core/java/android/text/Html.java

/*
 * Copyright (C) 2007 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 android.text;

import android.graphics.Color;
import com.android.internal.util.ArrayUtils;
import org.ccil.cowan.tagsoup.HTMLSchema;
import org.ccil.cowan.tagsoup.Parser;
import org.xml.sax.Attributes;
import org.xml.sax.ContentHandler;
import org.xml.sax.InputSource;
import org.xml.sax.Locator;
import org.xml.sax.SAXException;
import org.xml.sax.XMLReader;

import android.content.res.ColorStateList;
import android.content.res.Resources;
import android.graphics.Typeface;
import android.graphics.drawable.Drawable;
import android.text.style.AbsoluteSizeSpan;
import android.text.style.AlignmentSpan;
import android.text.style.CharacterStyle;
import android.text.style.ForegroundColorSpan;
import android.text.style.ImageSpan;
import android.text.style.ParagraphStyle;
import android.text.style.QuoteSpan;
import android.text.style.RelativeSizeSpan;
import android.text.style.StrikethroughSpan;
import android.text.style.StyleSpan;
import android.text.style.SubscriptSpan;
import android.text.style.SuperscriptSpan;
import android.text.style.TextAppearanceSpan;
import android.text.style.TypefaceSpan;
import android.text.style.URLSpan;
import android.text.style.UnderlineSpan;

import com.android.internal.util.XmlUtils;

import java.io.IOException;
import java.io.StringReader;
import java.util.HashMap;

/**
 * This class processes HTML strings into displayable styled text.
 * Not all HTML tags are supported.
 */
public class Html {
    /**
     * Retrieves images for HTML <img> tags.
     */
    public static interface ImageGetter {
        /**
         * This methos is called when the HTML parser encounters an
         * &lt;img&gt; tag.  The <code>source</code> argument is the
         * string from the "src" attribute; the return value should be
         * a Drawable representation of the image or <code>null</code>
         * for a generic replacement image.  Make sure you call
         * setBounds() on your Drawable if it doesn't already have
         * its bounds set.
         */
        public Drawable getDrawable(String source);
    }

    /**
     * Is notified when HTML tags are encountered that the parser does
     * not know how to interpret.
     */
    public static interface TagHandler {
        /**
         * This method will be called whenn the HTML parser encounters
         * a tag that it does not know how to interpret.
         */
        public void handleTag(boolean opening, String tag,
                                 Editable output, XMLReader xmlReader);
    }

    private Html() { }

    /**
     * Returns displayable styled text from the provided HTML string.
     * Any &lt;img&gt; tags in the HTML will display as a generic
     * replacement image which your program can then go through and
     * replace with real images.
     *
     * <p>This uses TagSoup to handle real HTML, including all of the brokenness found in the wild.
     */
    public static Spanned fromHtml(String source) {
        return fromHtml(source, null, null);
    }

    /**
     * Lazy initialization holder for HTML parser. This class will
     * a) be preloaded by the zygote, or b) not loaded until absolutely
     * necessary.
     */
    private static class HtmlParser {
        private static final HTMLSchema schema = new HTMLSchema();
    }

    /**
     * Returns displayable styled text from the provided HTML string.
     * Any &lt;img&gt; tags in the HTML will use the specified ImageGetter
     * to request a representation of the image (use null if you don't
     * want this) and the specified TagHandler to handle unknown tags
     * (specify null if you don't want this).
     *
     * <p>This uses TagSoup to handle real HTML, including all of the brokenness found in the wild.
     */
    public static Spanned fromHtml(String source, ImageGetter imageGetter,
                                   TagHandler tagHandler) {
        Parser parser = new Parser();
        try {
            parser.setProperty(Parser.schemaProperty, HtmlParser.schema);
        } catch (org.xml.sax.SAXNotRecognizedException e) {
            // Should not happen.
            throw new RuntimeException(e);
        } catch (org.xml.sax.SAXNotSupportedException e) {
            // Should not happen.
            throw new RuntimeException(e);
        }

