DBus reply data accessed via QString

micland · A amonR2 13 Jun 2016, 23:34

I get the following result: "", In other word an empty string. Sometimes my app crashes. Why?

Did you run the qdbusviewer (separate tool shipped with Qt) and inspect your interface manually? Does this viewer also get an empty string or anything else? If you get a valid answer using Qt I guess the backends is sending an empty string...

amonR2 · 15 Jun 2016, 03:25

@ Kshegunov, thanks for your help. To check if the obtained interface is valid, is this code correct:

iface_Clementine = new QDBusInterface("org.mpris.clementine", "/Player", "org.freedesktop.MediaPlayer", QDBusConnection::sessionBus(), this);

if (!iface_Clementine->isValid()) {
qCritical("The player is not nactive");
}
else{
replyClementine = iface_Clementine->call("GetMetadata");
QList<QVariant> args = replyClementine.arguments();

if (args.count() == 0) {
qCritical("Got no valid DBus response");
					    } 
else {
       QString a1 = args.at(1).toString();
       qDebug() << a1;
     }
}

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

@ micland, 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. Why the backend sends an empty string instead of the actual result? And in my code, if you notice, I check if the number of items in the list is null. Does Qt counts an empty string as an item?

kshegunov · A amonR2 15 Jun 2016, 02:57

@amonR2

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.