Avoid references to the Fragment's Views after onDestroyView()

BasicSample/app/src/main/java/com/example/android/persistence/ui/ProductFragment.java

@@ -87,6 +87,13 @@ private void subscribeToModel(final ProductViewModel model) {
         });
     }
 
+    @Override
+    public void onDestroyView() {
+        mBinding = null;
+        mCommentAdapter = null;
+        super.onDestroyView();
+    }
+
     /** Creates product fragment for specific product ID */
     public static ProductFragment forProduct(int productId) {
         ProductFragment fragment = new ProductFragment();

BasicSample/app/src/main/java/com/example/android/persistence/ui/ProductListFragment.java

@@ -90,6 +90,13 @@ private void subscribeUi(LiveData<List<ProductEntity>> liveData) {
         });
     }
 
+    @Override
+    public void onDestroyView() {
+        mBinding = null;
+        mProductAdapter = null;
+        super.onDestroyView();
+    }
+
     private final ProductClickCallback mProductClickCallback = product -> {
         if (getLifecycle().getCurrentState().isAtLeast(Lifecycle.State.STARTED)) {
             ((MainActivity) requireActivity()).show(product);

GithubBrowserSample/app/src/androidTest/java/com/android/example/github/util/AutoClearedValueTest.kt

@@ -15,6 +15,10 @@
  */
 package com.android.example.github.util
 
+import android.os.Bundle
+import android.view.LayoutInflater
+import android.view.View
+import android.view.ViewGroup
 import androidx.fragment.app.DialogFragment
 import androidx.fragment.app.Fragment
 import androidx.test.platform.app.InstrumentationRegistry
@@ -47,7 +51,6 @@ class AutoClearedValueTest {
 
     @Test
     fun clearOnReplace() {
-        testFragment.testValue = "foo"
         activityRule.activity.replaceFragment(TestFragment())
         InstrumentationRegistry.getInstrumentation().waitForIdleSync()
         try {
@@ -57,9 +60,22 @@ class AutoClearedValueTest {
         }
     }
 
+    @Test
+    fun clearOnReplaceBackStack() {
+        activityRule.activity.replaceFragment(TestFragment(), true)
+        InstrumentationRegistry.getInstrumentation().waitForIdleSync()
+        try {
+            testFragment.testValue
+            Assert.fail("should throw if accessed when not set")
+        } catch (ex: IllegalStateException) {
+        }
+        activityRule.activity.supportFragmentManager.popBackStack()
+        InstrumentationRegistry.getInstrumentation().waitForIdleSync()
+        assertThat(testFragment.testValue, `is`("foo"))
+    }
+
     @Test
     fun dontClearForChildFragment() {
-        testFragment.testValue = "foo"
         testFragment.childFragmentManager.beginTransaction()
             .add(Fragment(), "foo").commit()
         InstrumentationRegistry.getInstrumentation().waitForIdleSync()
@@ -68,15 +84,20 @@ class AutoClearedValueTest {
 
     @Test
     fun dontClearForDialog() {
-        testFragment.testValue = "foo"
         val dialogFragment = DialogFragment()
-        dialogFragment.show(testFragment.fragmentManager!!, "dialog")
+        dialogFragment.show(testFragment.parentFragmentManager, "dialog")
         dialogFragment.dismiss()
         InstrumentationRegistry.getInstrumentation().waitForIdleSync()
         assertThat(testFragment.testValue, `is`("foo"))
     }
 
     class TestFragment : Fragment() {
         var testValue by autoCleared<String>()
+
+        override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?,
+                savedInstanceState: Bundle?): View? {
+            testValue = "foo"
+            return View(context)
+        }
     }
 }

GithubBrowserSample/app/src/debug/java/com/android/example/github/testing/SingleFragmentActivity.kt

@@ -46,9 +46,14 @@ class SingleFragmentActivity : AppCompatActivity() {
             .commit()
     }
 
-    fun replaceFragment(fragment: Fragment) {
+    fun replaceFragment(fragment: Fragment, addToBackStack: Boolean = false) {
         supportFragmentManager.beginTransaction()
             .replace(R.id.container, fragment)
+            .apply {
+                if (addToBackStack) {
+                    addToBackStack(null)
+                }
+            }
             .commit()
     }
 }