        HtmlToSpannedConverter converter =
                new HtmlToSpannedConverter(source, imageGetter, tagHandler,
                        parser);
        return converter.convert();
    }

    /**
     * Returns an HTML representation of the provided Spanned text.
     */
    public static String toHtml(Spanned text) {
        StringBuilder out = new StringBuilder();
        withinHtml(out, text);
        return out.toString();
    }

    /**
     * Returns an HTML escaped representation of the given plain text.
     */
    public static String escapeHtml(CharSequence text) {
        StringBuilder out = new StringBuilder();
        withinStyle(out, text, 0, text.length());
        return out.toString();
    }

    private static void withinHtml(StringBuilder out, Spanned text) {
        int len = text.length();

        int next;
        for (int i = 0; i < text.length(); i = next) {
            next = text.nextSpanTransition(i, len, ParagraphStyle.class);
            ParagraphStyle[] style = text.getSpans(i, next, ParagraphStyle.class);
            String elements = " ";
            boolean needDiv = false;

            for(int j = 0; j < style.length; j++) {
                if (style[j] instanceof AlignmentSpan) {
                    Layout.Alignment align =
                        ((AlignmentSpan) style[j]).getAlignment();
                    needDiv = true;
                    if (align == Layout.Alignment.ALIGN_CENTER) {
                        elements = "align=\"center\" " + elements;
                    } else if (align == Layout.Alignment.ALIGN_OPPOSITE) {
                        elements = "align=\"right\" " + elements;
                    } else {
                        elements = "align=\"left\" " + elements;
                    }
                }
            }
            if (needDiv) {
                out.append("<div ").append(elements).append(">");
            }

            withinDiv(out, text, i, next);

            if (needDiv) {
                out.append("</div>");
            }
        }
    }

    private static void withinDiv(StringBuilder out, Spanned text,
            int start, int end) {
        int next;
        for (int i = start; i < end; i = next) {
            next = text.nextSpanTransition(i, end, QuoteSpan.class);
            QuoteSpan[] quotes = text.getSpans(i, next, QuoteSpan.class);

            for (QuoteSpan quote : quotes) {
                out.append("<blockquote>");
            }

            withinBlockquote(out, text, i, next);

            for (QuoteSpan quote : quotes) {
                out.append("</blockquote>\n");
            }
        }
    }

    private static String getOpenParaTagWithDirection(Spanned text, int start, int end) {
        final int len = end - start;
        final byte[] levels = new byte[ArrayUtils.idealByteArraySize(len)];
        final char[] buffer = TextUtils.obtain(len);
        TextUtils.getChars(text, start, end, buffer, 0);

        int paraDir = AndroidBidi.bidi(Layout.DIR_REQUEST_DEFAULT_LTR, buffer, levels, len,
                false /* no info */);
        switch(paraDir) {
            case Layout.DIR_RIGHT_TO_LEFT:
                return "<p dir=\"rtl\">";
            case Layout.DIR_LEFT_TO_RIGHT:
            default:
                return "<p dir=\"ltr\">";
        }
    }

    private static void withinBlockquote(StringBuilder out, Spanned text,
                                         int start, int end) {
        out.append(getOpenParaTagWithDirection(text, start, end));

        int next;
        for (int i = start; i < end; i = next) {
            next = TextUtils.indexOf(text, '\n', i, end);
            if (next < 0) {
                next = end;
            }

            int nl = 0;

            while (next < end && text.charAt(next) == '\n') {
                nl++;
                next++;
            }

            withinParagraph(out, text, i, next - nl, nl, next == end);
        }

        out.append("</p>\n");
    }

    private static void withinParagraph(StringBuilder out, Spanned text,
                                        int start, int end, int nl,
                                        boolean last) {
        int next;
        for (int i = start; i < end; i = next) {
            next = text.nextSpanTransition(i, end, CharacterStyle.class);
            CharacterStyle[] style = text.getSpans(i, next,
                                                   CharacterStyle.class);

