Add Player.getCurrentLiveOffset convenience method.

This returns the current offset to the live edge. The calculation is
non-intuitive enough to provide this convenience method.

PiperOrigin-RevId: 285171090

RELEASENOTES.md

@@ -24,6 +24,7 @@
 * Add `SpannedSubject` to testutils, for assertions on
   [Span-styled text]( https://developer.android.com/guide/topics/text/spans)
   (e.g. subtitles).
+* Add `Player.getCurrentLiveOffset` to conveniently return the live offset.
 
 ### 2.11.0 (2019-12-11) ###
 

library/core/src/main/java/com/google/android/exoplayer2/BasePlayer.java

@@ -144,6 +144,19 @@ public abstract class BasePlayer implements Player {
   }
 
   @Override
+  public final long getCurrentLiveOffset() {
+    Timeline timeline = getCurrentTimeline();
+    if (timeline.isEmpty()) {
+      return C.TIME_UNSET;
+    }
+    long windowStartTimeMs = timeline.getWindow(getCurrentWindowIndex(), window).windowStartTimeMs;
+    if (windowStartTimeMs == C.TIME_UNSET) {
+      return C.TIME_UNSET;
+    }
+    return System.currentTimeMillis() - window.windowStartTimeMs - getContentPosition();
+  }
+
+  @Override
   public final boolean isCurrentWindowSeekable() {
     Timeline timeline = getCurrentTimeline();
     return !timeline.isEmpty() && timeline.getWindow(getCurrentWindowIndex(), window).isSeekable;

library/core/src/main/java/com/google/android/exoplayer2/Player.java

@@ -1001,6 +1001,19 @@ public interface Player {
   boolean isCurrentWindowLive();
 
   /**
+   * Returns the offset of the current playback position from the live edge in milliseconds, or
+   * {@link C#TIME_UNSET} if the current window {@link #isCurrentWindowLive() isn't live} or the
+   * offset is unknown.
+   *
+   * <p>The offset is calculated as {@code currentTime - playbackPosition}, so should usually be
+   * positive.
+   *
+   * <p>Note that this offset may rely on an accurate local time, so this method may return an
+   * incorrect value if the difference between system clock and server clock is unknown.
+   */
+  long getCurrentLiveOffset();
+
+  /**
    * Returns whether the current window is seekable, or {@code false} if the {@link Timeline} is
    * empty.
    *