Implement seeking via a single code path.

When a seek is performed, renderers currently perform the
actions that they need to take in two places: Some changes
are performed in seekTo implementations. Other changes are
performed when discontinuities are read from the source.

In HLS we need to perform what is effectively a seek
originating in the source. To support this, this CL allows
discontinuities read from the source to modify the playback
position. All actions that renderers perform as a result
of a seek are moved to be performed when a discontinuity is
received.

Best way to understand CL:
- Look at SampleSource interface change and then at the
  concrete implementations, to make sure they've been
  changed properly.
- Look at SampleSourceTrackRenderer change.
- Look at concrete renderers. The general pattern is that
  code previously performed in seekTo and READ_DISCONTINUITY
  is merged into onDiscontinuity().

Note: This will be further untangled in V2.

Issue #676
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=112720746

extensions/opus/src/main/java/com/google/android/exoplayer/ext/opus/LibopusAudioTrackRenderer.java

@@ -145,19 +145,11 @@ public final class LibopusAudioTrackRenderer extends SampleSourceTrackRenderer
   }
 
   @Override
-  protected void onEnabled(int track, long positionUs, boolean joining)
+  protected void doSomeWork(long positionUs, long elapsedRealtimeUs, boolean sourceIsReady)
       throws ExoPlaybackException {
-    super.onEnabled(track, positionUs, joining);
-    seekToInternal(positionUs);
-  }
-
-  @Override
-  protected void doSomeWork(long positionUs, long elapsedRealtimeUs) throws ExoPlaybackException {
     if (outputStreamEnded) {
       return;
     }
-    sourceIsReady = continueBufferingSource(positionUs);
-    checkForDiscontinuity(positionUs);
 
     // Try and read a format if we don't have one already.
     if (format == null && !readFormat(positionUs)) {
@@ -275,14 +267,10 @@ public final class LibopusAudioTrackRenderer extends SampleSourceTrackRenderer
       }
     }
 
-    int result = readSource(positionUs, formatHolder, inputBuffer.sampleHolder, false);
+    int result = readSource(positionUs, formatHolder, inputBuffer.sampleHolder);
     if (result == SampleSource.NOTHING_READ) {
       return false;
     }
-    if (result == SampleSource.DISCONTINUITY_READ) {
-      flushDecoder();
-      return true;
-    }
     if (result == SampleSource.FORMAT_READ) {
       format = formatHolder.format;
       return true;
@@ -304,16 +292,6 @@ public final class LibopusAudioTrackRenderer extends SampleSourceTrackRenderer
     return true;
   }
 
-  private void checkForDiscontinuity(long positionUs) {
-    if (decoder == null) {
-      return;
-    }
-    int result = readSource(positionUs, formatHolder, null, true);
-    if (result == SampleSource.DISCONTINUITY_READ) {
-      flushDecoder();
-    }
-  }
-
   private void flushDecoder() {
     inputBuffer = null;
     outputBuffer = null;
@@ -344,18 +322,16 @@ public final class LibopusAudioTrackRenderer extends SampleSourceTrackRenderer
   }
 
   @Override
-  protected void seekTo(long positionUs) throws ExoPlaybackException {
+  protected void onDiscontinuity(long positionUs) {
-    super.seekTo(positionUs);
-    seekToInternal(positionUs);
-  }
-
-  private void seekToInternal(long positionUs) {
     audioTrack.reset();
     currentPositionUs = positionUs;
     allowPositionDiscontinuity = true;
     inputStreamEnded = false;
     outputStreamEnded = false;
     sourceIsReady = false;
+    if (decoder != null) {
+      flushDecoder();
+    }
   }
 
   @Override
@@ -387,7 +363,7 @@ public final class LibopusAudioTrackRenderer extends SampleSourceTrackRenderer
   }
 
   private boolean readFormat(long positionUs) {
-    int result = readSource(positionUs, formatHolder, null, false);
+    int result = readSource(positionUs, formatHolder, null);
     if (result == SampleSource.FORMAT_READ) {
       format = formatHolder.format;
       audioTrack.configure(format.getFrameworkMediaFormatV16(), false);

extensions/vp9/src/main/java/com/google/android/exoplayer/ext/vp9/LibvpxVideoTrackRenderer.java

@@ -175,12 +175,11 @@ public final class LibvpxVideoTrackRenderer extends SampleSourceTrackRenderer {
   }
 
   @Override
-  protected void doSomeWork(long positionUs, long elapsedRealtimeUs) throws ExoPlaybackException {
+  protected void doSomeWork(long positionUs, long elapsedRealtimeUs, boolean sourceIsReady)
+      throws ExoPlaybackException {
     if (outputStreamEnded) {
       return;
     }
-    sourceIsReady = continueBufferingSource(positionUs);
-    checkForDiscontinuity(positionUs);
 