            for (int j = 0; j < style.length; j++) {
                if (style[j] instanceof StyleSpan) {
                    int s = ((StyleSpan) style[j]).getStyle();

                    if ((s & Typeface.BOLD) != 0) {
                        out.append("<b>");
                    }
                    if ((s & Typeface.ITALIC) != 0) {
                        out.append("<i>");
                    }
                }
                if (style[j] instanceof TypefaceSpan) {
                    String s = ((TypefaceSpan) style[j]).getFamily();

                    if (s.equals("monospace")) {
                        out.append("<tt>");
                    }
                }
                if (style[j] instanceof SuperscriptSpan) {
                    out.append("<sup>");
                }
                if (style[j] instanceof SubscriptSpan) {
                    out.append("<sub>");
                }
                if (style[j] instanceof UnderlineSpan) {
                    out.append("<u>");
                }
                if (style[j] instanceof StrikethroughSpan) {
                    out.append("<strike>");
                }
                if (style[j] instanceof URLSpan) {
                    out.append("<a href=\"");
                    out.append(((URLSpan) style[j]).getURL());
                    out.append("\">");
                }
                if (style[j] instanceof ImageSpan) {
                    out.append("<img src=\"");
                    out.append(((ImageSpan) style[j]).getSource());
                    out.append("\">");

                    // Don't output the dummy character underlying the image.
                    i = next;
                }
                if (style[j] instanceof AbsoluteSizeSpan) {
                    out.append("<font size =\"");
                    out.append(((AbsoluteSizeSpan) style[j]).getSize() / 6);
                    out.append("\">");
                }
                if (style[j] instanceof ForegroundColorSpan) {
                    out.append("<font color =\"#");
                    String color = Integer.toHexString(((ForegroundColorSpan)
                            style[j]).getForegroundColor() + 0x01000000);
                    while (color.length() < 6) {
                        color = "0" + color;
                    }
                    out.append(color);
                    out.append("\">");
                }
            }

            withinStyle(out, text, i, next);

            for (int j = style.length - 1; j >= 0; j--) {
                if (style[j] instanceof ForegroundColorSpan) {
                    out.append("</font>");
                }
                if (style[j] instanceof AbsoluteSizeSpan) {
                    out.append("</font>");
                }
                if (style[j] instanceof URLSpan) {
                    out.append("</a>");
                }
                if (style[j] instanceof StrikethroughSpan) {
                    out.append("</strike>");
                }
                if (style[j] instanceof UnderlineSpan) {
                    out.append("</u>");
                }
                if (style[j] instanceof SubscriptSpan) {
                    out.append("</sub>");
                }
                if (style[j] instanceof SuperscriptSpan) {
                    out.append("</sup>");
                }
                if (style[j] instanceof TypefaceSpan) {
                    String s = ((TypefaceSpan) style[j]).getFamily();

                    if (s.equals("monospace")) {
                        out.append("</tt>");
                    }
                }
                if (style[j] instanceof StyleSpan) {
                    int s = ((StyleSpan) style[j]).getStyle();

                    if ((s & Typeface.BOLD) != 0) {
                        out.append("</b>");
                    }
                    if ((s & Typeface.ITALIC) != 0) {
                        out.append("</i>");
                    }
                }
            }
        }

        String p = last ? "" : "</p>\n" + getOpenParaTagWithDirection(text, start, end);

        if (nl == 1) {
            out.append("<br>\n");
        } else if (nl == 2) {
            out.append(p);
        } else {
            for (int i = 2; i < nl; i++) {
                out.append("<br>");
            }
            out.append(p);
        }
    }

    private static void withinStyle(StringBuilder out, CharSequence text,
                                    int start, int end) {
        for (int i = start; i < end; i++) {
            char c = text.charAt(i);

