Event based preparation.

- Removes the load delay that was previously present after
  source preparation.
- Prevents premature failure in the case that the buffering
  source fails to prepare.
- SampleSource.Callback will get a second method in a
  subsequent CL, approximately meaning "tell me if I can load
  more stuff". This will remove the need for LoadControl and
  the complexity that comes with it.
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=126172573

library/src/main/java/com/google/android/exoplayer/MultiSampleSource.java

@@ -16,7 +16,6 @@
 package com.google.android.exoplayer;
 
 import com.google.android.exoplayer.BufferingPolicy.LoadControl;
-import com.google.android.exoplayer.util.Assertions;
 
 import android.util.Pair;
 
@@ -28,57 +27,40 @@ import java.util.List;
 /**
  * Combines multiple {@link SampleSource} instances.
  */
-public final class MultiSampleSource implements SampleSource {
+public final class MultiSampleSource implements SampleSource, SampleSource.Callback {
 
   private final SampleSource[] sources;
   private final IdentityHashMap<TrackStream, SampleSource> trackStreamSources;
   private final int[] selectedTrackCounts;
 
-  private boolean prepared;
-  private boolean seenFirstTrackSelection;
+  private Callback callback;
+  private int pendingChildPrepareCount;
   private long durationUs;
   private TrackGroupArray trackGroups;
+
+  private boolean seenFirstTrackSelection;
   private SampleSource[] enabledSources;
 
   public MultiSampleSource(SampleSource... sources) {
     this.sources = sources;
+    pendingChildPrepareCount = sources.length;
     trackStreamSources = new IdentityHashMap<>();
     selectedTrackCounts = new int[sources.length];
   }
 
   @Override
-  public boolean prepare(long positionUs, LoadControl loadControl) throws IOException {
-    if (prepared) {
-      return true;
-    }
-    boolean sourcesPrepared = true;
+  public void prepare(Callback callback, LoadControl loadControl, long positionUs) {
+    this.callback = callback;
     for (SampleSource source : sources) {
-      sourcesPrepared &= source.prepare(positionUs, loadControl);
+      source.prepare(this, loadControl, positionUs);
     }
-    if (!sourcesPrepared) {
-      return false;
-    }
-    durationUs = 0;
-    int totalTrackGroupCount = 0;
-    for (SampleSource source : sources) {
-      totalTrackGroupCount += source.getTrackGroups().length;
-      if (durationUs != C.UNSET_TIME_US) {
-        long sourceDurationUs = source.getDurationUs();
-        durationUs = sourceDurationUs == C.UNSET_TIME_US
-            ? C.UNSET_TIME_US : Math.max(durationUs, sourceDurationUs);
-      }
-    }
-    TrackGroup[] trackGroupArray = new TrackGroup[totalTrackGroupCount];
-    int trackGroupIndex = 0;
+  }
+
+  @Override
+  public void maybeThrowPrepareError() throws IOException {
     for (SampleSource source : sources) {
-      int sourceTrackGroupCount = source.getTrackGroups().length;
-      for (int j = 0; j < sourceTrackGroupCount; j++) {
-        trackGroupArray[trackGroupIndex++] = source.getTrackGroups().get(j);
-      }
+      source.maybeThrowPrepareError();
     }
-    trackGroups = new TrackGroupArray(trackGroupArray);
-    prepared = true;
-    return true;
   }
 
   @Override
@@ -94,13 +76,12 @@ public final class MultiSampleSource implements SampleSource {
   @Override
   public TrackStream[] selectTracks(List<TrackStream> oldStreams,
       List<TrackSelection> newSelections, long positionUs) {
-    Assertions.checkState(prepared);
     TrackStream[] newStreams = new TrackStream[newSelections.size()];
     // Select tracks for each source.
     int enabledSourceCount = 0;
     for (int i = 0; i < sources.length; i++) {
       selectedTrackCounts[i] += selectTracks(sources[i], oldStreams, newSelections, positionUs,
-          newStreams);
+          newStreams, seenFirstTrackSelection);
       if (selectedTrackCounts[i] > 0) {
         enabledSourceCount++;
       }
@@ -166,10 +147,40 @@ public final class MultiSampleSource implements SampleSource {
     }
   }
 
+  // SampleSource.Callback implementation
+
+  @Override
+  public void onSourcePrepared(SampleSource ignored) {
+    if (--pendingChildPrepareCount > 0) {
+      return;
+    }
+    durationUs = 0;
+    int totalTrackGroupCount = 0;
+    for (SampleSource source : sources) {
+      totalTrackGroupCount += source.getTrackGroups().length;
+      if (durationUs != C.UNSET_TIME_US) {
+        long sourceDurationUs = source.getDurationUs();
+        durationUs = sourceDurationUs == C.UNSET_TIME_US
+            ? C.UNSET_TIME_US : Math.max(durationUs, sourceDurationUs);
+      }
+    }
+    TrackGroup[] trackGroupArray = new TrackGroup[totalTrackGroupCount];
+    int trackGroupIndex = 0;
+    for (SampleSource source : sources) {
+      int sourceTrackGroupCount = source.getTrackGroups().length;
+      for (int j = 0; j < sourceTrackGroupCount; j++) {
+        trackGroupArray[trackGroupIndex++] = source.getTrackGroups().get(j);
+      }
+    }
+    trackGroups = new TrackGroupArray(trackGroupArray);
+    callback.onSourcePrepared(this);
+  }
+
   // Internal methods.
 
