How to pass an Android class back to C++

[koahnig]

I have managed to get callbacks to C++ with this help https://www.kdab.com/qt-android-episode-5/
This is using a simple int therefore it is not very challenging.

With some java lines I am even able to pass back an object array. The object array is generated from a java callback object. However, when getting a java callback object what do I have to do in order to put into a callback to C++ and access it under C++?

[raven-worx]

@koahnig said in How to pass an Android class back to C++:

what do I have to do in order to put into a callback to C++ and access it under C++?

what exactly do you mean with "put into a callback to C++"?

If i understood you correct, you want to pass a JAVA object to C++ an call a method on it, right?

// JNI signature: "(Ljava/lang/Object;)V" - also could be "(Lmy/custom/Class;)V"
void fibonaciResult(JNIEnv */*env*/, jobject /*obj*/, jobject param)
{
    QAndroidJniObject callbackObj( param );
    callbackObj.callMethod( ... );
}

[ashajg]

hi @koahnig

may be this two post will be helpful....

1. https://forum.qt.io/topic/96836/native-android-code-with-qt
2. https://forum.qt.io/topic/97028/to-connect-android-broadcastreceiver-onreceive-method-with-cpp/2
   1st post is for calling a java function from cpp (Cpp to java)
   2nd is for broadcasting (observer pattern).(java to cpp)

[koahnig]

@raven-worx @ashajg

Thanks guys. I guess the problem sits between keyboard and chair. :(
It is the lack of enough understanding of java and the interface to cpp.

I have managed to get within java callbacks of location module. The callback can be read for basic pieces int, long, double and stuff. I am able to read and send those through a callback to C++, where I am more comfortable with. I am also able to create an array of objects in java and pass this C++.

I am digging deeper with the information you provided. Thanks for the leads.

[koahnig]

The callMethod is working. I missed the forest for the trees.

There is one remaining issue for a byte array.
This is the Android description:

That is my implementation where the compiler is complaining

    jbyteArray byteArray = nObject.callMethod<jbyteArray>("getData", "()[B");

...ing.cpp:761: error: undefined reference to '_jbyteArray* QAndroidJniObject::callMethod<_jbyteArray*>(char const*, char const*, ...) const'

[raven-worx]

@koahnig
try:

QAndroidJniObject arrObj = nObject.callObjectMethod<jbyteArray>("getData", "()[B");
jbyteArray arr = arrObj.object<jbyteArray>()

(jbyteArray is of type jarray, which is of type jobject)