     // Try and read a format if we don't have one already.
     if (format == null && !readFormat(positionUs)) {
@@ -311,15 +310,10 @@ public final class LibvpxVideoTrackRenderer extends SampleSourceTrackRenderer {
       }
     }
 
-    int result = readSource(positionUs, formatHolder, inputBuffer.sampleHolder,
+    int result = readSource(positionUs, formatHolder, inputBuffer.sampleHolder);
-        false);
     if (result == SampleSource.NOTHING_READ) {
       return false;
     }
-    if (result == SampleSource.DISCONTINUITY_READ) {
-      flushDecoder();
-      return true;
-    }
     if (result == SampleSource.FORMAT_READ) {
       format = formatHolder.format;
       return true;
@@ -339,16 +333,6 @@ public final class LibvpxVideoTrackRenderer extends SampleSourceTrackRenderer {
     return true;
   }
 
-  private void checkForDiscontinuity(long positionUs) {
-    if (decoder == null) {
-      return;
-    }
-    int result = readSource(positionUs, formatHolder, null, true);
-    if (result == SampleSource.DISCONTINUITY_READ) {
-      flushDecoder();
-    }
-  }
-
   private void flushDecoder() {
     inputBuffer = null;
     if (outputBuffer != null) {
@@ -369,23 +353,14 @@ public final class LibvpxVideoTrackRenderer extends SampleSourceTrackRenderer {
   }
 
   @Override
-  protected void seekTo(long positionUs) throws ExoPlaybackException {
+  protected void onDiscontinuity(long positionUs) {
-    super.seekTo(positionUs);
-    seekToInternal();
-  }
-
-  @Override
-  protected void onEnabled(int track, long positionUs, boolean joining)
-      throws ExoPlaybackException {
-    super.onEnabled(track, positionUs, joining);
-    seekToInternal();
-  }
-
-  private void seekToInternal() {
     sourceIsReady = false;
     inputStreamEnded = false;
     outputStreamEnded = false;
     renderedFirstFrame = false;
+    if (decoder != null) {
+      flushDecoder();
+    }
   }
 
   @Override
@@ -416,7 +391,7 @@ public final class LibvpxVideoTrackRenderer extends SampleSourceTrackRenderer {
   }
 
   private boolean readFormat(long positionUs) {
-    int result = readSource(positionUs, formatHolder, null, false);
+    int result = readSource(positionUs, formatHolder, null);
     if (result == SampleSource.FORMAT_READ) {
       format = formatHolder.format;
       return true;

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

@@ -80,7 +80,8 @@ public final class FrameworkSampleSource implements SampleSource, SampleSourceRe
   private int[] trackStates;
   private boolean[] pendingDiscontinuities;
 
-  private long seekPositionUs;
+  private long lastSeekPositionUs;
+  private long pendingSeekPositionUs;
 
   /**
    * Instantiates a new sample extractor reading from the specified {@code uri}.
@@ -184,16 +185,21 @@ public final class FrameworkSampleSource implements SampleSource, SampleSourceRe
     return true;
   }
 
+  @Override
+  public long readDiscontinuity(int track) {
+    if (pendingDiscontinuities[track]) {
+      pendingDiscontinuities[track] = false;
+      return lastSeekPositionUs;
+    }
+    return NO_DISCONTINUITY;
+  }
+
   @Override
   public int readData(int track, long positionUs, MediaFormatHolder formatHolder,
-      SampleHolder sampleHolder, boolean onlyReadDiscontinuity) {
+      SampleHolder sampleHolder) {
     Assertions.checkState(prepared);
     Assertions.checkState(trackStates[track] != TRACK_STATE_DISABLED);
     if (pendingDiscontinuities[track]) {
-      pendingDiscontinuities[track] = false;
-      return DISCONTINUITY_READ;
-    }
-    if (onlyReadDiscontinuity) {
       return NOTHING_READ;
     }
     if (trackStates[track] != TRACK_STATE_FORMAT_SENT) {
@@ -216,7 +222,7 @@ public final class FrameworkSampleSource implements SampleSource, SampleSourceRe
       if (sampleHolder.isEncrypted()) {
         sampleHolder.cryptoInfo.setFromExtractorV16(extractor);
       }
-      seekPositionUs = C.UNKNOWN_TIME_US;
+      pendingSeekPositionUs = C.UNKNOWN_TIME_US;
       extractor.advance();
       return SAMPLE_READ;
     } else {
@@ -285,8 +291,9 @@ public final class FrameworkSampleSource implements SampleSource, SampleSourceRe
   private void seekToUsInternal(long positionUs, boolean force) {
     // Unless forced, avoid duplicate calls to the underlying extractor's seek method in the case
     // that there have been no interleaving calls to readSample.
-    if (force || seekPositionUs != positionUs) {
+    if (force || pendingSeekPositionUs != positionUs) {
-      seekPositionUs = positionUs;
+      lastSeekPositionUs = positionUs;
+      pendingSeekPositionUs = positionUs;
       extractor.seekTo(positionUs, MediaExtractor.SEEK_TO_PREVIOUS_SYNC);
       for (int i = 0; i < trackStates.length; ++i) {
         if (trackStates[i] != TRACK_STATE_DISABLED) {

