Extract and re-write "front matter" in markdown documents

It's increasingly common for YAML to be used as metadata in front of
markdown documents. md4c does not handle this, so we need to remove
it ahead of time, lest md4c misinterpret it as heading text or so.

The --- fences are expected to be consistent regardless of the format of
what's between them, and the yaml (or whatever) parser does not need to
see them. So we remove them while reading, and QTextMarkdownWriter
writes them around the front matter if there is any.

If your application needs to parse this "front matter", just call
qtd->metaInformation(QTextDocument::FrontMatter).toUtf8() and feed that
to some parser that you've linked in, such as yaml-cpp.

Since YAML is used with GitHub Docs, we consider this feature to be part
of the GitHub dialect:
https://docs.github.com/en/contributing/writing-for-github-docs/using-yaml-frontmatter

[ChangeLog][QtGui][Text] Markdown "front matter" (usually YAML) is now
extracted during parsing (GitHub dialect) and can be retrieved from
QTextDocument::metaInformation(FrontMatter). QTextMarkdownWriter also
writes front matter (if any) to the output.

Fixes: QTBUG-120722
Change-Id: I220ddcd2b94c99453853643516ca7a36bb2bcd6f
Reviewed-by: Axel Spoerl <axel.spoerl@qt.io>

1 files changed, 9 insertions, 0 deletions

diff --git a/src/gui/text/qtextdocument.cpp b/src/gui/text/qtextdocument.cpp
index df00c809d1..38279b3064 100644
--- a/src/gui/text/qtextdocument.cpp
+++ b/src/gui/text/qtextdocument.cpp
@@ -1159,6 +1159,8 @@ QString QTextDocument::metaInformation(MetaInformation info) const
         return d->url;
     case CssMedia:
         return d->cssMedia;
+    case FrontMatter:
+        return d->frontMatter;
     }
     return QString();
 }
@@ -1182,6 +1184,9 @@ void QTextDocument::setMetaInformation(MetaInformation info, const QString &stri
     case CssMedia:
         d->cssMedia = string;
         break;
+    case FrontMatter:
+        d->frontMatter = string;
+        break;
     }
 }
 
@@ -1327,6 +1332,10 @@ void QTextDocument::setHtml(const QString &html)
     \value CssMedia         This value is used to select the corresponding '@media'
                             rule, if any, from a specified CSS stylesheet when setHtml()
                             is called. This enum value has been introduced in Qt 6.3.
+    \value FrontMatter      This value is used to select header material, if any was
+                            extracted during parsing of the source file (currently
+                            only from Markdown format). This enum value has been
+                            introduced in Qt 6.8.
 
     \sa metaInformation(), setMetaInformation(), setHtml()
 */