            if (c == '<') {
                out.append("&lt;");
            } else if (c == '>') {
                out.append("&gt;");
            } else if (c == '&') {
                out.append("&amp;");
            } else if (c >= 0xD800 && c <= 0xDFFF) {
                if (c < 0xDC00 && i + 1 < end) {
                    char d = text.charAt(i + 1);
                    if (d >= 0xDC00 && d <= 0xDFFF) {
                        i++;
                        int codepoint = 0x010000 | (int) c - 0xD800 << 10 | (int) d - 0xDC00;
                        out.append("&#").append(codepoint).append(";");
                    }
                }
            } else if (c > 0x7E || c < ' ') {
                out.append("&#").append((int) c).append(";");
            } else if (c == ' ') {
                while (i + 1 < end && text.charAt(i + 1) == ' ') {
                    out.append("&nbsp;");
                    i++;
                }

                out.append(' ');
            } else {
                out.append(c);
            }
        }
    }
}

class HtmlToSpannedConverter implements ContentHandler {

    private static final float[] HEADER_SIZES = {
        1.5f, 1.4f, 1.3f, 1.2f, 1.1f, 1f,
    };

    private String mSource;
    private XMLReader mReader;
    private SpannableStringBuilder mSpannableStringBuilder;
    private Html.ImageGetter mImageGetter;
    private Html.TagHandler mTagHandler;

    public HtmlToSpannedConverter(
            String source, Html.ImageGetter imageGetter, Html.TagHandler tagHandler,
            Parser parser) {
        mSource = source;
        mSpannableStringBuilder = new SpannableStringBuilder();
        mImageGetter = imageGetter;
        mTagHandler = tagHandler;
        mReader = parser;
    }

    public Spanned convert() {

        mReader.setContentHandler(this);
        try {
            mReader.parse(new InputSource(new StringReader(mSource)));
        } catch (IOException e) {
            // We are reading from a string. There should not be IO problems.
            throw new RuntimeException(e);
        } catch (SAXException e) {
            // TagSoup doesn't throw parse exceptions.
            throw new RuntimeException(e);
        }

        // Fix flags and range for paragraph-type markup.
        Object[] obj = mSpannableStringBuilder.getSpans(0, mSpannableStringBuilder.length(), ParagraphStyle.class);
        for (int i = 0; i < obj.length; i++) {
            int start = mSpannableStringBuilder.getSpanStart(obj[i]);
            int end = mSpannableStringBuilder.getSpanEnd(obj[i]);

            // If the last line of the range is blank, back off by one.
            if (end - 2 >= 0) {
                if (mSpannableStringBuilder.charAt(end - 1) == '\n' &&
                    mSpannableStringBuilder.charAt(end - 2) == '\n') {
                    end--;
                }
            }

            if (end == start) {
                mSpannableStringBuilder.removeSpan(obj[i]);
            } else {
                mSpannableStringBuilder.setSpan(obj[i], start, end, Spannable.SPAN_PARAGRAPH);
            }
        }

