WebVTT styling.

- parse webvtt cue
- remove all tags from string (supported or not)
- apply spans for b, i and u
- honor class names in tags to properly parse the cue but do not apply styles for them

library/src/main/java/com/google/android/exoplayer/text/webvtt/WebvttCueParser.java

+/*
+ * Copyright (C) 2014 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.exoplayer.text.webvtt;
+
+import android.graphics.Typeface;
+import android.text.SpannableStringBuilder;
+import android.text.Spanned;
+import android.text.style.StyleSpan;
+import android.text.style.UnderlineSpan;
+import android.util.Log;
+
+import java.util.Stack;
+
+/**
+ * Parser for webvtt cue text. (https://w3c.github.io/webvtt/#cue-text)
+ */
+/* package */ final class WebvttCueParser {
+
+  private static final char CHAR_LESS_THAN = '<';
+  private static final char CHAR_GREATER_THAN = '>';
+  private static final char CHAR_SLASH = '/';
+  private static final char CHAR_AMPERSAND = '&';
+  private static final char CHAR_SEMI_COLON = ';';
+  private static final char CHAR_SPACE = ' ';
+  private static final String SPACE = " ";
+
+  private static final String ENTITY_LESS_THAN = "lt";
+  private static final String ENTITY_GREATER_THAN = "gt";
+  private static final String ENTITY_AMPERSAND = "amp";
+  private static final String ENTITY_NON_BREAK_SPACE = "nbsp";
+
+  private static final String TAG_BOLD = "b";
+  private static final String TAG_ITALIC = "i";
+  private static final String TAG_UNDERLINE = "u";
+  private static final String TAG_CLASS = "c";
+  private static final String TAG_VOICE = "v";
+  private static final String TAG_LANG = "lang";
+
+  private static final int STYLE_BOLD = Typeface.BOLD;
+  private static final int STYLE_ITALIC = Typeface.ITALIC;
+
+  private static final String TAG = "WebvttCueParser";
+
+  public Spanned parse(String markup) {
+    SpannableStringBuilder spannedText = new SpannableStringBuilder();
+    Stack<StartTag> startTagStack = new Stack<>();
+    String[] tagTokens;
+    int pos = 0;
+    while (pos < markup.length()) {
+      char curr = markup.charAt(pos);
+      switch (curr) {
+        case CHAR_LESS_THAN:
+          if (pos + 1 >= markup.length()) {
+            pos++;
+            break; // avoid ArrayOutOfBoundsException
+          }
+          int ltPos = pos;
+          boolean isClosingTag = markup.charAt(ltPos + 1) == CHAR_SLASH;
+          pos = findEndOfTag(markup, ltPos + 1);
+          boolean isVoidTag = markup.charAt(pos - 2) == CHAR_SLASH;
+
+          tagTokens = tokenizeTag(markup.substring(
+              ltPos + (isClosingTag ? 2 : 1), isVoidTag ? pos - 2 : pos - 1));
+          if (tagTokens == null || !isSupportedTag(tagTokens[0])) {
+            continue;
+          }
+          if (isClosingTag) {
+            StartTag startTag;
+            do {
+              if (startTagStack.isEmpty()) {
+                break;
+              }
+              startTag = startTagStack.pop();
+              applySpansForTag(startTag, spannedText);
+            } while(!startTag.name.equals(tagTokens[0]));
+          } else if (!isVoidTag) {
+            startTagStack.push(new StartTag(tagTokens[0], spannedText.length()));
+          }
+          break;
+        case CHAR_AMPERSAND:
+          int semiColonEnd = markup.indexOf(CHAR_SEMI_COLON, pos + 1);
+          int spaceEnd = markup.indexOf(CHAR_SPACE, pos + 1);
+          int entityEnd = semiColonEnd == -1 ? spaceEnd
+              : spaceEnd == -1 ? semiColonEnd : Math.min(semiColonEnd, spaceEnd);
+          if (entityEnd != -1) {
+            applyEntity(markup.substring(pos + 1, entityEnd), spannedText);
+            if (entityEnd == spaceEnd) {
+              spannedText.append(" ");
+            }
+            pos = entityEnd + 1;
+          } else {
+            spannedText.append(curr);
+            pos++;
+          }
+          break;
+        default:
+          spannedText.append(curr);
+          pos++;
+          break;
+      }
+    }
+    // apply unclosed tags
+    while (!startTagStack.isEmpty()) {
+      applySpansForTag(startTagStack.pop(), spannedText);
+    }
+    return spannedText;
+  }
+
+  /**
+   * Find end of tag (&gt;). The position returned is the position of the &gt; plus one (exclusive).
+   *
+   * @param markup The webvtt cue markup to be parsed.
+   * @param startPos the position from where to start searching for the end of tag.
+   * @return the position of the end of tag plus 1 (one).
+   */
+  private int findEndOfTag(String markup, int startPos) {
+    int idx = markup.indexOf(CHAR_GREATER_THAN, startPos);
+    return idx == -1 ? markup.length() : idx + 1;
+  }
+
+  private void applyEntity(String entity, SpannableStringBuilder spannedText) {
+    switch (entity) {
+      case ENTITY_LESS_THAN:
+        spannedText.append('<');
+        break;
+      case ENTITY_GREATER_THAN:
+        spannedText.append('>');
+        break;
+      case ENTITY_NON_BREAK_SPACE:
+        spannedText.append(' ');
+        break;
+      case ENTITY_AMPERSAND:
+        spannedText.append('&');
+        break;
+      default:
+        Log.w(TAG, "ignoring unsupported entity: '&" + entity + ";'");
+        break;
+    }
+  }
+
+  private boolean isSupportedTag(String tagName) {
+    switch (tagName) {
+      case TAG_BOLD:
+      case TAG_CLASS:
+      case TAG_ITALIC:
+      case TAG_LANG:
+      case TAG_UNDERLINE:
+      case TAG_VOICE:
+        return true;
+      default:
+        return false;
+    }
+  }
+
+  private void applySpansForTag(StartTag startTag, SpannableStringBuilder spannedText) {
+    switch(startTag.name) {
+      case TAG_BOLD:
+        spannedText.setSpan(new StyleSpan(STYLE_BOLD), startTag.position,
+            spannedText.length(), Spanned.SPAN_EXCLUSIVE_EXCLUSIVE);
+        return;
+      case TAG_ITALIC:
+        spannedText.setSpan(new StyleSpan(STYLE_ITALIC), startTag.position,
+            spannedText.length(), Spanned.SPAN_EXCLUSIVE_EXCLUSIVE);
+        return;
+      case TAG_UNDERLINE:
+        spannedText.setSpan(new UnderlineSpan(), startTag.position,
+            spannedText.length(), Spanned.SPAN_EXCLUSIVE_EXCLUSIVE);
+        return;
+      default:
+        break;
+    }
+  }
+
+  /**
+   * Tokenizes a tag expression into tag name (pos 0) and classes (pos 1..n).
+   *
+   * @param fullTagExpression characters between &amp;lt: and &amp;gt; of a start or end tag
+   * @return an array of <code>String</code>s with the tag name at pos 0 followed by style classes
+   *    or null if it's an empty tag: '&lt;&gt;'
+   */
+  private String[] tokenizeTag(String fullTagExpression) {
+    fullTagExpression = fullTagExpression.replace("\\s+", " ").trim();
+    if (fullTagExpression.length() == 0) {
+      return null;
+    }
+    if (fullTagExpression.contains(SPACE)) {
+      fullTagExpression = fullTagExpression.substring(0, fullTagExpression.indexOf(SPACE));
+    }
+    return fullTagExpression.split("\\.");
+  }
+
+  private static final class StartTag {
+
+    public final String name;
+    public final int position;
+
+    public StartTag(String name, int position) {
+      this.position = position;
+      this.name = name;
+    }
+
+  }
+
+}

