Use more targeted listening in session PlayerActivity

The current metadata updates are triggered by item transitions,
but depending on the speed of loading the playlist, the first
metadata may only be known later via metadata-change callbacks.

Slow playlist loading also means the UI stays empty and it's
beneficial to show a placeholder to avoid the impressions the
UI hangs.

Finally, clean-up by removing unused string constants and merging
all listeners into onEvents

#minor-release

PiperOrigin-RevId: 571951529

demos/session/src/main/java/androidx/media3/demo/session/PlayerActivity.kt

@@ -30,11 +30,11 @@ import androidx.appcompat.app.AppCompatActivity
 import androidx.core.content.ContextCompat
 import androidx.media3.common.C.TRACK_TYPE_TEXT
 import androidx.media3.common.MediaItem
-import androidx.media3.common.MediaMetadata
 import androidx.media3.common.Player
 import androidx.media3.common.Player.EVENT_MEDIA_ITEM_TRANSITION
+import androidx.media3.common.Player.EVENT_MEDIA_METADATA_CHANGED
+import androidx.media3.common.Player.EVENT_TIMELINE_CHANGED
 import androidx.media3.common.Player.EVENT_TRACKS_CHANGED
-import androidx.media3.common.Timeline
 import androidx.media3.common.util.UnstableApi
 import androidx.media3.session.MediaController
 import androidx.media3.session.SessionToken
@@ -96,6 +96,7 @@ class PlayerActivity : AppCompatActivity() {
           SessionToken(this, ComponentName(this, PlaybackService::class.java))
         )
         .buildAsync()
+    updateMediaMetadataUI()
     controllerFuture.addListener({ setController() }, MoreExecutors.directExecutor())
   }
 
@@ -110,37 +111,43 @@ class PlayerActivity : AppCompatActivity() {
     playerView.player = controller
 
     updateCurrentPlaylistUI()
-    updateMediaMetadataUI(controller.mediaMetadata)
+    updateMediaMetadataUI()
     playerView.setShowSubtitleButton(controller.currentTracks.isTypeSupported(TRACK_TYPE_TEXT))
 
     controller.addListener(
       object : Player.Listener {
-        override fun onTimelineChanged(timeline: Timeline, reason: Int) {
-          if (reason == Player.TIMELINE_CHANGE_REASON_PLAYLIST_CHANGED) {
-            updateCurrentPlaylistUI()
-          }
-        }
-
         override fun onEvents(player: Player, events: Player.Events) {
-          if (events.contains(EVENT_MEDIA_ITEM_TRANSITION)) {
-            updateMediaMetadataUI(player.currentMediaItem?.mediaMetadata ?: MediaMetadata.EMPTY)
-          }
           if (events.contains(EVENT_TRACKS_CHANGED)) {
             playerView.setShowSubtitleButton(player.currentTracks.isTypeSupported(TRACK_TYPE_TEXT))
           }
+          if (events.contains(EVENT_TIMELINE_CHANGED)) {
+            updateCurrentPlaylistUI()
+          }
+          if (events.contains(EVENT_MEDIA_METADATA_CHANGED)) {
+            updateMediaMetadataUI()
+          }
+          if (events.contains(EVENT_MEDIA_ITEM_TRANSITION)) {
+            // Trigger adapter update to change highlight of current item.
+            mediaItemListAdapter.notifyDataSetChanged()
+          }
         }
       }
     )
   }
 
-  private fun updateMediaMetadataUI(mediaMetadata: MediaMetadata) {
+  private fun updateMediaMetadataUI() {
+    val controller = this.controller
+    if (controller == null || controller.mediaItemCount == 0) {
+      findViewById<TextView>(R.id.media_title).text = getString(R.string.waiting_for_metadata)
+      findViewById<TextView>(R.id.media_artist).text = ""
+      return
+    }
+
+    val mediaMetadata = controller.mediaMetadata
     val title: CharSequence = mediaMetadata.title ?: ""
 
     findViewById<TextView>(R.id.media_title).text = title
     findViewById<TextView>(R.id.media_artist).text = mediaMetadata.artist
-
-    // Trick to update playlist UI
-    mediaItemListAdapter.notifyDataSetChanged()
   }
 
   private fun updateCurrentPlaylistUI() {

demos/session/src/main/res/values/strings.xml

@@ -19,9 +19,6 @@
   <string name="open_player_content_description">Click to view your play list</string>
   <string name="added_media_item_format">Added %1$s to playlist</string>
   <string name="shuffle">Shuffle</string>
-  <string name="repeat">Repeat</string>
   <string name="play_button">Play</string>
-  <string name="no_item_prompt">
-    "! No media in the play list !\nPlease try to add more from browser"
-  </string>
+  <string name="waiting_for_metadata">Waiting for playlist to load…</string>
 </resources>