   private int selectTracks(SampleSource source, List<TrackStream> allOldStreams,
-      List<TrackSelection> allNewSelections, long positionUs, TrackStream[] allNewStreams) {
+      List<TrackSelection> allNewSelections, long positionUs, TrackStream[] allNewStreams,
+      boolean seenFirstTrackSelection) {
     // Get the subset of the old streams for the source.
     ArrayList<TrackStream> oldStreams = new ArrayList<>();
     for (int i = 0; i < allOldStreams.size(); i++) {

library/src/main/java/com/google/android/exoplayer/SampleSource.java

@@ -26,18 +26,45 @@ import java.util.List;
 public interface SampleSource {
 
   /**
-   * Prepares the source, or does nothing if the source is already prepared.
+   * A callback to be notified of {@link SampleSource} events.
+   */
+  interface Callback {
+
+    /**
+     * Invoked by the source when preparation completes.
+     * <p>
+     * May be called from any thread. After invoking this method, the source can expect
+     * {@link #selectTracks(List, List, long)} to be invoked when the initial track selection.
+     *
+     * @param source The prepared source.
+     */
+    void onSourcePrepared(SampleSource source);
+
+  }
+
+  /**
+   * Starts preparation of the source.
    * <p>
-   * {@link #selectTracks(List, List, long)} <b>must</b> be called after the source is prepared to
-   * make an initial track selection. This is true even if the caller does not wish to select any
-   * tracks.
+   * {@link Callback#onSourcePrepared(SampleSource)} is invoked when preparation completes. If
+   * preparation fails, {@link #maybeThrowPrepareError()} will throw an {@link IOException} if
+   * invoked.
    *
-   * @param positionUs The player's current playback position.
+   * @param callback A callback to receive updates from the source.
    * @param loadControl A {@link LoadControl} to determine when to load data.
+   * @param positionUs The player's current playback position.
    * @return True if the source is prepared, false otherwise.
-   * @throws IOException If there's an error preparing the source.
    */
-  boolean prepare(long positionUs, LoadControl loadControl) throws IOException;
+  void prepare(Callback callback, LoadControl loadControl, long positionUs);
+
+  /**
+   * Throws an error that's preventing the source from becoming prepared. Does nothing if no such
+   * error exists.
+   * <p>
+   * This method should only be called before the source has completed preparation.
+   *
+   * @throws IOException The underlying error.
+   */
+  void maybeThrowPrepareError() throws IOException;
 
   /**
    * Returns the duration of the source in microseconds, or {@link C#UNSET_TIME_US} if not known.

library/src/main/java/com/google/android/exoplayer/SingleSampleSource.java

@@ -109,9 +109,13 @@ public final class SingleSampleSource implements SampleSource, TrackStream,
   // SampleSource implementation.
 
   @Override
-  public boolean prepare(long positionUs, LoadControl loadControl) {
-    // TODO: Use the load control.
-    return true;
+  public void prepare(Callback callback, LoadControl loadControl, long positionUs) {
+    callback.onSourcePrepared(this);
+  }
+
+  @Override
+  public void maybeThrowPrepareError() throws IOException {
+    // Do nothing.
   }
 
   @Override

library/src/main/java/com/google/android/exoplayer/TrackRenderer.java

@@ -112,6 +112,10 @@ public abstract class TrackRenderer implements ExoPlayerComponent {
   private boolean readEndOfStream;
   private boolean streamIsFinal;
 
+  public TrackRenderer() {
+    readEndOfStream = true;
+  }
+
   /**
    * Sets the index of this renderer within the player.
    *

library/src/main/java/com/google/android/exoplayer/chunk/ChunkTrackStream.java

@@ -70,8 +70,7 @@ public class ChunkTrackStream<T extends ChunkSource> implements TrackStream,
    * @param eventDispatcher A dispatcher to notify of events.
    */
   public ChunkTrackStream(int trackType, T chunkSource, LoadControl loadControl, long positionUs,
-      int minLoadableRetryCount,
-      EventDispatcher eventDispatcher) {
+      int minLoadableRetryCount, EventDispatcher eventDispatcher) {
     this.trackType = trackType;
     this.chunkSource = chunkSource;
     this.loadControl = loadControl;

library/src/main/java/com/google/android/exoplayer/dash/DashSampleSource.java

@@ -39,7 +39,6 @@ import com.google.android.exoplayer.upstream.BandwidthMeter;
 import com.google.android.exoplayer.upstream.DataSource;
 import com.google.android.exoplayer.upstream.DataSourceFactory;
 import com.google.android.exoplayer.upstream.Loader;
-import com.google.android.exoplayer.upstream.Loader.Callback;
 import com.google.android.exoplayer.upstream.ParsingLoadable;
 import com.google.android.exoplayer.util.Util;
 