        return mSpannableStringBuilder;
    }

    private void handleStartTag(String tag, Attributes attributes) {
        if (tag.equalsIgnoreCase("br")) {
            // We don't need to handle this. TagSoup will ensure that there's a </br> for each <br>
            // so we can safely emite the linebreaks when we handle the close tag.
        } else if (tag.equalsIgnoreCase("p")) {
            handleP(mSpannableStringBuilder);
        } else if (tag.equalsIgnoreCase("div")) {
            handleP(mSpannableStringBuilder);
        } else if (tag.equalsIgnoreCase("strong")) {
            start(mSpannableStringBuilder, new Bold());
        } else if (tag.equalsIgnoreCase("b")) {
            start(mSpannableStringBuilder, new Bold());
        } else if (tag.equalsIgnoreCase("em")) {
            start(mSpannableStringBuilder, new Italic());
        } else if (tag.equalsIgnoreCase("cite")) {
            start(mSpannableStringBuilder, new Italic());
        } else if (tag.equalsIgnoreCase("dfn")) {
            start(mSpannableStringBuilder, new Italic());
        } else if (tag.equalsIgnoreCase("i")) {
            start(mSpannableStringBuilder, new Italic());
        } else if (tag.equalsIgnoreCase("big")) {
            start(mSpannableStringBuilder, new Big());
        } else if (tag.equalsIgnoreCase("small")) {
            start(mSpannableStringBuilder, new Small());
        } else if (tag.equalsIgnoreCase("font")) {
            startFont(mSpannableStringBuilder, attributes);
        } else if (tag.equalsIgnoreCase("blockquote")) {
            handleP(mSpannableStringBuilder);
            start(mSpannableStringBuilder, new Blockquote());
        } else if (tag.equalsIgnoreCase("tt")) {
            start(mSpannableStringBuilder, new Monospace());
        } else if (tag.equalsIgnoreCase("a")) {
            startA(mSpannableStringBuilder, attributes);
        } else if (tag.equalsIgnoreCase("u")) {
            start(mSpannableStringBuilder, new Underline());
        } else if (tag.equalsIgnoreCase("sup")) {
            start(mSpannableStringBuilder, new Super());
        } else if (tag.equalsIgnoreCase("sub")) {
            start(mSpannableStringBuilder, new Sub());
        } else if (tag.length() == 2 &&
                   Character.toLowerCase(tag.charAt(0)) == 'h' &&
                   tag.charAt(1) >= '1' && tag.charAt(1) <= '6') {
            handleP(mSpannableStringBuilder);
            start(mSpannableStringBuilder, new Header(tag.charAt(1) - '1'));
        } else if (tag.equalsIgnoreCase("img")) {
            startImg(mSpannableStringBuilder, attributes, mImageGetter);
        } else if (mTagHandler != null) {
            mTagHandler.handleTag(true, tag, mSpannableStringBuilder, mReader);
        }
    }

    private void handleEndTag(String tag) {
        if (tag.equalsIgnoreCase("br")) {
            handleBr(mSpannableStringBuilder);
        } else if (tag.equalsIgnoreCase("p")) {
            handleP(mSpannableStringBuilder);
        } else if (tag.equalsIgnoreCase("div")) {
            handleP(mSpannableStringBuilder);
        } else if (tag.equalsIgnoreCase("strong")) {
            end(mSpannableStringBuilder, Bold.class, new StyleSpan(Typeface.BOLD));
        } else if (tag.equalsIgnoreCase("b")) {
            end(mSpannableStringBuilder, Bold.class, new StyleSpan(Typeface.BOLD));
        } else if (tag.equalsIgnoreCase("em")) {
            end(mSpannableStringBuilder, Italic.class, new StyleSpan(Typeface.ITALIC));
        } else if (tag.equalsIgnoreCase("cite")) {
            end(mSpannableStringBuilder, Italic.class, new StyleSpan(Typeface.ITALIC));
        } else if (tag.equalsIgnoreCase("dfn")) {
            end(mSpannableStringBuilder, Italic.class, new StyleSpan(Typeface.ITALIC));
        } else if (tag.equalsIgnoreCase("i")) {
            end(mSpannableStringBuilder, Italic.class, new StyleSpan(Typeface.ITALIC));
        } else if (tag.equalsIgnoreCase("big")) {
            end(mSpannableStringBuilder, Big.class, new RelativeSizeSpan(1.25f));
        } else if (tag.equalsIgnoreCase("small")) {
            end(mSpannableStringBuilder, Small.class, new RelativeSizeSpan(0.8f));
        } else if (tag.equalsIgnoreCase("font")) {
            endFont(mSpannableStringBuilder);
        } else if (tag.equalsIgnoreCase("blockquote")) {
            handleP(mSpannableStringBuilder);
            end(mSpannableStringBuilder, Blockquote.class, new QuoteSpan());
        } else if (tag.equalsIgnoreCase("tt")) {
            end(mSpannableStringBuilder, Monospace.class,
                    new TypefaceSpan("monospace"));
        } else if (tag.equalsIgnoreCase("a")) {
            endA(mSpannableStringBuilder);
        } else if (tag.equalsIgnoreCase("u")) {
            end(mSpannableStringBuilder, Underline.class, new UnderlineSpan());
        } else if (tag.equalsIgnoreCase("sup")) {
            end(mSpannableStringBuilder, Super.class, new SuperscriptSpan());
        } else if (tag.equalsIgnoreCase("sub")) {
            end(mSpannableStringBuilder, Sub.class, new SubscriptSpan());
        } else if (tag.length() == 2 &&
                Character.toLowerCase(tag.charAt(0)) == 'h' &&
                tag.charAt(1) >= '1' && tag.charAt(1) <= '6') {
            handleP(mSpannableStringBuilder);
            endHeader(mSpannableStringBuilder);
        } else if (mTagHandler != null) {
            mTagHandler.handleTag(false, tag, mSpannableStringBuilder, mReader);
        }
    }

