For #4744 - Extract TransitionPreDrawListener

app/src/main/java/org/mozilla/fenix/home/HomeFragment.kt

@@ -13,7 +13,6 @@ import android.os.Bundle
 import android.view.LayoutInflater
 import android.view.View
 import android.view.ViewGroup
-import android.view.ViewTreeObserver
 import androidx.annotation.StringRes
 import androidx.appcompat.app.AlertDialog
 import androidx.appcompat.app.AppCompatActivity
@@ -113,21 +112,6 @@ class HomeFragment : Fragment(), AccountObserver {
         }
     }
 
-    private val preDrawListener = object : ViewTreeObserver.OnPreDrawListener {
-        override fun onPreDraw(): Boolean {
-            if (view != null) {
-                viewLifecycleOwner.lifecycleScope.launch {
-                    delay(ANIM_SCROLL_DELAY)
-                    restoreLayoutState()
-                    startPostponedEnterTransition()
-                }.invokeOnCompletion {
-                    sessionControlComponent.view.viewTreeObserver.removeOnPreDrawListener(this)
-                }
-            }
-            return true
-        }
-    }
-
     private var homeMenu: HomeMenu? = null
 
     private val sessionManager: SessionManager
@@ -201,20 +185,22 @@ class HomeFragment : Fragment(), AccountObserver {
         activity.themeManager.applyStatusBarTheme(activity)
 
         postponeEnterTransition()
-        sessionControlComponent.view.viewTreeObserver.addOnPreDrawListener(preDrawListener)
+        TransitionPreDrawListener(
+            fragment = this,
+            viewTreeObserver = sessionControlComponent.view.viewTreeObserver,
+            restoreLayoutState = {
+                val homeViewModel: HomeScreenViewModel by activityViewModels()
+                homeViewModel.layoutManagerState?.also { parcelable ->
+                    sessionControlComponent.view.layoutManager?.onRestoreInstanceState(parcelable)
+                }
+                homeLayout?.progress = homeViewModel.motionLayoutProgress
+                homeViewModel.layoutManagerState = null
+            }
+        )
 
         return view
     }
 
-    private fun restoreLayoutState() {
-        val homeViewModel: HomeScreenViewModel by activityViewModels()
-        homeViewModel.layoutManagerState?.also { parcelable ->
-            sessionControlComponent.view.layoutManager?.onRestoreInstanceState(parcelable)
-        }
-        homeLayout?.progress = homeViewModel.motionLayoutProgress
-        homeViewModel.layoutManagerState = null
-    }
-
     @SuppressWarnings("LongMethod")
     override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
         super.onViewCreated(view, savedInstanceState)
@@ -278,7 +264,6 @@ class HomeFragment : Fragment(), AccountObserver {
 
     override fun onDestroyView() {
         homeMenu = null
-        sessionControlComponent.view.viewTreeObserver.removeOnPreDrawListener(preDrawListener)
         super.onDestroyView()
     }
 

app/src/main/java/org/mozilla/fenix/home/TransitionPreDrawListener.kt

@@ -0,0 +1,52 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+package org.mozilla.fenix.home
+
+import android.view.ViewTreeObserver
+import androidx.fragment.app.Fragment
+import androidx.lifecycle.Lifecycle
+import androidx.lifecycle.LifecycleObserver
+import androidx.lifecycle.OnLifecycleEvent
+import androidx.lifecycle.lifecycleScope
+import kotlinx.coroutines.delay
+import kotlinx.coroutines.launch
+
+class TransitionPreDrawListener(
+    private val fragment: Fragment,
+    private val viewTreeObserver: ViewTreeObserver,
+    private val restoreLayoutState: () -> Unit
+) : ViewTreeObserver.OnPreDrawListener, LifecycleObserver {
+
+    init {
+        fragment.viewLifecycleOwner.lifecycle.addObserver(this)
+    }
+
+    @OnLifecycleEvent(Lifecycle.Event.ON_CREATE)
+    fun onCreateView() {
+        viewTreeObserver.addOnPreDrawListener(this)
+    }
+
+    @OnLifecycleEvent(Lifecycle.Event.ON_DESTROY)
+    fun onDestroyView() {
+        viewTreeObserver.removeOnPreDrawListener(this)
+    }
+
+    override fun onPreDraw(): Boolean {
+        if (fragment.view != null) {
+            fragment.viewLifecycleOwner.lifecycleScope.launch {
+                delay(ANIM_SCROLL_DELAY)
+                restoreLayoutState()
+                fragment.startPostponedEnterTransition()
+            }.invokeOnCompletion {
+                viewTreeObserver.removeOnPreDrawListener(this)
+            }
+        }
+        return true
+    }
+
+    companion object {
+        private const val ANIM_SCROLL_DELAY = 100L
+    }
+}

app/src/test/java/org/mozilla/fenix/home/TransitionPreDrawListenerTest.kt

@@ -0,0 +1,41 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+package org.mozilla.fenix.home
+
+import android.view.ViewTreeObserver
+import androidx.fragment.app.Fragment
+import io.mockk.mockk
+import io.mockk.verify
+import org.junit.Before
+import org.junit.Test
+
+class TransitionPreDrawListenerTest {
+
+    private lateinit var fragment: Fragment
+    private lateinit var viewTreeObserver: ViewTreeObserver
+
+    @Before
+    fun setup() {
+        fragment = mockk(relaxed = true)
+        viewTreeObserver = mockk(relaxed = true)
+    }
+
+    @Test
+    fun `adds observer when constructed`() {
+        val listener = TransitionPreDrawListener(fragment, viewTreeObserver) {}
+        verify { fragment.viewLifecycleOwner.lifecycle.addObserver(listener) }
+    }
+
+    @Test
+    fun `adds listener on create and removes on destroy`() {
+        val listener = TransitionPreDrawListener(fragment, viewTreeObserver) {}
+
+        listener.onCreateView()
+        verify { viewTreeObserver.addOnPreDrawListener(listener) }
+
+        listener.onDestroyView()
+        verify { viewTreeObserver.removeOnPreDrawListener(listener) }
+    }
+}