@@ -84,6 +83,7 @@ public final class DashSampleSource implements SampleSource {
   private long manifestLoadEndTimestamp;
   private MediaPresentationDescription manifest;
 
+  private Callback callback;
   private LoadControl loadControl;
   private boolean prepared;
   private long durationUs;
@@ -107,16 +107,15 @@ public final class DashSampleSource implements SampleSource {
   }
 
   @Override
-  public boolean prepare(long positionUs, LoadControl loadControl) throws IOException {
-    if (prepared) {
-      return true;
-    }
+  public void prepare(Callback callback, LoadControl loadControl, long positionUs) {
+    this.callback = callback;
     this.loadControl = loadControl;
+    startLoadingManifest();
+  }
+
+  @Override
+  public void maybeThrowPrepareError() throws IOException {
     loader.maybeThrowError();
-    if (!loader.isLoading() && manifest == null) {
-      startLoadingManifest();
-    }
-    return false;
   }
 
   @Override
@@ -231,6 +230,7 @@ public final class DashSampleSource implements SampleSource {
         resolveUtcTimingElement(manifest.utcTiming);
       } else {
         prepared = true;
+        callback.onSourcePrepared(this);
       }
     } else {
       for (ChunkTrackStream<DashChunkSource> trackStream : trackStreams) {
@@ -308,16 +308,18 @@ public final class DashSampleSource implements SampleSource {
   private void onUtcTimestampResolved(long elapsedRealtimeOffsetMs) {
     this.elapsedRealtimeOffset = elapsedRealtimeOffsetMs;
     prepared = true;
+    callback.onSourcePrepared(this);
   }
 
   private void onUtcTimestampResolutionError(IOException error) {
     Log.e(TAG, "Failed to resolve UtcTiming element.", error);
     // Be optimistic and continue in the hope that the device clock is correct.
     prepared = true;
+    callback.onSourcePrepared(this);
   }
 
-  private <T> void startLoading(ParsingLoadable<T> loadable, Callback<ParsingLoadable<T>> callback,
-      int minRetryCount) {
+  private <T> void startLoading(ParsingLoadable<T> loadable,
+      Loader.Callback<ParsingLoadable<T>> callback, int minRetryCount) {
     long elapsedRealtimeMs = loader.startLoading(loadable, callback, minRetryCount);
     eventDispatcher.loadStarted(loadable.dataSpec, loadable.type, elapsedRealtimeMs);
   }

library/src/main/java/com/google/android/exoplayer/extractor/DefaultTrackOutput.java

@@ -38,6 +38,20 @@ import java.util.concurrent.atomic.AtomicInteger;
  */
 public final class DefaultTrackOutput implements TrackOutput {
 
+  /**
+   * A listener for changes to the upstream format.
+   */
+  public interface UpstreamFormatChangedListener {
+
+    /**
+     * Invoked on the loading thread when an upstream format change occurs.
+     *
+     * @param format The new upstream format.
+     */
+    void onUpstreamFormatChanged(Format format);
+
+  }
+
   private static final int INITIAL_SCRATCH_SIZE = 32;
 
   private static final int STATE_ENABLED = 0;
@@ -64,6 +78,7 @@ public final class DefaultTrackOutput implements TrackOutput {
   private int lastAllocationOffset;
   private boolean needKeyframe;
   private boolean pendingSplice;
+  private UpstreamFormatChangedListener upstreamFormatChangeListener;
 
   /**
    * @param allocator An {@link Allocator} from which allocations for sample data can be obtained.
@@ -392,6 +407,15 @@ public final class DefaultTrackOutput implements TrackOutput {
   // Called by the loading thread.
 
   /**
+   * Sets a listener to be notified of changes to the upstream format.
+   *
+   * @param listener The listener.
+   */
+  public void setUpstreamFormatChangeListener(UpstreamFormatChangedListener listener) {
+    upstreamFormatChangeListener = listener;
+  }
+
+  /**
    * Like {@link #format(Format)}, but with an offset that will be added to the timestamps of
    * samples subsequently queued to the buffer. The offset is also used to adjust
    * {@link Format#subsampleOffsetUs} for both the {@link Format} passed and those subsequently
@@ -407,7 +431,11 @@ public final class DefaultTrackOutput implements TrackOutput {
 
   @Override
   public void format(Format format) {
-    infoQueue.format(getAdjustedSampleFormat(format, sampleOffsetUs));
+    Format adjustedFormat = getAdjustedSampleFormat(format, sampleOffsetUs);
+    infoQueue.format(adjustedFormat);
+    if (upstreamFormatChangeListener != null) {
+      upstreamFormatChangeListener.onUpstreamFormatChanged(adjustedFormat);
+    }
   }
 
