Cast a QObject* to QMediaPlayer with qobject_cast

davidino

Goodmorning,
I'm trying to cast a QObject pointer taken from QML into a QMediaPlayer one. I always get null using the method qobject_cast.

Here's the QML code, where volumeMeter has been made available to QML with context->setContextProperty("volumeMeter", &volume); from an object made in main.cpp

    MediaPlayer {
        id: playMusic
        objectName: "playMusic"
        autoLoad: true
        autoPlay: true
        playlist: Playlist{id:playerList}
        Component.onCompleted: { volumeMeter.initVolume(playMusic)}
    }

and here the volumeLevel class:

#ifndef VOLUMELEVEL_H
#define VOLUMELEVEL_H

#include <QObject>
#include <QAudioProbe>
#include <QMediaPlayer>
#include <QTimer>
#include <vector>
#include <memory>

#include "playListModel_global.h"

class PLAYLISTMODEL_EXPORT VolumeLevel : public QObject
{

    Q_OBJECT
public:
    explicit VolumeLevel(QObject *parent = nullptr);
    ~VolumeLevel();

    // Must call Q_INVOKABLE so that this function can be used in QML
    Q_INVOKABLE void initVolume(QObject* obj);
    void initVolume(QString qmlObjectName);

public slots:
    void processBuffer(QAudioBuffer buffer);
    void timerExpired();

private:
    QAudioProbe m_probe;
    QTimer m_timer;
    QMediaPlayer *m_player;
};

#include "volumelevel.h"
#include <QDebug>

#include <QAudioProbe>

VolumeLevel::VolumeLevel(QObject *parent) :
    QObject(parent)
{
    qRegisterMetaType<QMediaPlayer*>("QMediaPlayer");
}

void VolumeLevel::initVolume(QString qmlObjectName)
{
    Q_UNUSED(qmlObjectName)
}

void VolumeLevel::initVolume(QObject* obj)
{
    qDebug()<<obj;
    m_player = qobject_cast<QMediaPlayer*>(obj);
    if( m_player != nullptr )
    {
        qDebug()<<m_player;

        if(m_probe.setSource(m_player)) {
            connect(&m_probe, &QAudioProbe::audioBufferProbed, this, &VolumeLevel::processBuffer);
            connect(&m_timer, &QTimer::timeout, this, &VolumeLevel::timerExpired);
            m_timer.start(2000);
            qDebug()<<"Connection done";
        }
        else
            qDebug()<<"Connection not done";
    }
    else
        qDebug()<<"cast wrong";
}

void VolumeLevel::processBuffer(QAudioBuffer buffer)
{
    // With a 16bit sample buffer:
    const quint16 *data = buffer.constData<quint16>();
    qDebug()<<"Raw: "<<*data;
}

void VolumeLevel::timerExpired()
{
    qDebug()<<"probe is active: "<<m_probe.isActive();
}

VolumeLevel::~VolumeLevel()
{}

The output from console is:
QDeclarativeAudio(0x142c458, name = "playMusic")
cast wrong

So the object is correctly identify but the cast fails..
Can you advice?

Thanks.
Regards, Davide

davidino

Foudn solution here:
https://doc.qt.io/qt-5/qml-qtmultimedia-mediaplayer.html

QObject *qmlMediaPlayer; // The QML MediaPlayer object
QMediaPlayer *player = qvariant_cast<QMediaPlayer *>(qmlMediaPlayer->property("mediaObject"));

Thank you

jsulm

@davidino QDeclarativeAudio is not QMediaPlayer.
Shouldn't you cast to QDeclarativeAudio?

davidino

Hello @jsulm,
thank you for your answer.
As you can see in the code after the cast, m_probe.setSource() is called which requires QMediaObject* type as parameter.
QMediaPlayer subclasses QMediaObject, while QDeclarativeAudio doesn't.
Besides I'm not sure that I can use QDeclarativeAudio so unwary considering that in the class QDeclarativeAudio it's written:

//
// W A R N I N G
// -------------
//
// This file is not part of the Qt API. It exists for the convenience
// of other Qt classes. This header file may change from version to
// version without notice, or even be removed.
//
// We mean it.
//

Does it means that I cannot create in QML a MediaPlayer and pass it to c++ as QMediaPlayer?
Do I have to create a QMediaPlayer in c++ and then register the class for its usage in QML? It would be crazy..

jsulm

@davidino said in Cast a QObject* to QMediaPlayer with qobject_cast:

Do I have to create a QMediaPlayer in c++ and then register the class for its usage in QML? It would be crazy..

I'm not a QML expert but I think so, yes. I don't think MediaPlayer in QML is same as QMediaPlayer, but I may be wrong...

davidino

Foudn solution here:
https://doc.qt.io/qt-5/qml-qtmultimedia-mediaplayer.html

QObject *qmlMediaPlayer; // The QML MediaPlayer object
QMediaPlayer *player = qvariant_cast<QMediaPlayer *>(qmlMediaPlayer->property("mediaObject"));

Thank you