Allow merging of both periods and sources.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=127073129

library/src/main/java/com/google/android/exoplayer2/ConcatenatingMediaSource.java

@@ -16,7 +16,7 @@
 package com.google.android.exoplayer2;
 
 /**
- * A {@link MediaSource} that concatenates multiple {@link MediaSource}s.
+ * Concatenates multiple {@link MediaSource}s.
  */
 public final class ConcatenatingMediaSource implements MediaSource {
 

library/src/main/java/com/google/android/exoplayer2/MergingMediaPeriod.java

@@ -24,11 +24,10 @@ import java.util.ArrayList;
 import java.util.IdentityHashMap;
 import java.util.List;
 
-// TODO: Make this a MediaSource
 /**
  * Merges multiple {@link MediaPeriod} instances.
  */
-public final class MultiMediaPeriod implements MediaPeriod, MediaPeriod.Callback {
+public final class MergingMediaPeriod implements MediaPeriod, MediaPeriod.Callback {
 
   private final MediaPeriod[] periods;
   private final IdentityHashMap<TrackStream, MediaPeriod> trackStreamPeriods;
@@ -43,7 +42,7 @@ public final class MultiMediaPeriod implements MediaPeriod, MediaPeriod.Callback
   private MediaPeriod[] enabledPeriods;
   private SequenceableLoader sequenceableLoader;
 
-  public MultiMediaPeriod(MediaPeriod... periods) {
+  public MergingMediaPeriod(MediaPeriod... periods) {
     this.periods = periods;
     pendingChildPrepareCount = periods.length;
     trackStreamPeriods = new IdentityHashMap<>();

library/src/main/java/com/google/android/exoplayer2/MergingMediaSource.java

@@ -0,0 +1,60 @@
+/*
+ * Copyright (C) 2016 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.google.android.exoplayer2;
+
+import com.google.android.exoplayer2.util.Assertions;
+
+/**
+ * Merges multiple {@link MediaPeriod} instances.
+ * <p>
+ * The {@link MediaSource}s being merged must have known and equal period counts, and may not return
+ * {@code null} from {@link #createPeriod(int)}.
+ */
+public final class MergingMediaSource implements MediaSource {
+
+  private final MediaSource[] mediaSources;
+  private final int periodCount;
+
+  /**
+   * @param mediaSources The {@link MediaSource}s to merge.
+   */
+  public MergingMediaSource(MediaSource... mediaSources) {
+    this.mediaSources = mediaSources;
+    periodCount = mediaSources[0].getPeriodCount();
+    Assertions.checkState(periodCount != MediaSource.UNKNOWN_PERIOD_COUNT,
+        "Child sources must have known period counts");
+    for (int i = 1; i < mediaSources.length; i++) {
+      Assertions.checkState(mediaSources[i].getPeriodCount() == periodCount,
+          "Child sources must have equal period counts");
+    }
+  }
+
+  @Override
+  public int getPeriodCount() {
+    return periodCount;
+  }
+
+  @Override
+  public MediaPeriod createPeriod(int index) {
+    MediaPeriod[] periods = new MediaPeriod[mediaSources.length];
+    for (int i = 0; i < periods.length; i++) {
+      periods[i] = mediaSources[i].createPeriod(index);
+      Assertions.checkState(periods[i] != null, "Child source must not return null period");
+    }
+    return new MergingMediaPeriod(periods);
+  }
+
+}