   @Override

library/src/main/java/com/google/android/exoplayer/extractor/ExtractorSampleSource.java

@@ -18,6 +18,7 @@ package com.google.android.exoplayer.extractor;
 import com.google.android.exoplayer.BufferingPolicy.LoadControl;
 import com.google.android.exoplayer.C;
 import com.google.android.exoplayer.DecoderInputBuffer;
+import com.google.android.exoplayer.Format;
 import com.google.android.exoplayer.FormatHolder;
 import com.google.android.exoplayer.ParserException;
 import com.google.android.exoplayer.SampleSource;
@@ -25,6 +26,7 @@ import com.google.android.exoplayer.TrackGroup;
 import com.google.android.exoplayer.TrackGroupArray;
 import com.google.android.exoplayer.TrackSelection;
 import com.google.android.exoplayer.TrackStream;
+import com.google.android.exoplayer.extractor.DefaultTrackOutput.UpstreamFormatChangedListener;
 import com.google.android.exoplayer.upstream.BandwidthMeter;
 import com.google.android.exoplayer.upstream.DataSource;
 import com.google.android.exoplayer.upstream.DataSourceFactory;
@@ -71,7 +73,8 @@ import java.util.List;
  * from {@link Extractor#sniff(ExtractorInput)} will be used.
  */
 public final class ExtractorSampleSource implements SampleSource, ExtractorOutput,
-    Loader.Callback<ExtractorSampleSource.ExtractingLoadable> {
+    Loader.Callback<ExtractorSampleSource.ExtractingLoadable>,
+    UpstreamFormatChangedListener {
 
   /**
    * Interface definition for a callback to be notified of {@link ExtractorSampleSource} events.
@@ -129,11 +132,12 @@ public final class ExtractorSampleSource implements SampleSource, ExtractorOutpu
   private final Loader loader;
   private final ExtractorHolder extractorHolder;
 
-  private volatile boolean tracksBuilt;
-  private volatile SeekMap seekMap;
-
+  private Callback callback;
   private LoadControl loadControl;
+  private SeekMap seekMap;
+  private boolean tracksBuilt;
   private boolean prepared;
+
   private boolean seenFirstTrackSelection;
   private boolean notifyReset;
   private int enabledTrackCount;
@@ -304,31 +308,16 @@ public final class ExtractorSampleSource implements SampleSource, ExtractorOutpu
   // SampleSource implementation.
 
   @Override
-  public boolean prepare(long positionUs, LoadControl loadControl) throws IOException {
-    if (prepared) {
-      return true;
-    }
+  public void prepare(Callback callback, LoadControl loadControl, long positionUs) {
+    this.callback = callback;
     this.loadControl = loadControl;
-    if (seekMap != null && tracksBuilt && haveFormatsForAllTracks()) {
-      loadCondition.close();
-      int trackCount = sampleQueues.length;
-      TrackGroup[] trackArray = new TrackGroup[trackCount];
-      trackEnabledStates = new boolean[trackCount];
-      durationUs = seekMap.getDurationUs();
-      for (int i = 0; i < trackCount; i++) {
-        trackArray[i] = new TrackGroup(sampleQueues[i].getUpstreamFormat());
-      }
-      tracks = new TrackGroupArray(trackArray);
-      prepared = true;
-      return true;
-    }
-    // We're not prepared.
+    loadCondition.open();
+    startLoading();
+  }
+
+  @Override
+  public void maybeThrowPrepareError() throws IOException {
     maybeThrowError();
-    if (!loader.isLoading()) {
-      loadCondition.open();
-      startLoading();
-    }
-    return false;
   }
 
   @Override
@@ -521,6 +510,7 @@ public final class ExtractorSampleSource implements SampleSource, ExtractorOutpu
   public TrackOutput track(int id) {
     sampleQueues = Arrays.copyOf(sampleQueues, sampleQueues.length + 1);
     DefaultTrackOutput sampleQueue = new DefaultTrackOutput(loadControl.getAllocator());
+    sampleQueue.setUpstreamFormatChangeListener(this);
     sampleQueues[sampleQueues.length - 1] = sampleQueue;
     return sampleQueue;
   }
@@ -528,15 +518,46 @@ public final class ExtractorSampleSource implements SampleSource, ExtractorOutpu
   @Override
   public void endTracks() {
     tracksBuilt = true;
+    maybeFinishPrepare();
   }
 
   @Override
   public void seekMap(SeekMap seekMap) {
     this.seekMap = seekMap;
+    maybeFinishPrepare();
+  }
+
+  // UpstreamFormatChangedListener implementation
+
+  @Override
+  public void onUpstreamFormatChanged(Format format) {
+    maybeFinishPrepare();
   }
 
   // Internal methods.
 
