DBus reply data accessed via QString

kshegunov · A amonR2 15 Jun 2016, 02:57

To check if the obtained interface is valid, is this code correct

Yes.

But how do you do to check if the reply is valid?

I personally check it through a QDBusReply object (e.g. here). If using the dbus message class use QDBusAbstractInterface::lastError instead.

micland · A amonR2 15 Jun 2016, 02:57

@amonR2 said:

yes I ran qdbusviewer and obtained a message like the one I put in my first post://Received reply from :1.156 Arguments: [Argument: a{sv} {"album" = [Variant(QString): "In Search of..."], "artist" = [Variant(QString): "N.E.R.D."],...] . The only difference was the name of the service that I changed in my code but I still get an empty string as result.

Uhm, that's a bit strange. Can you try to inspect the reply with the debugger or send the arguments to debug out (qDebug() << args;) just to see if all arguments are empty or what Qt has interpreted from the message?

amonR2 · 16 Jun 2016, 06:44

@kshegunov , thank you very much.

@micland , I don't know how to inspect the reply with the debugger so I prefer to give you the result of qDebug() << args; which is: (QVariant(, )). Should I set Qt or my header in a certain way? So far I use the following includes:

#include <QtGui> 
#include <QtDBus/QtDBus>

Is it ok? And what about the Q_DECLARE_METATYPE() that I don't use? Should I use it here? If yes, how?

micland · A amonR2 16 Jun 2016, 03:12

@amonR2 said:

I don't know how to inspect the reply with the debugger so I prefer to give you the result of qDebug() << args; which is: (QVariant(, )). Should I set Qt or my header in a certain way?

The order of the includes should not be relevant.
According to your debug output your program does not get the same answer as qdbusviewer gets. But I don't think that it's a bug in Qt because qdbusviewer uses the same logic and should be affected by that bug, too.
Could you check for reply->errorMessage(); and double check if there is any typo in the specified DBus interface / object / method in your code?

amonR2 · 17 Jun 2016, 08:10

@micland said:

Could you check for reply->errorMessage();

There sorry but I am not sure what you are talking about, did you mean to do this instead: qDebug() << replyClementine.errroMessage();? If yes, the output is still an empty string (""). For the typo in the DBus interface, objet and method in my code I don't think there is one as qDebug() << replyClementine gives me a correct response. But why this response is not "translated" into a list of variants and strings? What may "corrupt" the process here?

micland · A amonR2 17 Jun 2016, 03:42

@amonR2
Yes, I wanted to see that there is no error message provided (or: I was hoping to see a message that gave us a hint ;-) ). I'm a bit confused that there is no error AND no usable reply.
Another idea: If you don't known how to debug the arguments, can you print out args.count()? Perhaps there is just one entry in the list and this entry might be a new list (or hash map) - and you're accessing the (not existing) second argument.... (the output (QVariant(, )) might indicate that there is just one argument...).

amonR2 · 20 Jun 2016, 06:38

The output of qDebug << args.count(); gives me 1. There is something I have changed in my code's environment: previously it was in release mode and I have just thought to put it in Debug one's which gives more info. With this code:

QString a1 = args.at(1).toString();
qDebug() << a1;

my app crashes and the IDE gives me this message: ASSERT failure in QList<T>::at: "index out of range", file /usr/include/qt4/QtCore/qlist.h, line 469. But when I use this one:

QString a1 = args.at(0).toString();
qDebug() << a1;

my app doesn't crash and qDebug() gives me an empty string.

micland · 21 Jun 2016, 01:58

@amonR2
Ok, the argument list contains just one argument - that's why you get the "index out of range" error because you try to access the second argument. Please add the line qDebug() << args.at(0).typeName(); to see the type of the first argument. Maybe it's a list where you find all your expected data in...

amonR2 · 21 Jun 2016, 12:50

The reply from qDebug() << args.at(0).typeName(); gives me: QDBusArgument.

micland · A amonR2 21 Jun 2016, 01:58

@amonR2
Huh, a nested QDBusArgument? Well check the inner argument if it serves the expected information:
args.at(0).value<QDBusArgument>().args().
(You should really try to inspect the reply using the debugger and iterate through the object tree to see what's encapsulated in the arguments. That's a lot easier than poking with qDebug() - perhaps this link will help you getting started? http://doc.qt.io/qtcreator/creator-debug-mode.html)

amonR2 · 22 Jun 2016, 07:05

@micland said:

Huh, a nested QDBusArgument?

sorry I don't understand what you mean there but that's all qDebug displays.
Then when I try to qDebug thisargs.at(0).value<QDBusArgument>().args();, the compiler says: 'class QDBusArgument' has no member named 'args'. So I tried with this: args.at(0).value<QDBusArgument>() and received the following from the compiler: no match for 'operator<<' in 'qDebug()() << QVariant::value() const [with T = QDBusArgument]()' [...] no known conversion for argument 1 from 'QDebug' to 'QDBusArgument&'.
Finally, by puting a breakpoint (thank you for the link) at QList<QVariant> args = replyClementine.arguments(); in my code and launching the debug-mode, the compiler stops and shows me this through the "Locals and Expressions" window:

	args                 	<inaccessible>	QList<QVariant>
	this	 @0x83f87d8	FenetreNNS
		QWidget		QWidget
		Deviselabel	0x0	QLabel *
		MainLabel	 @0x844d210	QLabel
		Nextm	 @0x8451f28	QPushButton
		NoTrackLabel	 @0x8454628	QLabel
		Play	 @0x844f628	QPushButton
		Previous	 @0x8451088	QPushButton
		TemoinSousTitre	false	bool
		Volume	 @0x8453a48	QSlider
		VolumeLabel	 @0x844d160	QLabel
		boutonDeviseBaht	 @0x8439978	QPushButton
		boutonDeviseDenar	 @0x843cce0	QPushButton
		boutonDeviseDollar	 @0x843f968	QPushButton
		boutonDeviseEuro	 @0x8440418	QPushButton
		boutonDeviseLira	 @0x843e608	QPushButton
		boutonDevisePeso	 @0x8442098	QPushButton
		boutonDevisePound	 @0x8444e58	QPushButton
		boutonDeviseRuble	 @0x843b078	QPushButton
		boutonDeviseRupee	 @0x844a4b0	QPushButton
		boutonDeviseWon	 @0x844b2e0	QPushButton
		boutonDeviseYen	 @0x8448a28	QPushButton
		boutonDeviseYuan	 @0x8449e70	QPushButton
		iface_Clementine	 @0x845e788	QDBusInterface
		machine	 @0x8455420	QStateMachine
		principalefen	 @0xbffff7b4	FenPrincipale
		replyClementine		QDBusMessage
			d_ptr	 @0x845d098	QDBusMessagePrivate
		signMapper	 @0x8454ac0	QSignalMapper
		signMapper2	 @0x84553e8	QSignalMapper
		sliderSousTitre	 @0x844d998	QSlider
		state1	 @0x8451f60	QState
		state2	 @0x8451f70	QState
		str1	"Currency"	QString
		tempo	 @0x84505b0	QTimer

Should I show the content of some other windows?

micland · 23 Jun 2016, 02:24

@amonR2 said:

Huh, a nested QDBusArgument?
sorry I don't understand what you mean there but that's all qDebug displays.

Aye, sorry - I was wrong, read your answer too fast and got confused...
It's hard to find this error without reproducing it. If I find some time at the weekend I'll try to debug it. Can you tell me which software you installed? will say: what's that player your communicating with?

amonR2 · 27 Jun 2016, 07:48

@micland
The name of the player is "Clementine" and I am using Qt 4.8 . No worries, thank you a lot for your help anyway. I will keep looking for a solution. If I can't find anything I will use one of the other APIs. Cheers again.

micland · A amonR2 23 Jun 2016, 02:24

@amonR2
I'm a little bit closer, but did non really succeed...
First: Qt 4.8 is some days older, I used 5.6 for my experiements. Inspecting the DBus interface using qdbusviewer (from Qt5.6) ended with the error message "Unable to find method GetMetadata on path /Player in interface org.freedesktop.MediaPlayer" (the method is listed but I can't call it) - but the cli client qdbus provided me the expected data. I can't say if that's a bug in Qt or a "conformity problem", but it shows that calling the clementine DBus interface from Qt works not straight forward.

I tried to access the interface using a simple Qt program (like the example code you posted) and got the same reply as you did. Inspecting the received QDBusArgument showed that its currentyType() is a MapType (4) which means that the arguments are organized as a key/value list (the same says the MPRIS spec: see "Metadata" is an array of dict entries in the form (string, variant) eg: {sv}., https://xmms2.org/wiki/MPRIS#.22Metadata.22)

I think you have to extract the values manually and play arround with beginMap(), beginMapEntry(), endMapEntry(), endMap() of QDBusArgument. (I spent just a little time but did not succeed ...)
But I'm interested if that's the right way so if you find a solution please share it here! (If I find some time I will try it again, too...)

micland · 30 Jun 2016, 22:57

@amonR2
out of curiousity I was looking for a solution (I didn't know before that maps and structs are supported by DBus). And I found a way - using Qt 5.6, but should work with Qt 4.8 as well:

    // first the same code as you did already
    QDBusInterface *iface_Clementine = new QDBusInterface("org.mpris.clementine", "/Player", "org.freedesktop.MediaPlayer", QDBusConnection::sessionBus());
    if(iface_Clementine->isValid()) {
        QDBusMessage replyClementine = iface_Clementine->call("GetMetadata");
        qDebug() << replyClementine;
        // there is just one argument and this argument is a map
        QList<QVariant> args = replyClementine.arguments();
        const QDBusArgument &arg = args[0].value<QDBusArgument>();
        // the map has to be extracted entry by entry which is a tuple of key and value
        // in this case, the key is a string but the type of the value depends on the key (string, int, ...)
        arg.beginMap();
        while (!arg.atEnd()) {
            QString key;
            QDBusVariant value;
            arg.beginMapEntry();
            arg >> key >> value;
            arg.endMapEntry();
            qDebug() << "Key:" << key << "\t\tValue:" << value.variant();
        }
        arg.endMap();
    }

You still have to inspect the type of the value (depends on what the entry represents: album or track number, or whatever). And the argument parsing can be wrapped in a stream operator, see http://doc.qt.io/qt-5/qdbusargument.html#beginMap-1

Some further helpul information can be found here: http://stackoverflow.com/questions/20206376/how-do-i-extract-the-returned-data-from-qdbusmessage-in-a-qt-dbus-call

I have no clue why the qdbusviewer of Qt 5.6 is unable to call the DBus method but the code snippet above is working fine.

amonR2 · wrote on 30 Jun 2016, 22:57

@micland

WOOW!!! Thank you sooo much micland!! It works!!!
I tried a code similar to yours but some data were missing. Just needed to do a very tiny modification on your code:

qDebug() << key << value.variant().toString();

or directly

qDebug() << value.variant().toString();

Except it, it is perfect! Thank you again.
@micland said:

I have no clue why the qdbusviewer of Qt 5.6 is unable to call the DBus method but the code snippet above is working fine.

I hope it is going to change as I intend to use it or the 5.7 version very soon.

Cheers again.