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

@@ -239,13 +239,6 @@ public class MediaCodecAudioTrackRenderer extends MediaCodecTrackRenderer implem
     return this;
   }
 
-  @Override
-  protected void onEnabled(int track, long positionUs, boolean joining)
-      throws ExoPlaybackException {
-    super.onEnabled(track, positionUs, joining);
-    seekToInternal(positionUs);
-  }
-
   @Override
   protected void onOutputFormatChanged(android.media.MediaFormat outputFormat) {
     boolean passthrough = passthroughMediaFormat != null;
@@ -312,13 +305,8 @@ public class MediaCodecAudioTrackRenderer extends MediaCodecTrackRenderer implem
   }
 
   @Override
-  protected void seekTo(long positionUs) throws ExoPlaybackException {
+  protected void onDiscontinuity(long positionUs) throws ExoPlaybackException {
-    super.seekTo(positionUs);
+    super.onDiscontinuity(positionUs);
-    seekToInternal(positionUs);
-  }
-
-  private void seekToInternal(long positionUs) {
-    // TODO: Try and re-use the same AudioTrack instance once [Internal: b/7941810] is fixed.
     audioTrack.reset();
     currentPositionUs = positionUs;
     allowPositionDiscontinuity = true;
@@ -376,7 +364,7 @@ public class MediaCodecAudioTrackRenderer extends MediaCodecTrackRenderer implem
 
     // If we are out of sync, allow currentPositionUs to jump backwards.
     if ((handleBufferResult & AudioTrack.RESULT_POSITION_DISCONTINUITY) != 0) {
-      handleDiscontinuity();
+      handleAudioTrackDiscontinuity();
       allowPositionDiscontinuity = true;
     }
 
@@ -395,7 +383,7 @@ public class MediaCodecAudioTrackRenderer extends MediaCodecTrackRenderer implem
     audioTrack.handleEndOfStream();
   }
 
-  protected void handleDiscontinuity() {
+  protected void handleAudioTrackDiscontinuity() {
     // Do nothing
   }
 

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

@@ -262,13 +262,6 @@ public abstract class MediaCodecTrackRenderer extends SampleSourceTrackRenderer
     codecReinitializationState = REINITIALIZATION_STATE_NONE;
   }
 
-  @Override
-  protected void onEnabled(int track, long positionUs, boolean joining)
-      throws ExoPlaybackException {
-    super.onEnabled(track, positionUs, joining);
-    seekToInternal();
-  }
-
   @Override
   protected final boolean handlesTrack(MediaFormat mediaFormat) throws DecoderQueryException {
     return handlesTrack(mediaCodecSelector, mediaFormat);
@@ -458,15 +451,13 @@ public abstract class MediaCodecTrackRenderer extends SampleSourceTrackRenderer
   }
 
   @Override
-  protected void seekTo(long positionUs) throws ExoPlaybackException {
+  protected void onDiscontinuity(long positionUs) throws ExoPlaybackException {
-    super.seekTo(positionUs);
-    seekToInternal();
-  }
-
-  private void seekToInternal() {
     sourceState = SOURCE_STATE_NOT_READY;
     inputStreamEnded = false;
     outputStreamEnded = false;
+    if (codec != null) {
+      flushCodec();
+    }
   }
 
   @Override
@@ -480,11 +471,11 @@ public abstract class MediaCodecTrackRenderer extends SampleSourceTrackRenderer
   }
 
   @Override
-  protected void doSomeWork(long positionUs, long elapsedRealtimeUs) throws ExoPlaybackException {
+  protected void doSomeWork(long positionUs, long elapsedRealtimeUs, boolean sourceIsReady)
-    sourceState = continueBufferingSource(positionUs)
+      throws ExoPlaybackException {
+    sourceState = sourceIsReady
         ? (sourceState == SOURCE_STATE_NOT_READY ? SOURCE_STATE_READY : sourceState)
         : SOURCE_STATE_NOT_READY;
-    checkForDiscontinuity(positionUs);
     if (format == null) {
       readFormat(positionUs);
     }
@@ -501,22 +492,12 @@ public abstract class MediaCodecTrackRenderer extends SampleSourceTrackRenderer
   }
 
   private void readFormat(long positionUs) throws ExoPlaybackException {
-    int result = readSource(positionUs, formatHolder, sampleHolder, false);
+    int result = readSource(positionUs, formatHolder, null);
     if (result == SampleSource.FORMAT_READ) {
       onInputFormatChanged(formatHolder);
     }
   }
 