library/src/main/java/com/google/android/exoplayer/text/webvtt/WebvttParser.java

@@ -21,7 +21,6 @@ import com.google.android.exoplayer.text.Cue;
 import com.google.android.exoplayer.text.SubtitleParser;
 import com.google.android.exoplayer.util.MimeTypes;
 
-import android.text.Html;
 import android.text.Layout.Alignment;
 import android.text.TextUtils;
 import android.util.Log;
@@ -48,10 +47,12 @@ public final class WebvttParser implements SubtitleParser {
   private static final Pattern CUE_HEADER = Pattern.compile("^(\\S+)\\s+-->\\s+(\\S+)(.*)?$");
   private static final Pattern CUE_SETTING = Pattern.compile("(\\S+?):(\\S+)");
 
+  private final WebvttCueParser cueParser;
   private final PositionHolder positionHolder;
   private final StringBuilder textBuilder;
 
   public WebvttParser() {
+    this.cueParser = new WebvttCueParser();
     positionHolder = new PositionHolder();
     textBuilder = new StringBuilder();
   }
@@ -130,11 +131,12 @@ public final class WebvttParser implements SubtitleParser {
       String line;
       while ((line = webvttData.readLine()) != null && !line.isEmpty()) {
         if (textBuilder.length() > 0) {
-          textBuilder.append("<br>");
+          textBuilder.append("\n");
         }
         textBuilder.append(line.trim());
       }
-      CharSequence cueText = Html.fromHtml(textBuilder.toString());
+
+      CharSequence cueText = cueParser.parse(textBuilder.toString());
 
       WebvttCue cue = new WebvttCue(cueStartTime, cueEndTime, cueText, cueTextAlignment, cueLine,
           cueLineType, cueLineAnchor, cuePosition, cuePositionAnchor, cueWidth);