Android Service using QAndroidService



  • Hello,

    I am trying to create an Android Service in QT that will give other Android apps the chance to communicate with it. Given that we have the https://doc.qt.io/qt-5/qandroidservice.html class in QT I thought that it is enough to extend it, un-comment the manifest service part and run the app. Otherwise I am not sure why do we have that class or how to use it.
    If I try to connect to the custom service from another app, I can at least start it using the bindService method which returns true(so I guess it works) but the onBind method from QT of my service never gets called.

    I used some of the code written here: https://github.com/bog-dan-ro/qt_services

    My main file looks like this:

    class MyBinder : public QAndroidBinder
    {
    public:
        bool onTransact(int code, const QAndroidParcel &data, const QAndroidParcel &reply, QAndroidBinder::CallType flags) override
        {
           // never gets called but it is no surprise for me since the onBind method is not called either
            qDebug() << "~~~  onTransact" << code << int(flags);
            switch (code) {
            case 1: {
                QAndroidBinder binder = data.readBinder();
                qDebug() << "~~~  onTransact client sent its binder !";
                break;
            default:
                qDebug() << "~~~  onTransact client sent unknow data" << data.readData();
                reply.writeVariant(555);
                break;
            }
            return true;
        }
    };
    
    // my service, I would like to do stuff in this service on the QT side if possible
    class MyService : public QAndroidService
    {
    public:
        MyService(int &argc, char **argv) : QAndroidService(argc, argv) {}
    
        QAndroidBinder *onBind(const QAndroidIntent &/*intent*/) override
        {
            // this method is not called ever
            qInfo() << "ServiceXXXL -> ~~~ create onBind !!!";
            return new MyBinder();
        }
    };
    
    int main(int argc, char *argv[])
    {
          // this method gets called whenever I call bindService() from another android app
    
         MyService app(argc, argv); // I am pretty sure this is not ok
    
        return app.exec();
    }
    

    Is there a working sample of how one can create a Service in QT and actually do stuff in it while allowing other apps or Java native code to bind to it?

    Thank you very much for any help,
    Catalin

    PS: I am new to QT, usually I write Java/Kotlin Android and Swift iOS apps so I am sorry if the question is too "light" :)


  • Moderators

    @CatalinP
    QAndroidService class is just a substitution for a QCoreApplication, but doesn't create a android service.

    see this: https://www.kdab.com/qt-android-create-android-service-using-qt/



  • Hello @raven-worx and thanks for your answer. Ok I got it wrong then, I thought I could use the onBind method somehow and link that MyService QT class to the MyService.java service or to the QtService.java service.

    Since it wraps most of the Service methods, including onBind, how can I make the onBind method to fire?

    I already checked that blog post but it does not cover what I need, I am already able to start the QtService from another android app but I can't figure out how to also bind to it in QT.

    Any thoughts are much appreciated! :)



  • Hello, there's no other ideas on this? :)


  • Moderators

    @CatalinP
    What exactly are you trying to achieve?
    Does this help? https://doc.qt.io/archives/qt-5.11/qandroidserviceconnection.html



  • Hello @raven-worx and thanks once again for your time.

    I am trying to create an Android Service using QT creator that will be able to communicate through bluetooth to some devices and that will enable other Android applications to use it to get certain data from it.

    Like I said above, I managed to enable a Service from QT Creator and also start it from another Android application but unfortunately it is the java QService that is being generated by QT creator at build time and I can't actually handle my bluetooth/logic stuff in it because I don't have access to it from QT (or I don't know how to implement it properly).
    I hoped that by extending the QAndroidService I will be able to override the onBind and do the business logic there but it seems that it doesn't work as I thought and I can't see a good article(that works) on how to communicate to a Service in QT.

    I hope I explained clearly what I would like to achieve, if not please let me know :).


  • Moderators

    @CatalinP
    so it seems you want a bound service right?
    If so it should be enough to sublcass QAndroidService and in it's onBind() method return a custom QAndroidBinder instance which does the RPC call results.

    I've never done that yet though.



  • @raven-worx yep, that's what I wanted and I already tried that but the onBind method doesn't get called for some reasons... I pasted what I tried above in the post and also the github repo that I copied.



  • Have you found the solution? Is the QtAndroid::bindService working for you?

    What about implementing it directly in Java? I'm trying to do it but because of my lack of Java and Android knowledge it is not as easy as I thought.



  • Unfortunately all my attempts failed so far. :-(

    In Java I can bind the service but the onBind function somehow returns a wrong type. In the ServiceConnection function onServiceConnected(ComponentName className, IBinder service) I need to get the class. But my application crashes when calling

    MyBinder binder = (MyBinder) service;
    

    with following error:

    10-14 22:02:08.154  7908  7908 E AndroidRuntime: java.lang.ClassCastException: android.os.BinderProxy cannot be cast to cz.jech.muzika.MyService$MyBinder
    10-14 22:02:08.154  7908  7908 E AndroidRuntime: at cz.jech.muzika.Activity$1.onServiceConnected(Activity.java:53)
    

    So I tried to do it in Qt (in the C++ part). This is my code:

    auto serviceIntent = QAndroidIntent(QtAndroid::androidActivity().object(), "cz.jech.muzika.MyService");
    ServiceConnection serviceConnection;
    QtAndroid::bindService(serviceIntent, serviceConnection, QtAndroid::BindFlag::AutoCreate);
    

    The ServiceConnection is my own class which I derived from QAndroidServiceConnection and implemented the virtual functions.

    But if I execute the QtAndroid::bindService(serviceIntent, serviceConnection, QtAndroid::BindFlag::AutoCreate); function, my application crashes:

    10-14 23:28:43.616 19647 19647 F libc    : Fatal signal 11 (SIGSEGV), code 1 (SEGV_MAPERR), fault addr 0x0 in tid 19647 (cz.jech.muzika), pid 19647 (cz.jech.muzika)
    

    Do you have any ideas what else should I try? Thank you.



  • @vlada I don't really understand what you are trying to do...

    Do you want to create a Android background service application or do you want to connect your activity to a background service?

    Have to read this nice article on KDAB blog? ==> https://www.kdab.com/qt-android-create-android-service-using-qt/



  • @KroMignon I read the article already a few years ago and I have a working service implemented for a long time. This is not the issue I have.

    Since Android 8 if you create a background service and then switch to another application, your service gets almost immediately killed by system. To avoid this, you have two options. The first one is to create a foreground service, which is the one I use. But this requires to have a permanent notification shown.

    The second option is to have a bound service. The service then runs as long as your application runs. This is what I need, because I want the service to get killed together with the activity. If only the service gets killed, after restarting it, it doesn't work correctly anymore.

    So what I'm trying to do is to bind the service to the activity. I just found a correct way to do it:

    auto serviceIntent = QAndroidIntent(QtAndroid::androidActivity().object(), "cz.jech.muzika.MyService");
    ServiceConnection *serviceConnection = new ServiceConnection();
    QtAndroid::bindService(serviceIntent, *serviceConnection, QtAndroid::BindFlag::AutoCreate);
    

    Now I also need to find a way to unbind the service, because QtAndroid::unbindService doesn't exist. But I hope this can be done on the Android side in Java.