-  private void checkForDiscontinuity(long positionUs) throws ExoPlaybackException {
-    if (codec == null) {
-      return;
-    }
-    int result = readSource(positionUs, formatHolder, sampleHolder, true);
-    if (result == SampleSource.DISCONTINUITY_READ) {
-      flushCodec();
-    }
-  }
-
   private void flushCodec() throws ExoPlaybackException {
     codecHotswapTimeMs = -1;
     inputIndex = -1;
@@ -598,7 +579,7 @@ public abstract class MediaCodecTrackRenderer extends SampleSourceTrackRenderer
         }
         codecReconfigurationState = RECONFIGURATION_STATE_QUEUE_PENDING;
       }
-      result = readSource(positionUs, formatHolder, sampleHolder, false);
+      result = readSource(positionUs, formatHolder, sampleHolder);
       if (firstFeed && sourceState == SOURCE_STATE_READY && result == SampleSource.NOTHING_READ) {
         sourceState = SOURCE_STATE_READY_READ_MAY_FAIL;
       }
@@ -607,10 +588,6 @@ public abstract class MediaCodecTrackRenderer extends SampleSourceTrackRenderer
     if (result == SampleSource.NOTHING_READ) {
       return false;
     }
-    if (result == SampleSource.DISCONTINUITY_READ) {
-      flushCodec();
-      return true;
-    }
     if (result == SampleSource.FORMAT_READ) {
       if (codecReconfigurationState == RECONFIGURATION_STATE_QUEUE_PENDING) {
         // We received two formats in a row. Clear the current buffer of any reconfiguration data

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

@@ -227,8 +227,6 @@ public class MediaCodecVideoTrackRenderer extends MediaCodecTrackRenderer {
   protected void onEnabled(int track, long positionUs, boolean joining)
       throws ExoPlaybackException {
     super.onEnabled(track, positionUs, joining);
-    renderedFirstFrame = false;
-    consecutiveDroppedFrameCount = 0;
     if (joining && allowedJoiningTimeUs > 0) {
       joiningDeadlineUs = SystemClock.elapsedRealtime() * 1000L + allowedJoiningTimeUs;
     }
@@ -236,8 +234,8 @@ public class MediaCodecVideoTrackRenderer extends MediaCodecTrackRenderer {
   }
 
   @Override
-  protected void seekTo(long positionUs) throws ExoPlaybackException {
+  protected void onDiscontinuity(long positionUs) throws ExoPlaybackException {
-    super.seekTo(positionUs);
+    super.onDiscontinuity(positionUs);
     renderedFirstFrame = false;
     consecutiveDroppedFrameCount = 0;
     joiningDeadlineUs = -1;

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

@@ -46,9 +46,9 @@ public interface SampleSource {
    */
   public static final int FORMAT_READ = -4;
   /**
-   * A discontinuity in the sample stream.
+   * Returned from {@link SampleSourceReader#readDiscontinuity(int)} to indicate no discontinuity.
    */
-  public static final int DISCONTINUITY_READ = -5;
+  public static final long NO_DISCONTINUITY = Long.MIN_VALUE;
 
   /**
    * A consumer of samples should call this method to register themselves and gain access to the
@@ -104,7 +104,7 @@ public interface SampleSource {
      * (i.e. @link {@link MediaFormat#adaptive} is true). Hence the track formats returned through
      * this method should not be used to configure decoders. Decoder configuration should be
      * performed using the formats obtained when reading the media stream through calls to
-     * {@link #readData(int, long, MediaFormatHolder, SampleHolder, boolean)}.
+     * {@link #readData(int, long, MediaFormatHolder, SampleHolder)}.
      * <p>
      * This method should only be called after the source has been prepared.
      *
@@ -115,7 +115,7 @@ public interface SampleSource {
 
     /**
      * Enable the specified track. This allows the track's format and samples to be read from
-     * {@link #readData(int, long, MediaFormatHolder, SampleHolder, boolean)}.
+     * {@link #readData(int, long, MediaFormatHolder, SampleHolder)}.
      * <p>
      * This method should only be called after the source has been prepared, and when the specified
      * track is disabled.
@@ -138,13 +138,27 @@ public interface SampleSource {
     public boolean continueBuffering(int track, long positionUs);
 
