Start better documenting track selection

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=162601990

library/core/src/main/java/com/google/android/exoplayer2/trackselection/MappingTrackSelector.java

@@ -20,6 +20,7 @@ import android.util.SparseArray;
 import android.util.SparseBooleanArray;
 import com.google.android.exoplayer2.C;
 import com.google.android.exoplayer2.ExoPlaybackException;
+import com.google.android.exoplayer2.Renderer;
 import com.google.android.exoplayer2.RendererCapabilities;
 import com.google.android.exoplayer2.RendererConfiguration;
 import com.google.android.exoplayer2.source.TrackGroup;
@@ -31,7 +32,8 @@ import java.util.Map;
 
 /**
  * Base class for {@link TrackSelector}s that first establish a mapping between {@link TrackGroup}s
- * and renderers, and then from that mapping create a {@link TrackSelection} for each renderer.
+ * and {@link Renderer}s, and then from that mapping create a {@link TrackSelection} for each
+ * renderer.
  */
 public abstract class MappingTrackSelector extends TrackSelector {
 

library/core/src/main/java/com/google/android/exoplayer2/trackselection/TrackSelector.java

@@ -16,19 +16,74 @@
 package com.google.android.exoplayer2.trackselection;
 
 import com.google.android.exoplayer2.ExoPlaybackException;
+import com.google.android.exoplayer2.ExoPlayer;
+import com.google.android.exoplayer2.Renderer;
 import com.google.android.exoplayer2.RendererCapabilities;
+import com.google.android.exoplayer2.RendererConfiguration;
 import com.google.android.exoplayer2.source.TrackGroupArray;
 
-/** Selects tracks to be consumed by available renderers. */
+/**
+ * The component of an {@link ExoPlayer} responsible for selecting tracks to be consumed by each of
+ * the player's {@link Renderer}s. The {@link DefaultTrackSelector} implementation should be
+ * suitable for most use cases.
+ *
+ * <h3>Interactions with the player</h3>
+ * The following interactions occur between the player and its track selector during playback.
+ * <p>
+ * <ul>
+ *     <li>When the player is created it will initialize the track selector by calling
+ *     {@link #init(InvalidationListener)}.</li>
+ *     <li>When the player needs to make a track selection it will call
+ *     {@link #selectTracks(RendererCapabilities[], TrackGroupArray)}. This typically occurs at the
+ *     start of playback, when the player starts to buffer a new period of the media being played,
+ *     and when the track selector invalidates its previous selections.</li>
+ *     <li>The player may perform a track selection well in advance of the selected tracks becoming
+ *     active, where active is defined to mean that the renderers are actually consuming media
+ *     corresponding to the selection that was made. For example when playing media containing
+ *     multiple periods, the track selection for a period is made when the player starts to buffer
+ *     that period. Hence if the player's buffering policy is to maintain a 30 second buffer, the
+ *     selection will occur approximately 30 seconds in advance of it becoming active. In fact the
+ *     selection may never become active, for example if the user seeks to some other period of the
+ *     media during the 30 second gap. The player indicates to the track selector when a selection
+ *     it has previously made becomes active by calling {@link #onSelectionActivated(Object)}.</li>
+ *     <li>If the track selector wishes to indicate to the player that selections it has previously
+ *     made are invalid, it can do so by calling
+ *     {@link InvalidationListener#onTrackSelectionsInvalidated()} on the
+ *     {@link InvalidationListener} that was passed to {@link #init(InvalidationListener)}. A
+ *     track selector may wish to do this if its configuration has changed, for example if it now
+ *     wishes to prefer audio tracks in a particular language. This will trigger the player to make
+ *     new track selections. Note that the player will have to re-buffer in the case that the new
+ *     track selection for the currently playing period differs from the one that was invalidated.
+ *     </li>
+ * </ul>
+ *
+ * <h3>Renderer configuration</h3>
+ * The {@link TrackSelectorResult} returned by
+ * {@link #selectTracks(RendererCapabilities[], TrackGroupArray)} contains not only
+ * {@link TrackSelection}s for each renderer, but also {@link RendererConfiguration}s defining
+ * configuration parameters that the renderers should apply when consuming the corresponding media.
+ * Whilst it may seem counter-intuitive for a track selector to also specify renderer configuration
+ * information, in practice the two are tightly bound together. It may only be possible to play a
+ * certain combination tracks if the renderers are configured in a particular way. Equally, it may
+ * only be possible to configure renderers in a particular way if certain tracks are selected. Hence
+ * it makes sense to determined the track selection and corresponding renderer configurations in a
+ * single step.
+ *
+ * <h3>Threading model</h3>
+ * All calls made by the player into the track selector are on the player's internal playback
+ * thread. The track selector may call {@link InvalidationListener#onTrackSelectionsInvalidated()}
+ * from any thread.
+ */
 public abstract class TrackSelector {
 