+  private void maybeFinishPrepare() {
+    if (seekMap == null || !tracksBuilt) {
+      return;
+    }
+    for (DefaultTrackOutput sampleQueue : sampleQueues) {
+      if (sampleQueue.getUpstreamFormat() == null) {
+        return;
+      }
+    }
+    loadCondition.close();
+    int trackCount = sampleQueues.length;
+    TrackGroup[] trackArray = new TrackGroup[trackCount];
+    trackEnabledStates = new boolean[trackCount];
+    durationUs = seekMap.getDurationUs();
+    for (int i = 0; i < trackCount; i++) {
+      trackArray[i] = new TrackGroup(sampleQueues[i].getUpstreamFormat());
+    }
+    tracks = new TrackGroupArray(trackArray);
+    prepared = true;
+    callback.onSourcePrepared(this);
+  }
+
   private void copyLengthFromLoader(ExtractingLoadable loadable) {
     if (length == C.LENGTH_UNBOUNDED) {
       length = loadable.length;
@@ -618,15 +639,6 @@ public final class ExtractorSampleSource implements SampleSource, ExtractorOutpu
     return largestQueuedTimestampUs;
   }
 
-  private boolean haveFormatsForAllTracks() {
-    for (DefaultTrackOutput sampleQueue : sampleQueues) {
-      if (sampleQueue.getUpstreamFormat() == null) {
-        return false;
-      }
-    }
-    return true;
-  }
-
   private boolean isPendingReset() {
     return pendingResetPositionUs != C.UNSET_TIME_US;
   }

library/src/main/java/com/google/android/exoplayer/hls/HlsSampleSource.java

@@ -53,7 +53,7 @@ import java.util.List;
  * A {@link SampleSource} for HLS streams.
  */
 public final class HlsSampleSource implements SampleSource,
-    Loader.Callback<ParsingLoadable<HlsPlaylist>> {
+    Loader.Callback<ParsingLoadable<HlsPlaylist>>, HlsTrackStreamWrapper.Callback {
 
   /**
    * The minimum number of times to retry loading data prior to failing.
@@ -70,7 +70,11 @@ public final class HlsSampleSource implements SampleSource,
   private final DataSource manifestDataSource;
   private final HlsPlaylistParser manifestParser;
 
+  private Callback callback;
   private LoadControl loadControl;
+  private long preparePositionUs;
+  private int pendingPrepareCount;
+
   private boolean seenFirstTrackSelection;
   private long durationUs;
   private boolean isLive;
@@ -96,50 +100,26 @@ public final class HlsSampleSource implements SampleSource,
   }
 
   @Override
-  public boolean prepare(long positionUs, LoadControl loadControl) throws IOException {
-    if (trackGroups != null) {
-      return true;
-    }
-
+  public void prepare(Callback callback, LoadControl loadControl, long positionUs) {
+    this.callback = callback;
     this.loadControl = loadControl;
+    this.preparePositionUs = positionUs;
+    ParsingLoadable<HlsPlaylist> loadable = new ParsingLoadable<>(manifestDataSource,
+        manifestUri, C.DATA_TYPE_MANIFEST, manifestParser);
+    long elapsedRealtimeMs = manifestFetcher.startLoading(loadable, this,
+        MIN_LOADABLE_RETRY_COUNT);
+    eventDispatcher.loadStarted(loadable.dataSpec, loadable.type, elapsedRealtimeMs);
+  }
+
+  @Override
+  public void maybeThrowPrepareError() throws IOException {
     if (trackStreamWrappers == null) {
       manifestFetcher.maybeThrowError();
-      if (!manifestFetcher.isLoading()) {
-        ParsingLoadable<HlsPlaylist> loadable = new ParsingLoadable<>(manifestDataSource,
-            manifestUri, C.DATA_TYPE_MANIFEST, manifestParser);
-        long elapsedRealtimeMs = manifestFetcher.startLoading(loadable, this,
-            MIN_LOADABLE_RETRY_COUNT);
-        eventDispatcher.loadStarted(loadable.dataSpec, loadable.type, elapsedRealtimeMs);
-      }
-      return false;
-    }
-
-    boolean trackStreamWrappersPrepared = true;
-    for (HlsTrackStreamWrapper trackStreamWrapper : trackStreamWrappers) {
-      trackStreamWrappersPrepared &= trackStreamWrapper.prepare(positionUs);
-    }
-    if (!trackStreamWrappersPrepared) {
-      return false;
-    }
-
-    // The wrapper at index 0 is the one of type TRACK_TYPE_DEFAULT.
-    durationUs = trackStreamWrappers[0].getDurationUs();
-    isLive = trackStreamWrappers[0].isLive();
-
-    int totalTrackGroupCount = 0;
-    for (HlsTrackStreamWrapper trackStreamWrapper : trackStreamWrappers) {
-      totalTrackGroupCount += trackStreamWrapper.getTrackGroups().length;
-    }
-    TrackGroup[] trackGroupArray = new TrackGroup[totalTrackGroupCount];
-    int trackGroupIndex = 0;
-    for (HlsTrackStreamWrapper trackStreamWrapper : trackStreamWrappers) {
-      int wrapperTrackGroupCount = trackStreamWrapper.getTrackGroups().length;
-      for (int j = 0; j < wrapperTrackGroupCount; j++) {
-        trackGroupArray[trackGroupIndex++] = trackStreamWrapper.getTrackGroups().get(j);
+    } else {
+      for (HlsTrackStreamWrapper trackStreamWrapper : trackStreamWrappers) {
+        trackStreamWrapper.maybeThrowPrepareError();
       }
     }
-    trackGroups = new TrackGroupArray(trackGroupArray);
-    return true;
   }
 