     /**
-     * Attempts to read either a sample, a new format or or a discontinuity from the source.
+     * Attempts to read a pending discontinuity from the source.
+     * <p>
+     * This method should only be called when the specified track is enabled.
+     *
+     * @param track The track from which to read.
+     * @return If a discontinuity was read then the playback position after the discontinuity. Else
+     *     {@link #NO_DISCONTINUITY}.
+     */
+    public long readDiscontinuity(int track);
+
+    /**
+     * Attempts to read a sample or a new format from the source.
      * <p>
      * This method should only be called when the specified track is enabled.
      * <p>
      * Note that where multiple tracks are enabled, {@link #NOTHING_READ} may be returned if the
      * next piece of data to be read from the {@link SampleSource} corresponds to a different track
      * than the one for which data was requested.
+     * <p>
+     * This method will always return {@link #NOTHING_READ} in the case that there's a pending
+     * discontinuity to be read from {@link #readDiscontinuity(int)} for the specified track.
      *
      * @param track The track from which to read.
      * @param positionUs The current playback position.
@@ -153,13 +167,11 @@ public interface SampleSource {
      * @param sampleHolder A {@link SampleHolder} object to populate in the case of a new sample.
      *     If the caller requires the sample data then it must ensure that {@link SampleHolder#data}
      *     references a valid output buffer.
-     * @param onlyReadDiscontinuity Whether to only read a discontinuity. If true, only
-     *     {@link #DISCONTINUITY_READ} or {@link #NOTHING_READ} can be returned.
      * @return The result, which can be {@link #SAMPLE_READ}, {@link #FORMAT_READ},
-     *     {@link #DISCONTINUITY_READ}, {@link #NOTHING_READ} or {@link #END_OF_STREAM}.
+     *     {@link #NOTHING_READ} or {@link #END_OF_STREAM}.
      */
     public int readData(int track, long positionUs, MediaFormatHolder formatHolder,
-        SampleHolder sampleHolder, boolean onlyReadDiscontinuity);
+        SampleHolder sampleHolder);
 
     /**
      * Seeks to the specified time in microseconds.

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

@@ -48,7 +48,7 @@ public abstract class SampleSourceTrackRenderer extends TrackRenderer {
   }
 
   @Override
-  protected boolean doPrepare(long positionUs) throws ExoPlaybackException {
+  protected final boolean doPrepare(long positionUs) throws ExoPlaybackException {
     boolean allSourcesPrepared = true;
     for (int i = 0; i < sources.length; i++) {
       allSourcesPrepared &= sources[i].prepare(positionUs);
@@ -103,26 +103,30 @@ public abstract class SampleSourceTrackRenderer extends TrackRenderer {
     return true;
   }
 
-  /**
-   * Returns whether this renderer is capable of handling the provided track.
-   *
-   * @param mediaFormat The format of the track.
-   * @return True if the renderer can handle the track, false otherwise.
-   * @throws DecoderQueryException Thrown if there was an error querying decoders.
-   */
-  protected abstract boolean handlesTrack(MediaFormat mediaFormat) throws DecoderQueryException;
-
   @Override
   protected void onEnabled(int track, long positionUs, boolean joining)
       throws ExoPlaybackException {
+    positionUs = shiftInputPosition(positionUs);
     enabledSource = sources[handledSourceIndices[track]];
     enabledSourceTrackIndex = handledSourceTrackIndices[track];
     enabledSource.enable(enabledSourceTrackIndex, positionUs);
+    onDiscontinuity(positionUs);
   }
 
   @Override
-  protected void seekTo(long positionUs) throws ExoPlaybackException {
+  protected final void seekTo(long positionUs) throws ExoPlaybackException {
+    positionUs = shiftInputPosition(positionUs);
     enabledSource.seekToUs(positionUs);
+    checkForDiscontinuity(positionUs);
+  }
+
+  @Override
+  protected final void doSomeWork(long positionUs, long elapsedRealtimeUs)
+      throws ExoPlaybackException {
+    positionUs = shiftInputPosition(positionUs);
+    boolean sourceIsReady = enabledSource.continueBuffering(enabledSourceTrackIndex, positionUs);
+    positionUs = checkForDiscontinuity(positionUs);
+    doSomeWork(positionUs, elapsedRealtimeUs, sourceIsReady);
   }
 
   @Override
@@ -147,14 +151,6 @@ public abstract class SampleSourceTrackRenderer extends TrackRenderer {
     }
   }
 
