Resolve the memory leaks in demo short-form app

Issue: androidx/media#1839
#cherrypick
PiperOrigin-RevId: 696080063

RELEASENOTES.md

@@ -85,6 +85,8 @@
         Composable UI elements to `demo-compose` utilizing
         `PlayPauseButtonState`, `NextButtonState`, `PreviousButtonState`,
         `RepeatButtonState`, `ShuffleButtonState`.
+    *   Resolve the memory leaks in demo short-form app
+        ([#1839](https://github.com/androidx/media/issues/1839)).
 *   Remove deprecated symbols:
     *   Remove deprecated `AudioMixer.create()` method. Use
         `DefaultAudioMixer.Factory().create()` instead.

demos/shortform/src/main/java/androidx/media3/demo/shortform/viewpager/ViewPagerActivity.kt

@@ -25,7 +25,7 @@ import androidx.viewpager2.widget.ViewPager2
 
 class ViewPagerActivity : AppCompatActivity() {
   private lateinit var viewPagerView: ViewPager2
-  private lateinit var adapter: ViewPagerMediaAdapter
+  private lateinit var onPageChangeCallback: ViewPager2.OnPageChangeCallback
   private var numberOfPlayers = 3
   private var mediaItemDatabase = MediaItemDatabase()
 
@@ -40,23 +40,24 @@ class ViewPagerActivity : AppCompatActivity() {
     Log.d(TAG, "Using a pool of $numberOfPlayers players")
     viewPagerView = findViewById(R.id.viewPager)
     viewPagerView.offscreenPageLimit = 1
-    viewPagerView.registerOnPageChangeCallback(
+  }
+
+  override fun onStart() {
+    super.onStart()
+    val adapter = ViewPagerMediaAdapter(mediaItemDatabase, numberOfPlayers, applicationContext)
+    viewPagerView.adapter = adapter
+    onPageChangeCallback =
       object : ViewPager2.OnPageChangeCallback() {
         override fun onPageSelected(position: Int) {
           adapter.onPageSelected(position)
         }
       }
-    )
-  }
-
-  override fun onStart() {
-    super.onStart()
-    adapter = ViewPagerMediaAdapter(mediaItemDatabase, numberOfPlayers, this)
-    viewPagerView.adapter = adapter
+    viewPagerView.registerOnPageChangeCallback(onPageChangeCallback)
   }
 
   override fun onStop() {
-    adapter.onDestroy()
+    viewPagerView.unregisterOnPageChangeCallback(onPageChangeCallback)
+    viewPagerView.adapter = null
     super.onStop()
   }
 }

demos/shortform/src/main/java/androidx/media3/demo/shortform/viewpager/ViewPagerMediaAdapter.kt

@@ -43,7 +43,7 @@ class ViewPagerMediaAdapter(
   private val currentMediaItemsAndIndexes: ArrayDeque<Pair<MediaItem, Int>> = ArrayDeque()
   private var playerPool: PlayerPool
   private val holderMap: MutableMap<Int, ViewPagerMediaHolder>
-  private var currentPlayingIndex: Int = C.INDEX_UNSET
+  private val preloadControl: DefaultPreloadControl
 
   companion object {
     private const val TAG = "ViewPagerMediaAdapter"
@@ -65,8 +65,10 @@ class ViewPagerMediaAdapter(
         )
         .setPrioritizeTimeOverSizeThresholds(true)
         .build()
+    preloadControl = DefaultPreloadControl()
     val preloadManagerBuilder =
-      DefaultPreloadManager.Builder(context, DefaultPreloadControl()).setLoadControl(loadControl)
+      DefaultPreloadManager.Builder(context.applicationContext, preloadControl)
+        .setLoadControl(loadControl)
     playerPool = PlayerPool(numberOfPlayers, preloadManagerBuilder)
     holderMap = mutableMapOf()
     preloadManager = preloadManagerBuilder.build()
@@ -76,6 +78,13 @@ class ViewPagerMediaAdapter(
     preloadManager.invalidate()
   }
 
+  override fun onDetachedFromRecyclerView(recyclerView: RecyclerView) {
+    playerPool.destroyPlayers()
+    preloadManager.release()
+    holderMap.clear()
+    super.onDetachedFromRecyclerView(recyclerView)
+  }
+
   override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): ViewPagerMediaHolder {
     val view =
       LayoutInflater.from(parent.context).inflate(R.layout.media_item_view_pager, parent, false)
@@ -128,14 +137,9 @@ class ViewPagerMediaAdapter(
     return Int.MAX_VALUE
   }
 
-  fun onDestroy() {
-    playerPool.destroyPlayers()
-    preloadManager.release()
-  }
-
   fun onPageSelected(position: Int) {
-    currentPlayingIndex = position
     holderMap[position]?.playIfPossible()
+    preloadControl.currentPlayingIndex = position
     preloadManager.setCurrentPlayingIndex(position)
     preloadManager.invalidate()
   }
@@ -168,7 +172,9 @@ class ViewPagerMediaAdapter(
     preloadManager.remove(itemAndIndex.first)
   }
 
-  inner class DefaultPreloadControl : TargetPreloadStatusControl<Int> {
+  inner class DefaultPreloadControl(var currentPlayingIndex: Int = C.INDEX_UNSET) :
+    TargetPreloadStatusControl<Int> {
+
     override fun getTargetPreloadStatus(rankingData: Int): DefaultPreloadManager.Status? {
       if (abs(rankingData - currentPlayingIndex) == 2) {
         return DefaultPreloadManager.Status(STAGE_LOADED_FOR_DURATION_MS, 500L)