   @Override
@@ -237,6 +217,10 @@ public final class HlsSampleSource implements SampleSource,
     trackStreamWrappers = new HlsTrackStreamWrapper[trackStreamWrapperList.size()];
     trackStreamWrapperList.toArray(trackStreamWrappers);
     selectedTrackCounts = new int[trackStreamWrappers.length];
+    pendingPrepareCount = trackStreamWrappers.length;
+    for (HlsTrackStreamWrapper trackStreamWrapper : trackStreamWrappers) {
+      trackStreamWrapper.prepare();
+    }
   }
 
   @Override
@@ -255,6 +239,34 @@ public final class HlsSampleSource implements SampleSource,
     return isFatal ? Loader.DONT_RETRY_FATAL : Loader.RETRY;
   }
 
+  // HlsTrackStreamWrapper callback.
+
+  @Override
+  public void onPrepared() {
+    if (--pendingPrepareCount > 0) {
+      return;
+    }
+
+    // The wrapper at index 0 is the one of type TRACK_TYPE_DEFAULT.
+    durationUs = trackStreamWrappers[0].getDurationUs();
+    isLive = trackStreamWrappers[0].isLive();
+
+    int totalTrackGroupCount = 0;
+    for (HlsTrackStreamWrapper trackStreamWrapper : trackStreamWrappers) {
+      totalTrackGroupCount += trackStreamWrapper.getTrackGroups().length;
+    }
+    TrackGroup[] trackGroupArray = new TrackGroup[totalTrackGroupCount];
+    int trackGroupIndex = 0;
+    for (HlsTrackStreamWrapper trackStreamWrapper : trackStreamWrappers) {
+      int wrapperTrackGroupCount = trackStreamWrapper.getTrackGroups().length;
+      for (int j = 0; j < wrapperTrackGroupCount; j++) {
+        trackGroupArray[trackGroupIndex++] = trackStreamWrapper.getTrackGroups().get(j);
+      }
+    }
+    trackGroups = new TrackGroupArray(trackGroupArray);
+    callback.onSourcePrepared(this);
+  }
+
   // Internal methods.
 
   private List<HlsTrackStreamWrapper> buildTrackStreamWrappers(HlsPlaylist playlist) {
@@ -296,16 +308,18 @@ public final class HlsSampleSource implements SampleSource,
     } else {
       // Leave the enabled variants unchanged. They're likely either all video or all audio.
     }
-    Variant[] variants = new Variant[selectedVariants.size()];
-    selectedVariants.toArray(variants);
-    trackStreamWrappers.add(buildTrackStreamWrapper(C.TRACK_TYPE_DEFAULT, baseUri, variants,
-        new FormatEvaluator.AdaptiveEvaluator(bandwidthMeter), masterPlaylist.muxedAudioFormat,
-        masterPlaylist.muxedCaptionFormat));
+    if (!selectedVariants.isEmpty()) {
+      Variant[] variants = new Variant[selectedVariants.size()];
+      selectedVariants.toArray(variants);
+      trackStreamWrappers.add(buildTrackStreamWrapper(C.TRACK_TYPE_DEFAULT, baseUri, variants,
+          new FormatEvaluator.AdaptiveEvaluator(bandwidthMeter), masterPlaylist.muxedAudioFormat,
+          masterPlaylist.muxedCaptionFormat));
+    }
 
