Using QtLibrary on Android application.

Juan Garcia

I'm using native Qt Lib that is doing several network calls and send/recv signals into Android Studio app. I'm not being able to work with it because I'm not receiving any signal in slot. I can imagine that this is sth related with the EventLoop so I have tried to start a QCoreApplication into JNI_OnLoad method but I'm getting a crash. This is the code:

// C++ code
#include <jni.h>
#include <android/log.h>
#include <QDebug>
#include "mytimer.h"
#include <QCoreApplication>
#include <QtConcurrentRun>
#include <thread>


class S {

public:
    static S& getInstance()
    {
        static S instance;
        return instance;
    }

private:
    S() {}                    // Constructor? (the {} brackets) are needed here.

    jobject * m_ref;
    JNIEnv * m_jniEnv;

public:
    S(S const&)               = delete;
    void operator=(S const&)  = delete;
    void setRef(jobject * obj) {
        m_ref = obj;
    }
    void setEnv(JNIEnv * env) {
        m_jniEnv = env;
    }
    void met() {}

    void callJava(JNIEnv *env, jobject obj) {

        __android_log_print(ANDROID_LOG_VERBOSE, "S", "Object instance");


        // Construct a String
        jstring jstr = env->NewStringUTF("This string comes from JNI");
        // First get the class that contains the method you need to call
        jclass clazz = env->FindClass("com/example/mylibrary3/MyClass");
        // Get the method that you want to call
        jmethodID messageMe = env->GetMethodID(clazz, "messageMe", "(Ljava/lang/String;)Ljava/lang/String;");
        // Call the method on the object
        jobject result = env->CallObjectMethod(obj, messageMe, jstr);
        // Get a C-style string
        const char* str = env->GetStringUTFChars((jstring) result, NULL);

        // Clean up
        env->ReleaseStringUTFChars(jstr, str);
    }

};

void runQCoreApplication()
{
   // Instantiate QCoreApplication.
   int i = 1;
   char* c[1] = {"MyLib"};
   QCoreApplication a(i, c);

//   // Start the QTimer.
   __android_log_print(ANDROID_LOG_INFO, "LibTag", "Starting QTimer!");
   MyTimer timer;

   // Start the event loop.
   a.exec();
}

// define our native method
static void printHello(JNIEnv *env, jobject obj)
{
    __android_log_print(ANDROID_LOG_VERBOSE, "MyApp", "Hello world"); 

    S::getInstance().callJava(env, obj);
//    MyTimer timer;
}

static void callCallback(JNIEnv */*env*/, jobject /*obj*/, jobject paradisoObject)
{
    __android_log_print(ANDROID_LOG_VERBOSE, "MyApp", "Hello world");
}


static JNINativeMethod methods[] = {
    { "printHello", "()V", (void *)printHello }
};

JNIEXPORT jint JNI_OnLoad(JavaVM* vm, void* /*reserved*/)
{
    JNIEnv* env;
    if (vm->GetEnv(reinterpret_cast<void**>(&env), JNI_VERSION_1_6)
           != JNI_OK) {
        return JNI_ERR;
    }

    jclass javaClass = env->FindClass("com/example/mylibrary3/MyClass");
    if (!javaClass)
        return JNI_ERR;

    if (env->RegisterNatives(javaClass, methods,
                            sizeof(methods) / sizeof(methods[0])) < 0) {
        return JNI_ERR;
    }

    QtConcurrent::run(runQCoreApplication);


    return JNI_VERSION_1_6;
}

This is the Java part:

package com.example.mylibrary3;

interface ICallback {
    void Result(String url);
}

class MyCallback implements ICallback {

    @Override
    public void Result(String url) {
        System.out.println("Callback" + url);
    }
}

public class MyClass {

    private ICallback callback;

    static {
        System.out.println("HOLA 2");
        System.loadLibrary("QtLibExample");
    }

    public MyClass() {
        this.callback = new MyCallback();
    }

    public native void printHello();

    public String messageMe(String text) {
        System.out.println("This is the text" + text);

        callback.Result(text);
        return text;
    }
}

And I'm getting this when app starts:

V/S: Object instance
I/System.out: This is the textThis string comes from JNI
    CallbackThis string comes from JNI
W/: WARNING: QApplication was not created in the main() thread.
A/libc: Fatal signal 11 (SIGSEGV), code 1, fault addr 0x0 in tid 26361 (Thread (pooled))

Any idea on how to solve it?