-  private void maybeThrowError(SampleSourceReader source) throws ExoPlaybackException {
-    try {
-      source.maybeThrowError();
-    } catch (IOException e) {
-      throw new ExoPlaybackException(e);
-    }
-  }
-
   @Override
   protected void onDisabled() throws ExoPlaybackException {
     enabledSource.disable(enabledSourceTrackIndex);
@@ -169,16 +165,6 @@ public abstract class SampleSourceTrackRenderer extends TrackRenderer {
     }
   }
 
-  protected final boolean continueBufferingSource(long positionUs) {
-    return enabledSource.continueBuffering(enabledSourceTrackIndex, positionUs);
-  }
-
-  protected final int readSource(long positionUs, MediaFormatHolder formatHolder,
-      SampleHolder sampleHolder, boolean onlyReadDiscontinuity) {
-    return enabledSource.readData(enabledSourceTrackIndex, positionUs, formatHolder, sampleHolder,
-        onlyReadDiscontinuity);
-  }
-
   @Override
   protected final int getTrackCount() {
     return handledSourceTrackIndices.length;
@@ -190,4 +176,92 @@ public abstract class SampleSourceTrackRenderer extends TrackRenderer {
     return source.getFormat(handledSourceTrackIndices[track]);
   }
 
+  /**
+   * Shifts positions passed to {@link #onEnabled(int, long, boolean)}, {@link #seekTo(long)} and
+   * {@link #doSomeWork(long, long)}.
+   * <p>
+   * The default implementation does not modify the position. Except in very specific cases,
+   * subclasses should not override this method.
+   *
+   * @param positionUs The position in microseconds.
+   * @return The adjusted position in microseconds.
+   */
+  protected long shiftInputPosition(long positionUs) {
+    return positionUs;
+  }
+
+  // Methods to be called by subclasses.
+
+  /**
+   * Reads from the enabled upstream source.
+   *
+   * @param positionUs The current playback position.
+   * @param formatHolder A {@link MediaFormatHolder} object to populate in the case of a new format.
+   * @param sampleHolder A {@link SampleHolder} object to populate in the case of a new sample.
+   *     If the caller requires the sample data then it must ensure that {@link SampleHolder#data}
+   *     references a valid output buffer.
+   * @return The result, which can be {@link SampleSource#SAMPLE_READ},
+   *     {@link SampleSource#FORMAT_READ}, {@link SampleSource#NOTHING_READ} or
+   *     {@link SampleSource#END_OF_STREAM}.
+   */
+  protected final int readSource(long positionUs, MediaFormatHolder formatHolder,
+      SampleHolder sampleHolder) {
+    return enabledSource.readData(enabledSourceTrackIndex, positionUs, formatHolder, sampleHolder);
+  }
+
+  // Abstract methods.
+
+  /**
+   * Returns whether this renderer is capable of handling the provided track.
+   *
+   * @param mediaFormat The format of the track.
+   * @return True if the renderer can handle the track, false otherwise.
+   * @throws DecoderQueryException Thrown if there was an error querying decoders.
+   */
+  protected abstract boolean handlesTrack(MediaFormat mediaFormat) throws DecoderQueryException;
+
+  /**
+   * Invoked when a discontinuity is encountered. Also invoked when the renderer is enabled, for
+   * convenience.
+   *
+   * @param positionUs The playback position after the discontinuity, or the position at which
+   *     the renderer is being enabled.
+   * @throws ExoPlaybackException If an error occurs handling the discontinuity.
+   */
+  protected abstract void onDiscontinuity(long positionUs) throws ExoPlaybackException;
+
+  /**
+   * Called by {@link #doSomeWork(long, long)}.
+   *
+   * @param positionUs The current media time in microseconds, measured at the start of the
+   *     current iteration of the rendering loop.
+   * @param elapsedRealtimeUs {@link android.os.SystemClock#elapsedRealtime()} in microseconds,
+   *     measured at the start of the current iteration of the rendering loop.
+   * @param sourceIsReady The result of the most recent call to
+   *     {@link SampleSourceReader#continueBuffering(int, long)}.
+   * @throws ExoPlaybackException If an error occurs.
+   * @throws ExoPlaybackException
+   */
+  protected abstract void doSomeWork(long positionUs, long elapsedRealtimeUs, boolean sourceIsReady)
+      throws ExoPlaybackException;
+
+  // Private methods.
+
+  private long checkForDiscontinuity(long positionUs) throws ExoPlaybackException {
+    long discontinuityPositionUs = enabledSource.readDiscontinuity(enabledSourceTrackIndex);
+    if (discontinuityPositionUs != SampleSource.NO_DISCONTINUITY) {
+      onDiscontinuity(discontinuityPositionUs);
+      return discontinuityPositionUs;
+    }
+    return positionUs;
+  }
+
+  private void maybeThrowError(SampleSourceReader source) throws ExoPlaybackException {
+    try {
+      source.maybeThrowError();
+    } catch (IOException e) {
+      throw new ExoPlaybackException(e);
+    }
+  }
+
 }