     // Build the audio stream wrapper if applicable.
     List<Variant> audioVariants = masterPlaylist.audios;
     if (!audioVariants.isEmpty()) {
-      variants = new Variant[audioVariants.size()];
+      Variant[] variants = new Variant[audioVariants.size()];
       audioVariants.toArray(variants);
       trackStreamWrappers.add(buildTrackStreamWrapper(C.TRACK_TYPE_AUDIO, baseUri, variants, null,
           null, null));
@@ -314,7 +328,7 @@ public final class HlsSampleSource implements SampleSource,
     // Build the text stream wrapper if applicable.
     List<Variant> subtitleVariants = masterPlaylist.subtitles;
     if (!subtitleVariants.isEmpty()) {
-      variants = new Variant[subtitleVariants.size()];
+      Variant[] variants = new Variant[subtitleVariants.size()];
       subtitleVariants.toArray(variants);
       trackStreamWrappers.add(buildTrackStreamWrapper(C.TRACK_TYPE_TEXT, baseUri, variants, null,
           null, null));
@@ -329,8 +343,9 @@ public final class HlsSampleSource implements SampleSource,
     DataSource dataSource = dataSourceFactory.createDataSource(bandwidthMeter);
     HlsChunkSource defaultChunkSource = new HlsChunkSource(baseUri, variants, dataSource,
         timestampAdjusterProvider, formatEvaluator);
-    return new HlsTrackStreamWrapper(trackType, defaultChunkSource, loadControl, muxedAudioFormat,
-        muxedCaptionFormat, MIN_LOADABLE_RETRY_COUNT, eventDispatcher);
+    return new HlsTrackStreamWrapper(trackType, this, defaultChunkSource, loadControl,
+        preparePositionUs, muxedAudioFormat, muxedCaptionFormat, MIN_LOADABLE_RETRY_COUNT,
+        eventDispatcher);
   }
 
   private int selectTracks(HlsTrackStreamWrapper trackStreamWrapper,

library/src/main/java/com/google/android/exoplayer/hls/HlsTrackStreamWrapper.java

@@ -28,6 +28,7 @@ import com.google.android.exoplayer.TrackStream;
 import com.google.android.exoplayer.chunk.Chunk;
 import com.google.android.exoplayer.chunk.ChunkHolder;
 import com.google.android.exoplayer.extractor.DefaultTrackOutput;
+import com.google.android.exoplayer.extractor.DefaultTrackOutput.UpstreamFormatChangedListener;
 import com.google.android.exoplayer.extractor.ExtractorOutput;
 import com.google.android.exoplayer.extractor.SeekMap;
 import com.google.android.exoplayer.upstream.Loader;
@@ -44,7 +45,20 @@ import java.util.List;
  * Loads {@link HlsMediaChunk}s obtained from a {@link HlsChunkSource}, and provides
  * {@link TrackStream}s from which the loaded media can be consumed.
  */
-/* package */ final class HlsTrackStreamWrapper implements Loader.Callback<Chunk>, ExtractorOutput {
+/* package */ final class HlsTrackStreamWrapper implements Loader.Callback<Chunk>, ExtractorOutput,
+    UpstreamFormatChangedListener {
+
+  /**
+   * A callback to be notified of events.
+   */
+  public interface Callback {
+
+    /**
+     * Invoked when the wrapper has been prepared.
+     */
+    void onPrepared();
+
+  }
 
   private static final int PRIMARY_TYPE_NONE = 0;
   private static final int PRIMARY_TYPE_TEXT = 1;
@@ -52,6 +66,7 @@ import java.util.List;
   private static final int PRIMARY_TYPE_VIDEO = 3;
 
   private final int trackType;
+  private final Callback callback;
   private final HlsChunkSource chunkSource;
   private final LoadControl loadControl;
   private final Format muxedAudioFormat;
@@ -85,8 +100,10 @@ import java.util.List;
 
   /**
    * @param trackType The type of the track. One of the {@link C} {@code TRACK_TYPE_*} constants.
+   * @param callback A callback for the wrapper.
    * @param chunkSource A {@link HlsChunkSource} from which chunks to load are obtained.
    * @param loadControl Controls when the source is permitted to load data.
+   * @param positionUs The position from which to start loading media.
    * @param muxedAudioFormat If HLS master playlist indicates that the stream contains muxed audio,
    *     this is the audio {@link Format} as defined by the playlist.
    * @param muxedCaptionFormat If HLS master playlist indicates that the stream contains muxed
@@ -95,10 +112,11 @@ import java.util.List;
    *     before propagating an error.
    * @param eventDispatcher A dispatcher to notify of events.
    */
-  public HlsTrackStreamWrapper(int trackType, HlsChunkSource chunkSource, LoadControl loadControl,
-      Format muxedAudioFormat, Format muxedCaptionFormat, int minLoadableRetryCount,
-      EventDispatcher eventDispatcher) {
+  public HlsTrackStreamWrapper(int trackType, Callback callback, HlsChunkSource chunkSource,
+      LoadControl loadControl, long positionUs, Format muxedAudioFormat, Format muxedCaptionFormat,
+      int minLoadableRetryCount, EventDispatcher eventDispatcher) {
     this.trackType = trackType;
+    this.callback = callback;
     this.chunkSource = chunkSource;
     this.loadControl = loadControl;
     this.muxedAudioFormat = muxedAudioFormat;
@@ -110,44 +128,16 @@ import java.util.List;
     sampleQueues = new SparseArray<>();
     mediaChunks = new LinkedList<>();
     readingEnabled = true;
-    pendingResetPositionUs = C.UNSET_TIME_US;
+    pendingResetPositionUs = positionUs;
+    downstreamPositionUs = positionUs;
   }
 