GithubBrowserSample/app/src/main/java/com/android/example/github/util/AutoClearedValue.kt

@@ -16,26 +16,31 @@
 
 package com.android.example.github.util
 
-import androidx.lifecycle.Lifecycle
-import androidx.lifecycle.LifecycleObserver
-import androidx.lifecycle.OnLifecycleEvent
 import androidx.fragment.app.Fragment
+import androidx.lifecycle.DefaultLifecycleObserver
+import androidx.lifecycle.LifecycleOwner
+import androidx.lifecycle.observe
 import kotlin.properties.ReadWriteProperty
 import kotlin.reflect.KProperty
 
 /**
- * A lazy property that gets cleaned up when the fragment is destroyed.
+ * A lazy property that gets cleaned up when the fragment's view is destroyed.
  *
- * Accessing this variable in a destroyed fragment will throw NPE.
+ * Accessing this variable while the fragment's view is destroyed will throw NPE.
  */
 class AutoClearedValue<T : Any>(val fragment: Fragment) : ReadWriteProperty<Fragment, T> {
     private var _value: T? = null
 
     init {
-        fragment.lifecycle.addObserver(object : LifecycleObserver {
-            @OnLifecycleEvent(Lifecycle.Event.ON_DESTROY)
-            fun onDestroy() {
-                _value = null
+        fragment.lifecycle.addObserver(object: DefaultLifecycleObserver {
+            override fun onCreate(owner: LifecycleOwner) {
+                fragment.viewLifecycleOwnerLiveData.observe(fragment) { viewLifecycleOwner ->
+                    viewLifecycleOwner?.lifecycle?.addObserver(object: DefaultLifecycleObserver {
+                        override fun onDestroy(owner: LifecycleOwner) {
+                            _value = null
+                        }
+                    })
+                }
             }
         })
     }

NavigationAdvancedSample/app/src/main/java/com/example/android/navigationadvancedsample/listscreen/Leaderboard.kt

@@ -33,17 +33,14 @@ import com.example.android.navigationadvancedsample.R
  */
 class Leaderboard : Fragment() {
 
-    private lateinit var recyclerView: RecyclerView
-    private lateinit var viewAdapter: RecyclerView.Adapter<*>
-
     override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?,
                               savedInstanceState: Bundle?): View? {
         // Inflate the layout for this fragment
         val view = inflater.inflate(R.layout.fragment_leaderboard, container, false)
 
-        viewAdapter = MyAdapter(Array(10) { "Person ${it + 1}" })
+        val viewAdapter = MyAdapter(Array(10) { "Person ${it + 1}" })
 
-        recyclerView = view.findViewById<RecyclerView>(R.id.leaderboard_list).apply {
+        view.findViewById<RecyclerView>(R.id.leaderboard_list).run {
             // use this setting to improve performance if you know that changes
             // in content do not change the layout size of the RecyclerView
             setHasFixedSize(true)

NavigationBasicSample/app/src/main/java/com/example/android/navigationsample/Leaderboard.kt

@@ -32,24 +32,20 @@ import androidx.navigation.Navigation
  */
 class Leaderboard : Fragment() {
 
-    private lateinit var recyclerView: RecyclerView
-    private lateinit var viewAdapter: RecyclerView.Adapter<*>
-
     override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?,
                               savedInstanceState: Bundle?): View? {
         // Inflate the layout for this fragment
         val view = inflater.inflate(R.layout.fragment_leaderboard, container, false)
 
-        viewAdapter = MyAdapter(arrayOf("Flo", "Ly", "Jo"))
+        val viewAdapter = MyAdapter(arrayOf("Flo", "Ly", "Jo"))
 
-        recyclerView = view.findViewById<RecyclerView>(R.id.leaderboard_list).apply {
+        view.findViewById<RecyclerView>(R.id.leaderboard_list).run {
             // use this setting to improve performance if you know that changes
             // in content do not change the layout size of the RecyclerView
             setHasFixedSize(true)
 
             // specify an viewAdapter (see also next example)
             adapter = viewAdapter
-
         }
         return view
     }