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

@@ -119,12 +119,15 @@ public final class SingleSampleSource implements SampleSource, SampleSourceReade
     }
   }
 
+  @Override
+  public long readDiscontinuity(int track) {
+    return NO_DISCONTINUITY;
+  }
+
   @Override
   public int readData(int track, long positionUs, MediaFormatHolder formatHolder,
-      SampleHolder sampleHolder, boolean onlyReadDiscontinuity) {
+      SampleHolder sampleHolder) {
-    if (onlyReadDiscontinuity) {
+    if (state == STATE_END_OF_STREAM) {
-      return NOTHING_READ;
-    } else if (state == STATE_END_OF_STREAM) {
       return END_OF_STREAM;
     } else if (state == STATE_SEND_FORMAT) {
       formatHolder.format = format;

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

@@ -222,22 +222,22 @@ public class ChunkSampleSource implements SampleSource, SampleSourceReader, Load
     return loadingFinished || !sampleQueue.isEmpty();
   }
 
+  @Override
+  public long readDiscontinuity(int track) {
+    if (pendingDiscontinuity) {
+      pendingDiscontinuity = false;
+      return lastSeekPositionUs;
+    }
+    return NO_DISCONTINUITY;
+  }
+
   @Override
   public int readData(int track, long positionUs, MediaFormatHolder formatHolder,
-      SampleHolder sampleHolder, boolean onlyReadDiscontinuity) {
+      SampleHolder sampleHolder) {
     Assertions.checkState(state == STATE_ENABLED);
     downstreamPositionUs = positionUs;
 
-    if (pendingDiscontinuity) {
+    if (pendingDiscontinuity || isPendingReset()) {
-      pendingDiscontinuity = false;
-      return DISCONTINUITY_READ;
-    }
-
-    if (onlyReadDiscontinuity) {
-      return NOTHING_READ;
-    }
-
-    if (isPendingReset()) {
       return NOTHING_READ;
     }
 

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

@@ -384,16 +384,20 @@ public final class ExtractorSampleSource implements SampleSource, SampleSourceRe
   }
 
   @Override
-  public int readData(int track, long playbackPositionUs, MediaFormatHolder formatHolder,
+  public long readDiscontinuity(int track) {
-      SampleHolder sampleHolder, boolean onlyReadDiscontinuity) {
-    downstreamPositionUs = playbackPositionUs;
-
     if (pendingDiscontinuities[track]) {
       pendingDiscontinuities[track] = false;
-      return DISCONTINUITY_READ;
+      return lastSeekPositionUs;
     }
+    return NO_DISCONTINUITY;
+  }
 
-    if (onlyReadDiscontinuity || isPendingReset()) {
+  @Override
+  public int readData(int track, long playbackPositionUs, MediaFormatHolder formatHolder,
+      SampleHolder sampleHolder) {
+    downstreamPositionUs = playbackPositionUs;
+
+    if (pendingDiscontinuities[track] || isPendingReset()) {
       return NOTHING_READ;
     }
 

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

@@ -281,22 +281,22 @@ public final class HlsSampleSource implements SampleSource, SampleSourceReader,
     return false;
   }
 
+  @Override
+  public long readDiscontinuity(int track) {
+    if (pendingDiscontinuities[track]) {
+      pendingDiscontinuities[track] = false;
+      return lastSeekPositionUs;
+    }
+    return NO_DISCONTINUITY;
+  }
+
   @Override
   public int readData(int track, long playbackPositionUs, MediaFormatHolder formatHolder,
-      SampleHolder sampleHolder, boolean onlyReadDiscontinuity) {
+      SampleHolder sampleHolder) {
     Assertions.checkState(prepared);
     downstreamPositionUs = playbackPositionUs;
 
-    if (pendingDiscontinuities[track]) {
+    if (pendingDiscontinuities[track] || isPendingReset()) {
-      pendingDiscontinuities[track] = false;
-      return DISCONTINUITY_READ;
-    }
-
-    if (onlyReadDiscontinuity) {
-      return NOTHING_READ;
-    }
-
-    if (isPendingReset()) {
       return NOTHING_READ;
     }
 

library/src/main/java/com/google/android/exoplayer/metadata/MetadataTrackRenderer.java

@@ -93,29 +93,17 @@ public final class MetadataTrackRenderer<T> extends SampleSourceTrackRenderer im
   }
 
