Add RubySpan

This will be used when parsing Ruby info from WebVTT and TTML/IMSC
subtitles.

PiperOrigin-RevId: 288280181

library/core/src/main/java/com/google/android/exoplayer2/text/span/RubySpan.java

+/*
+ * Copyright (C) 2020 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.text.span;
+
+import static java.lang.annotation.RetentionPolicy.SOURCE;
+
+import androidx.annotation.IntDef;
+import java.lang.annotation.Documented;
+import java.lang.annotation.Retention;
+
+/**
+ * A styling span for ruby text.
+ *
+ * <p>The text covered by this span is known as the "base text", and the ruby text is stored in
+ * {@link #rubyText}.
+ *
+ * <p>More information on <a href="https://en.wikipedia.org/wiki/Ruby_character">ruby characters</a>
+ * and <a href="https://developer.android.com/guide/topics/text/spans">span styling</a>.
+ */
+// NOTE: There's no Android layout support for rubies, so this span currently doesn't extend any
+// styling superclasses (e.g. MetricAffectingSpan). The only way to render these rubies is to
+// extract the spans and do the layout manually.
+// TODO: Consider adding support for parenthetical text to be used when rendering doesn't support
+// rubies (e.g. HTML <rp> tag).
+public final class RubySpan {
+
+  /** The ruby position is unknown. */
+  public static final int POSITION_UNKNOWN = -1;
+
+  /**
+   * The ruby text should be positioned above the base text.
+   *
+   * <p>For vertical text it should be positioned to the right, same as CSS's <a
+   * href="https://developer.mozilla.org/en-US/docs/Web/CSS/ruby-position">ruby-position</a>.
+   */
+  public static final int POSITION_OVER = 1;
+
+  /**
+   * The ruby text should be positioned below the base text.
+   *
+   * <p>For vertical text it should be positioned to the left, same as CSS's <a
+   * href="https://developer.mozilla.org/en-US/docs/Web/CSS/ruby-position">ruby-position</a>.
+   */
+  public static final int POSITION_UNDER = 2;
+
+  /**
+   * The possible positions of the ruby text relative to the base text.
+   *
+   * <p>One of:
+   *
+   * <ul>
+   *   <li>{@link #POSITION_UNKNOWN}
+   *   <li>{@link #POSITION_OVER}
+   *   <li>{@link #POSITION_UNDER}
+   * </ul>
+   */
+  @Documented
+  @Retention(SOURCE)
+  @IntDef({POSITION_UNKNOWN, POSITION_OVER, POSITION_UNDER})
+  public @interface Position {}
+
+  /** The ruby text, i.e. the smaller explanatory characters. */
+  public final String rubyText;
+
+  /** The position of the ruby text relative to the base text. */
+  @Position public final int position;
+
+  public RubySpan(String rubyText, @Position int position) {
+    this.rubyText = rubyText;
+    this.position = position;
+  }
+}