-Re-added loading for items prior to current index in MediaSourceManager to allow faster access time.

-Added some null checks annotation.

app/src/main/java/org/schabi/newpipe/player/playback/MediaSourceManager.java

@@ -326,8 +326,10 @@ public class MediaSourceManager {
         maybeLoadItem(currentItem);
 
         // The rest are just for seamless playback
-        final int leftBound = currentIndex + 1;
+        // Although timeline is not updated prior to the current index, these sources are still
-        final int rightLimit = leftBound + WINDOW_SIZE;
+        // loaded into the cache for faster retrieval at a potentially later time.
+        final int leftBound = Math.max(0, currentIndex - WINDOW_SIZE);
+        final int rightLimit = currentIndex + WINDOW_SIZE + 1;
         final int rightBound = Math.min(playQueue.size(), rightLimit);
         final List<PlayQueueItem> items = new ArrayList<>(
                 playQueue.getStreams().subList(leftBound,rightBound));
@@ -343,10 +345,9 @@ public class MediaSourceManager {
         }
     }
 
-    private void maybeLoadItem(@Nullable final PlayQueueItem item) {
+    private void maybeLoadItem(@NonNull final PlayQueueItem item) {
         if (DEBUG) Log.d(TAG, "maybeLoadItem() called.");
-
+        if (sources == null) return;
-        if (sources == null || item == null) return;
 
         final int index = playQueue.indexOf(item);
         if (index > sources.getSize() - 1) return;
@@ -355,7 +356,11 @@ public class MediaSourceManager {
             if (DEBUG) Log.d(TAG, " Loaded: [" + item.getTitle() +
                     "] with url: " + item.getUrl());
 
-            if (isCorrectionNeeded(item)) update(playQueue.indexOf(item), mediaSource);
+            final int itemIndex = playQueue.indexOf(item);
+            // Only update the playlist timeline for items at the current index or after.
+            if (itemIndex >= playQueue.getIndex() && isCorrectionNeeded(item)) {
+                update(itemIndex, mediaSource);
+            }
 
             loadingItems.remove(item);
             tryUnblock();
@@ -449,7 +454,7 @@ public class MediaSourceManager {
      * with position * in respect to the play queue only if no {@link MediaSource}
      * already exists at the given index.
      * */
-    private void emplace(final int index, final MediaSource source) {
+    private void emplace(final int index, @NonNull final MediaSource source) {
         if (sources == null) return;
         if (index < 0 || index < sources.getSize()) return;
 
@@ -489,7 +494,7 @@ public class MediaSourceManager {
      * this will modify the playback timeline prior to the index and cause desynchronization
      * on the playing item between {@link PlayQueue} and {@link DynamicConcatenatingMediaSource}.
      * */
-    private synchronized void update(final int index, final MediaSource source) {
+    private synchronized void update(final int index, @NonNull final MediaSource source) {
         if (sources == null) return;
         if (index < 0 || index >= sources.getSize()) return;
 

app/src/main/java/org/schabi/newpipe/playlist/PlayQueue.java

@@ -45,7 +45,7 @@ public abstract class PlayQueue implements Serializable {
 
     private ArrayList<PlayQueueItem> backup;
     private ArrayList<PlayQueueItem> streams;
-    private final AtomicInteger queueIndex;
+    @NonNull private final AtomicInteger queueIndex;
 
     private transient BehaviorSubject<PlayQueueEvent> eventBroadcast;
     private transient Flowable<PlayQueueEvent> broadcastReceiver;
@@ -133,7 +133,7 @@ public abstract class PlayQueue implements Serializable {
      * Returns the index of the given item using referential equality.
      * May be null despite play queue contains identical item.
      * */
-    public int indexOf(final PlayQueueItem item) {
+    public int indexOf(@NonNull final PlayQueueItem item) {
         // referential equality, can't think of a better way to do this
         // todo: better than this
         return streams.indexOf(item);
@@ -213,7 +213,7 @@ public abstract class PlayQueue implements Serializable {
      *
      * @see #append(List items)
      * */
-    public synchronized void append(final PlayQueueItem... items) {
+    public synchronized void append(@NonNull final PlayQueueItem... items) {
         append(Arrays.asList(items));
     }
 
@@ -225,7 +225,7 @@ public abstract class PlayQueue implements Serializable {
      *
      * Will emit a {@link AppendEvent} on any given context.
      * */
-    public synchronized void append(final List<PlayQueueItem> items) {
+    public synchronized void append(@NonNull final List<PlayQueueItem> items) {
         List<PlayQueueItem> itemList = new ArrayList<>(items);
 
         if (isShuffled()) {
@@ -393,7 +393,7 @@ public abstract class PlayQueue implements Serializable {
     // Rx Broadcast
     //////////////////////////////////////////////////////////////////////////*/
 
-    private void broadcast(final PlayQueueEvent event) {
+    private void broadcast(@NonNull final PlayQueueEvent event) {
         if (eventBroadcast != null) {
             eventBroadcast.onNext(event);
         }

app/src/main/java/org/schabi/newpipe/util/NavigationHelper.java

@@ -61,6 +61,7 @@ public class NavigationHelper {
     // Players
     //////////////////////////////////////////////////////////////////////////*/
 
+    @NonNull
     public static Intent getPlayerIntent(@NonNull final Context context,
                                          @NonNull final Class targetClazz,
                                          @NonNull final PlayQueue playQueue,
@@ -74,12 +75,14 @@ public class NavigationHelper {
         return intent;
     }
 
+    @NonNull
     public static Intent getPlayerIntent(@NonNull final Context context,
                                          @NonNull final Class targetClazz,
                                          @NonNull final PlayQueue playQueue) {
         return getPlayerIntent(context, targetClazz, playQueue, null);
     }
 
+    @NonNull
     public static Intent getPlayerEnqueueIntent(@NonNull final Context context,
                                                 @NonNull final Class targetClazz,
                                                 @NonNull final PlayQueue playQueue,
@@ -89,6 +92,7 @@ public class NavigationHelper {
                 .putExtra(BasePlayer.SELECT_ON_APPEND, selectOnAppend);
     }
 
+    @NonNull
     public static Intent getPlayerIntent(@NonNull final Context context,
                                          @NonNull final Class targetClazz,
                                          @NonNull final PlayQueue playQueue,