   /**
-   * Notified when previous selections by a {@link TrackSelector} are no longer valid.
+   * Notified when selections previously made by a {@link TrackSelector} are no longer valid.
    */
   public interface InvalidationListener {
 
     /**
-     * Called by a {@link TrackSelector} when previous selections are no longer valid.
+     * Called by a {@link TrackSelector} to indicate that selections it has previously made are no
+     * longer valid. May be called from any thread.
      */
     void onTrackSelectionsInvalidated();
 
@@ -37,16 +92,17 @@ public abstract class TrackSelector {
   private InvalidationListener listener;
 
   /**
-   * Initializes the selector.
+   * Called by the player to initialize the selector.
    *
-   * @param listener A listener for the selector.
+   * @param listener An invalidation listener that the selector can call to indicate that selections
+   *     it has previously made are no longer valid.
    */
   public final void init(InvalidationListener listener) {
     this.listener = listener;
   }
 
   /**
-   * Performs a track selection for renderers.
+   * Called by the player to perform a track selection.
    *
    * @param rendererCapabilities The {@link RendererCapabilities} of the renderers for which tracks
    *     are to be selected.
@@ -58,15 +114,16 @@ public abstract class TrackSelector {
       TrackGroupArray trackGroups) throws ExoPlaybackException;
 
   /**
-   * Called when a {@link TrackSelectorResult} previously generated by
+   * Called by the player when a {@link TrackSelectorResult} previously generated by
    * {@link #selectTracks(RendererCapabilities[], TrackGroupArray)} is activated.
    *
-   * @param info The value of {@link TrackSelectorResult#info} in the activated result.
+   * @param info The value of {@link TrackSelectorResult#info} in the activated selection.
    */
   public abstract void onSelectionActivated(Object info);
 
   /**
-   * Invalidates all previously generated track selections.
+   * Calls {@link InvalidationListener#onTrackSelectionsInvalidated()} to invalidate all previously
+   * generated track selections.
    */
   protected final void invalidate() {
     if (listener != null) {

library/core/src/main/java/com/google/android/exoplayer2/trackselection/TrackSelectorResult.java

@@ -25,11 +25,11 @@ import com.google.android.exoplayer2.util.Util;
 public final class TrackSelectorResult {
 
   /**
-   * The groups provided to the {@link TrackSelector}.
+   * The track groups that were provided to the {@link TrackSelector}.
    */
   public final TrackGroupArray groups;
   /**
-   * A {@link TrackSelectionArray} containing the selection for each renderer.
+   * A {@link TrackSelectionArray} containing the track selection for each renderer.
    */
   public final TrackSelectionArray selections;
   /**
@@ -43,10 +43,10 @@ public final class TrackSelectorResult {
   public final RendererConfiguration[] rendererConfigurations;
 
   /**
-   * @param groups The groups provided to the {@link TrackSelector}.
+   * @param groups The track groups provided to the {@link TrackSelector}.
    * @param selections A {@link TrackSelectionArray} containing the selection for each renderer.
    * @param info An opaque object that will be returned to
-   *     {@link TrackSelector#onSelectionActivated(Object)} should the selections be activated.
+   *     {@link TrackSelector#onSelectionActivated(Object)} should the selection be activated.
    * @param rendererConfigurations A {@link RendererConfiguration} for each renderer, to be used
    *     with the selections.
    */
@@ -62,7 +62,7 @@ public final class TrackSelectorResult {
    * Returns whether this result is equivalent to {@code other} for all renderers.
    *
    * @param other The other {@link TrackSelectorResult}. May be null, in which case {@code false}
-   *     will be returned in all cases.
+   *     will be returned.
    * @return Whether this result is equivalent to {@code other} for all renderers.
    */
   public boolean isEquivalent(TrackSelectorResult other) {
@@ -83,9 +83,10 @@ public final class TrackSelectorResult {
    * renderer.
    *
    * @param other The other {@link TrackSelectorResult}. May be null, in which case {@code false}
-   *     will be returned in all cases.
+   *     will be returned.
    * @param index The renderer index to check for equivalence.
-   * @return Whether this result is equivalent to {@code other} for all renderers.
+   * @return Whether this result is equivalent to {@code other} for the renderer at the specified
+   *     index.
    */
   public boolean isEquivalent(TrackSelectorResult other, int index) {
     if (other == null) {