentException) {
+            assertWithMessage("adding a fragment to the back stack manually should fail")
+                .that(e.message)
+                .contains("The fragment " + independentFragment + " is unknown to the " +
+                    "FragmentNavigator. Please use the navigate() function to add fragments to " +
+                    "the FragmentNavigator managed FragmentManager.")
+        }
+    }
+
     @LargeTest
     @UiThreadTest
     @Test

navigation/navigation-fragment/src/main/java/androidx/navigation/fragment/FragmentNavigator.kt

@@ -143,6 +143,13 @@ public open class FragmentNavigator(
                 val entry = (state.backStack.value + state.transitionsInProgress.value).lastOrNull {
                     it.id == fragment.tag
                 }
+                if (!pop) {
+                    requireNotNull(entry) {
+                        "The fragment " + fragment + " is unknown to the FragmentNavigator. " +
+                            "Please use the navigate() function to add fragments to the " +
+                            "FragmentNavigator managed FragmentManager."
+                    }
+                }
                 if (entry != null) {
                     // In case we get a fragment that was never attached to the fragment manager,
                     // we need to make sure we still return the entries to their proper final state.