[Interest] Subclassing QMacPasteboardMime

Adam Light aclight at gmail.com
Thu May 24 22:52:26 CEST 2012


This question applies to Mac OSX. I'm using Qt 4.8.1 on Mac OS 10.6.8.

In my application, I wish to handle tiff clipboard data (of UTI type
public.tiff) different than the implementation in QMacPasteboardMimeTiff (I
don't want the data to pass through a QImage).

I have written my own subclass of QMacPasteboardMime that does exactly what
I want, and it works correctly.

The documentation of QMacPasteboardMime (
http://qt-project.org/doc/qt-4.8/qmacpasteboardmime.html) states:
"When working with MIME data, Qt will interate through all instances of
QMacPasteboardMime to find an instance that can convert to, or from, a
specific MIME type. It will do this by calling canConvert() on each
instance, starting with (and choosing) the last created instance first. The
actual conversions will be done by using convertToMime()
and convertFromMime()."

I am instantiating my subclass of QMacPasteboardMime (call it
MyMacPasteboardMimeTiff) after my application's QApplication object is
created (the base QMacPasteboardMime class is created in the QApplication
initialization code).

My problem is that my MyMacPasteboardMimeTiff::mimeFor(), canConvert(),
etc. methods never get called. This is
because QMacPasteboardMime::flavorToMime(), which gets called when
QMimeData::formats() is called, uses this code:

QString QMacPasteboardMime::flavorToMime(uchar t, QString flav)


    MimeList *mimes = globalMimeList();

    for(MimeList::const_iterator it = mimes->constBegin(); it !=
mimes->constEnd(); ++it) {


        qDebug("QMacMIme::flavorToMime: attempting %s (%d) for flavor
%d[%c%c%c%c] [%s]",


               (*it)->type & t, flav, (flav >> 24) & 0xFF, (flav >>
16) & 0xFF, (flav >> 8) & 0xFF, (flav) & 0xFF,



        if((*it)->type & t) {

            QString mimeType = (*it)->mimeFor(flav);


                return mimeType;



    return QString();


So contrary to what the documentation
states/implies, QMacPasteboardMimeTiff::mimeFor() ends up getting called
before MyMacPasteboardMimeTiff::mimeFor() gets called.
Since QMacPasteboardMimeTiff supports the public.tiff UTI, it ends up being
used to do the conversion from the clipboard's data to MIME data.

If I instantiate my MyMacPasteboardMimeTiff class before the QApplication
object is initialized, then my class gets called to do conversion of
public.tiff data on the clipboard (that's how I know that my class works).
However, if any QMacPasteboardMime subclasses are instantiated
before QMacPasteboardMime::initialize() is called, none of Qt's
QMacPasteboardMime subclasses will get instantiated (though I might be able
to instantiate them all on my own, but that is a hack and even then the
cleanup_mimes function wouldn't get called to clean them all up).

Am I missing something? It really seems to me that
QMacPasteboardMime::flavorToMime() should be iterating in reverse, not
forward, as the documentation suggests. But I suspect that "fixing" that
behavior could break the behavior of working applications so won't happen.

Thanks for any help
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