QMediaPlayer::metaData always returns QVariant(Invalid) - qt

Question

I used Qt on MAC OS and try to retrieve the metadata of media. So i took the Qt's Media Player Example (Qt 5.8.0) and modified a little bit:
Instead of:
connect(controls, SIGNAL(play()), player, SLOT(play()));
i used:
connect(controls, &PlayerControls::play, player,
[this]{
qDebug() << player->isMetaDataAvailable();
qDebug() << player->metaData(QMediaMetaData::Size);
player->play();
qDebug() << player->isMetaDataAvailable();
qDebug() << player->metaData(QMediaMetaData::Resolution);
});
The results are:
false
QVariant(Invalid)
false
QVariant(Invalid)
1st question: why is the metadata not available even after the media started playing?
2nd question: i added some codes in the destructor:
Player::~Player()
{
qDebug() << player->isMetaDataAvailable();
qDebug() << player->metaData(QMediaMetaData::Duration);
}
Now the metadata is available, but no data is returned:
true
QVariant(Invalid)
Can anyone help me please?
Update Question 1:
Instead of
connect(controls, SIGNAL(pause()), player, SLOT(pause()));
i used:
connect(controls, &PlayerControls::pause, player,
[this]{
qDebug() << player->isMetaDataAvailable();
qDebug() << player->metaData(QMediaMetaData::Resolution);
});
And after about 3-5 seconds of playing, the metadata is already available, but it also returns nothing:
true
QVariant(Invalid)

Answer 1

This is not the exact answer for your question but this may help you. You can check which metadata available with this code:
QMetaDataReaderControl *c = qobject_cast<QMetaDataReaderControl*>(player_->service()->requestControl(QMetaDataReaderControl_iid));
if(c) {
connect(c, &QMetaDataReaderControl::metaDataAvailableChanged, [c](bool) {
qDebug() << c->availableMetaData();
});
}

Answer 2

I', not sure, but I think that qDebug cannot handle QVariant, you should use for example:
qDebug() << (player->metaData(QMediaMetaData::Title)).toString();