I tried to implement a notification of SD card removal in my application on Android. There is a sample app and guide to do it. The sample works for me, but I stops working if I try to implement it in my application.

I tried to change the sample app to meet my needs and at one point it stopped working too. Basically the Java part works OK and even the callback to C++ on Android UI thread works. Originally it is implemented like this:

// native.cpp
#include <jni.h>
 #include <QMetaObject>
 #include "mainwindow.h"
 
/static void onReceiveNativeMounted(JNIEnv * /*env*/, jobject /*obj*/)
{
    QMetaObject::invokeMethod(&MainWindow::instance(), "onReceiveMounted"
                              , Qt::QueuedConnection);
}

Then there is the mainwindow class:

// mainwindow.h
#ifndef MAINWINDOW_H
#define MAINWINDOW_H
 
#include <QMainWindow>
 
namespace Ui {
class MainWindow;
}
class MainWindow : public QMainWindow
{
    Q_OBJECT
 
public:
    static MainWindow &instance(QWidget *parent = 0);
 
public slots:
    void onReceiveMounted();
    void onReceiveUnmounted();
 
private:
    explicit MainWindow(QWidget *parent = 0);
    ~MainWindow();
 
private:
    Ui::MainWindow *ui;
};

This construction works. Now all I did was to create my own class called AndroidLib and call it from the native.cpp. The modified part looks like this:

#include "androidlib.h"
 
static void onReceiveNativeMounted(JNIEnv * /*env*/, jobject /*obj*/)
{
    QMetaObject::invokeMethod(&AndroidLib::instance(), "onReceiveMounted"
                              , Qt::QueuedConnection);
}

The AndroidLib class looks almost identical to the former MainWindows class:

#ifndef ANDROIDLIB_H
#define ANDROIDLIB_H

#include <QObject>
#include <QDebug>

class AndroidLib : public QObject
{
    Q_OBJECT
public:
    explicit AndroidLib(QObject *parent = nullptr);
    static AndroidLib &instance(QObject *parent = 0);

/*private:
    explicit AndroidLib(QObject *parent = nullptr);*/

public slots:
    void onReceiveMounted();
    void onReceiveUnmounted();

signals:
    void sdMount(const bool mounted);
};

#endif // ANDROIDLIB_H

The function in native.cpp is called. Even the function AndroidLib::instance() is called. But the function onReceiveMounted() in AndroidLib is not called.

Is there something wrong in the command

/QMetaObject::invokeMethod(&AndroidLib::instance(), "onReceiveMounted", Qt::QueuedConnection);

which causes that the function is not executed?

Thanks a lot for any advice regarding this issue.