   @Override
-  protected void onEnabled(int track, long positionUs, boolean joining)
+  protected void onDiscontinuity(long positionUs) {
-      throws ExoPlaybackException {
-    super.onEnabled(track, positionUs, joining);
-    seekToInternal();
-  }
-
-  @Override
-  protected void seekTo(long positionUs) throws ExoPlaybackException {
-    super.seekTo(positionUs);
-    seekToInternal();
-  }
-
-  private void seekToInternal() {
     pendingMetadata = null;
     inputStreamEnded = false;
   }
 
   @Override
-  protected void doSomeWork(long positionUs, long elapsedRealtimeUs) throws ExoPlaybackException {
+  protected void doSomeWork(long positionUs, long elapsedRealtimeUs, boolean sourceIsReady)
-    continueBufferingSource(positionUs);
+      throws ExoPlaybackException {
     if (!inputStreamEnded && pendingMetadata == null) {
       sampleHolder.clearData();
-      int result = readSource(positionUs, formatHolder, sampleHolder, false);
+      int result = readSource(positionUs, formatHolder, sampleHolder);
       if (result == SampleSource.SAMPLE_READ) {
         pendingMetadataTimestamp = sampleHolder.timeUs;
         try {

library/src/main/java/com/google/android/exoplayer/text/TextTrackRenderer.java

@@ -185,26 +185,22 @@ public final class TextTrackRenderer extends SampleSourceTrackRenderer implement
     parserThread = new HandlerThread("textParser");
     parserThread.start();
     parserHelper = new SubtitleParserHelper(parserThread.getLooper(), subtitleParsers[parserIndex]);
-    seekToInternal();
   }
 
   @Override
-  protected void seekTo(long positionUs) throws ExoPlaybackException {
+  protected void onDiscontinuity(long positionUs) {
-    super.seekTo(positionUs);
-    seekToInternal();
-  }
-
-  private void seekToInternal() {
     inputStreamEnded = false;
     subtitle = null;
     nextSubtitle = null;
-    parserHelper.flush();
     clearTextRenderer();
+    if (parserHelper != null) {
+      parserHelper.flush();
+    }
   }
 
   @Override
-  protected void doSomeWork(long positionUs, long elapsedRealtimeUs) throws ExoPlaybackException {
+  protected void doSomeWork(long positionUs, long elapsedRealtimeUs, boolean sourceIsReady)
-    continueBufferingSource(positionUs);
+      throws ExoPlaybackException {
     if (nextSubtitle == null) {
       try {
         nextSubtitle = parserHelper.getAndClearResult();
@@ -247,7 +243,7 @@ public final class TextTrackRenderer extends SampleSourceTrackRenderer implement
       // Try and read the next subtitle from the source.
       SampleHolder sampleHolder = parserHelper.getSampleHolder();
       sampleHolder.clearData();
-      int result = readSource(positionUs, formatHolder, sampleHolder, false);
+      int result = readSource(positionUs, formatHolder, sampleHolder);
       if (result == SampleSource.FORMAT_READ) {
         parserHelper.setFormat(formatHolder.format);
       } else if (result == SampleSource.SAMPLE_READ) {

library/src/main/java/com/google/android/exoplayer/text/eia608/Eia608TrackRenderer.java

@@ -97,16 +97,10 @@ public final class Eia608TrackRenderer extends SampleSourceTrackRenderer impleme
   protected void onEnabled(int track, long positionUs, boolean joining)
       throws ExoPlaybackException {
     super.onEnabled(track, positionUs, joining);
-    seekToInternal();
   }
 
   @Override
-  protected void seekTo(long positionUs) throws ExoPlaybackException {
+  protected void onDiscontinuity(long positionUs) {
-    super.seekTo(positionUs);
-    seekToInternal();
-  }
-
-  private void seekToInternal() {
     inputStreamEnded = false;
     pendingCaptionLists.clear();
     clearPendingSample();
@@ -116,15 +110,15 @@ public final class Eia608TrackRenderer extends SampleSourceTrackRenderer impleme
   }
 
   @Override
-  protected void doSomeWork(long positionUs, long elapsedRealtimeUs) throws ExoPlaybackException {
+  protected void doSomeWork(long positionUs, long elapsedRealtimeUs, boolean sourceIsReady)
-    continueBufferingSource(positionUs);
+      throws ExoPlaybackException {
     if (isSamplePending()) {
       maybeParsePendingSample(positionUs);
     }
 
     int result = inputStreamEnded ? SampleSource.END_OF_STREAM : SampleSource.SAMPLE_READ;
     while (!isSamplePending() && result == SampleSource.SAMPLE_READ) {
-      result = readSource(positionUs, formatHolder, sampleHolder, false);
+      result = readSource(positionUs, formatHolder, sampleHolder);
       if (result == SampleSource.SAMPLE_READ) {
         maybeParsePendingSample(positionUs);
       } else if (result == SampleSource.END_OF_STREAM) {