    private static void handleP(SpannableStringBuilder text) {
        int len = text.length();

        if (len >= 1 && text.charAt(len - 1) == '\n') {
            if (len >= 2 && text.charAt(len - 2) == '\n') {
                return;
            }

            text.append("\n");
            return;
        }

        if (len != 0) {
            text.append("\n\n");
        }
    }

    private static void handleBr(SpannableStringBuilder text) {
        text.append("\n");
    }

    private static Object getLast(Spanned text, Class kind) {
        /*
         * This knows that the last returned object from getSpans()
         * will be the most recently added.
         */
        Object[] objs = text.getSpans(0, text.length(), kind);

        if (objs.length == 0) {
            return null;
        } else {
            return objs[objs.length - 1];
        }
    }

    private static void start(SpannableStringBuilder text, Object mark) {
        int len = text.length();
        text.setSpan(mark, len, len, Spannable.SPAN_MARK_MARK);
    }

    private static void end(SpannableStringBuilder text, Class kind,
                            Object repl) {
        int len = text.length();
        Object obj = getLast(text, kind);
        int where = text.getSpanStart(obj);

        text.removeSpan(obj);

        if (where != len) {
            text.setSpan(repl, where, len, Spannable.SPAN_EXCLUSIVE_EXCLUSIVE);
        }
    }

    private static void startImg(SpannableStringBuilder text,
                                 Attributes attributes, Html.ImageGetter img) {
        String src = attributes.getValue("", "src");
        Drawable d = null;

        if (img != null) {
            d = img.getDrawable(src);
        }

        if (d == null) {
            d = Resources.getSystem().
                    getDrawable(com.android.internal.R.drawable.unknown_image);
            d.setBounds(0, 0, d.getIntrinsicWidth(), d.getIntrinsicHeight());
        }

        int len = text.length();
        text.append("\uFFFC");

        text.setSpan(new ImageSpan(d, src), len, text.length(),
                     Spannable.SPAN_EXCLUSIVE_EXCLUSIVE);
    }

    private static void startFont(SpannableStringBuilder text,
                                  Attributes attributes) {
        String color = attributes.getValue("", "color");
        String face = attributes.getValue("", "face");

        int len = text.length();
        text.setSpan(new Font(color, face), len, len, Spannable.SPAN_MARK_MARK);
    }

    private static void endFont(SpannableStringBuilder text) {
        int len = text.length();
        Object obj = getLast(text, Font.class);
        int where = text.getSpanStart(obj);

        text.removeSpan(obj);

        if (where != len) {
            Font f = (Font) obj;

            if (!TextUtils.isEmpty(f.mColor)) {
                if (f.mColor.startsWith("@")) {
                    Resources res = Resources.getSystem();
                    String name = f.mColor.substring(1);
                    int colorRes = res.getIdentifier(name, "color", "android");
                    if (colorRes != 0) {
                        ColorStateList colors = res.getColorStateList(colorRes);
                        text.setSpan(new TextAppearanceSpan(null, 0, 0, colors, null),
                                where, len,
                                Spannable.SPAN_EXCLUSIVE_EXCLUSIVE);
                    }
                } else {
                    int c = Color.getHtmlColor(f.mColor);
                    if (c != -1) {
                        text.setSpan(new ForegroundColorSpan(c | 0xFF000000),
                                where, len,
                                Spannable.SPAN_EXCLUSIVE_EXCLUSIVE);
                    }
                }
            }