-  public boolean prepare(long positionUs) throws IOException {
-    if (prepared) {
-      return true;
-    }
-    if (chunkSource.getTrackCount() == 0) {
-      trackGroups = new TrackGroupArray();
-      prepared = true;
-      return true;
-    }
-    if (sampleQueuesBuilt) {
-      boolean canBuildTracks = true;
-      int sampleQueueCount = sampleQueues.size();
-      for (int i = 0; i < sampleQueueCount; i++) {
-        if (sampleQueues.valueAt(i).getUpstreamFormat() == null) {
-          canBuildTracks = false;
-          break;
-        }
-      }
-      if (canBuildTracks) {
-        buildTracks();
-        prepared = true;
-        return true;
-      }
-    }
-    // We're not prepared.
+  public void prepare() {
+    maybeStartLoading();
+  }
+
+  public void maybeThrowPrepareError() throws IOException {
     maybeThrowError();
-    if (!loader.isLoading()) {
-      // We're going to have to start loading a chunk to get what we need for preparation. We should
-      // attempt to load the chunk at positionUs, so that we'll already be loading the correct chunk
-      // in the common case where the renderer is subsequently enabled at this position.
-      pendingResetPositionUs = positionUs;
-      downstreamPositionUs = positionUs;
-      maybeStartLoading();
-    }
-    return false;
   }
 
   public long getDurationUs() {
@@ -376,6 +366,7 @@ import java.util.List;
       return sampleQueues.get(id);
     }
     DefaultTrackOutput trackOutput = new DefaultTrackOutput(loadControl.getAllocator());
+    trackOutput.setUpstreamFormatChangeListener(this);
     sampleQueues.put(id, trackOutput);
     return trackOutput;
   }
@@ -383,6 +374,7 @@ import java.util.List;
   @Override
   public void endTracks() {
     sampleQueuesBuilt = true;
+    maybeFinishPrepare();
   }
 
   @Override
@@ -390,8 +382,30 @@ import java.util.List;
     // Do nothing.
   }
 
+  // UpstreamFormatChangedListener implementation.
+
+  @Override
+  public void onUpstreamFormatChanged(Format format) {
+    maybeFinishPrepare();
+  }
+
   // Internal methods.
 
+  private void maybeFinishPrepare() {
+    if (!sampleQueuesBuilt) {
+      return;
+    }
+    int sampleQueueCount = sampleQueues.size();
+    for (int i = 0; i < sampleQueueCount; i++) {
+      if (sampleQueues.valueAt(i).getUpstreamFormat() == null) {
+        return;
+      }
+    }
+    buildTracks();
+    prepared = true;
+    callback.onPrepared();
+  }
+
   /**
    * Builds tracks that are exposed by this {@link HlsTrackStreamWrapper} instance, as well as
    * internal data-structures required for operation.

library/src/main/java/com/google/android/exoplayer/smoothstreaming/SmoothStreamingSampleSource.java

@@ -73,6 +73,7 @@ public final class SmoothStreamingSampleSource implements SampleSource,
   private long manifestLoadStartTimestamp;
   private SmoothStreamingManifest manifest;
 
+  private Callback callback;
   private LoadControl loadControl;
   private boolean prepared;
   private long durationUs;
@@ -97,16 +98,15 @@ public final class SmoothStreamingSampleSource implements SampleSource,
   }
 
   @Override
-  public boolean prepare(long positionUs, LoadControl loadControl) throws IOException {
-    if (prepared) {
-      return true;
-    }
+  public void prepare(Callback callback, LoadControl loadControl, long positionUs) {
+    this.callback = callback;
     this.loadControl = loadControl;
+    startLoadingManifest();
+  }
+
+  @Override
+  public void maybeThrowPrepareError() throws IOException {
     manifestLoader.maybeThrowError();
-    if (!manifestLoader.isLoading()) {
-      startLoadingManifest();
-    }
-    return false;
   }
 
   @Override
@@ -218,6 +218,7 @@ public final class SmoothStreamingSampleSource implements SampleSource,
             new TrackEncryptionBox(true, INITIALIZATION_VECTOR_SIZE, keyId)};
       }
       prepared = true;
+      callback.onSourcePrepared(this);
     } else {
       for (ChunkTrackStream<SmoothStreamingChunkSource> trackStream : trackStreams) {
         trackStream.getChunkSource().updateManifest(manifest);

library/src/main/java/com/google/android/exoplayer/upstream/Loader.java

@@ -46,7 +46,7 @@ public final class Loader {
   }
 
   /**
-   * Interface definition of an object that can be loaded using a {@link Loader}.
+   * An object that can be loaded using a {@link Loader}.
    */
   public interface Loadable {
 
@@ -73,7 +73,7 @@ public final class Loader {
   }
 
   /**
-   * Interface definition for a callback to be notified of {@link Loader} events.
+   * A callback to be notified of {@link Loader} events.
    */
   public interface Callback<T extends Loadable> {