            if (f.mFace != null) {
                text.setSpan(new TypefaceSpan(f.mFace), where, len,
                             Spannable.SPAN_EXCLUSIVE_EXCLUSIVE);
            }
        }
    }

    private static void startA(SpannableStringBuilder text, Attributes attributes) {
        String href = attributes.getValue("", "href");

        int len = text.length();
        text.setSpan(new Href(href), len, len, Spannable.SPAN_MARK_MARK);
    }

    private static void endA(SpannableStringBuilder text) {
        int len = text.length();
        Object obj = getLast(text, Href.class);
        int where = text.getSpanStart(obj);

        text.removeSpan(obj);

        if (where != len) {
            Href h = (Href) obj;

            if (h.mHref != null) {
                text.setSpan(new URLSpan(h.mHref), where, len,
                             Spannable.SPAN_EXCLUSIVE_EXCLUSIVE);
            }
        }
    }

    private static void endHeader(SpannableStringBuilder text) {
        int len = text.length();
        Object obj = getLast(text, Header.class);

        int where = text.getSpanStart(obj);

        text.removeSpan(obj);

        // Back off not to change only the text, not the blank line.
        while (len > where && text.charAt(len - 1) == '\n') {
            len--;
        }

        if (where != len) {
            Header h = (Header) obj;

            text.setSpan(new RelativeSizeSpan(HEADER_SIZES[h.mLevel]),
                         where, len, Spannable.SPAN_EXCLUSIVE_EXCLUSIVE);
            text.setSpan(new StyleSpan(Typeface.BOLD),
                         where, len, Spannable.SPAN_EXCLUSIVE_EXCLUSIVE);
        }
    }

    public void setDocumentLocator(Locator locator) {
    }

    public void startDocument() throws SAXException {
    }

    public void endDocument() throws SAXException {
    }

    public void startPrefixMapping(String prefix, String uri) throws SAXException {
    }

    public void endPrefixMapping(String prefix) throws SAXException {
    }

    public void startElement(String uri, String localName, String qName, Attributes attributes)
            throws SAXException {
        handleStartTag(localName, attributes);
    }

    public void endElement(String uri, String localName, String qName) throws SAXException {
        handleEndTag(localName);
    }

    public void characters(char ch[], int start, int length) throws SAXException {
        StringBuilder sb = new StringBuilder();

        /*
         * Ignore whitespace that immediately follows other whitespace;
         * newlines count as spaces.
         */

        for (int i = 0; i < length; i++) {
            char c = ch[i + start];

            if (c == ' ' || c == '\n') {
                char pred;
                int len = sb.length();

                if (len == 0) {
                    len = mSpannableStringBuilder.length();

                    if (len == 0) {
                        pred = '\n';
                    } else {
                        pred = mSpannableStringBuilder.charAt(len - 1);
                    }
                } else {
                    pred = sb.charAt(len - 1);
                }

                if (pred != ' ' && pred != '\n') {
                    sb.append(' ');
                }
            } else {
                sb.append(c);
            }
        }

        mSpannableStringBuilder.append(sb);
    }

    public void ignorableWhitespace(char ch[], int start, int length) throws SAXException {
    }

    public void processingInstruction(String target, String data) throws SAXException {
    }

    public void skippedEntity(String name) throws SAXException {
    }

    private static class Bold { }
    private static class Italic { }
    private static class Underline { }
    private static class Big { }
    private static class Small { }
    private static class Monospace { }
    private static class Blockquote { }
    private static class Super { }
    private static class Sub { }

    private static class Font {
        public String mColor;
        public String mFace;

        public Font(String color, String face) {
            mColor = color;
            mFace = face;
        }
    }

    private static class Href {
        public String mHref;

        public Href(String href) {
            mHref = href;
        }
    }

    private static class Header {
        private int mLevel;

        public Header(int level) {
            mLevel = level;
        }
    }
}