Do I have to create static attributes to update UI in JNI?

Question

My Target

• I'm coding an android studio project with some c++ code
• I want to update UI within c++ code

My Code

The code is as follows:

• MainActivity.java:

public class MainActivity extends AppCompatActivity {

    // Used to load the 'davdmnn' library on application startup.
    static {
        System.loadLibrary("davdmnn");
    }

    private ActivityMainBinding binding;
    private TextView tv;
    private Button btn;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        binding = ActivityMainBinding.inflate(getLayoutInflater());
        setContentView(binding.getRoot());

        // Example of a call to a native method
        tv = binding.sampleText;

        String path = this.getFilesDir() + File.separator;
        stringFromJNI(path);
    }

    /**
     * A native method that is implemented by the 'davdmnn' native library,
     * which is packaged with this application.
     */
    public native String stringFromJNI(String path);

    public void sendMessage(int id) {
        tv.setText(id);
    }
}

• native-lib.cpp

extern "C" JNIEXPORT jstring JNICALL
Java_com_example_davdmnn_MainActivity_stringFromJNI(
        JNIEnv* env,
        jobject /* this */, jstring path) {

    for (int i = 0; i < 100; i++){
        // doing sth
        // update tv, like tv.setText(i)
    }

}

My Question

• I notice my question is related to this one. It uses a handler object to udpate UI, which is good for me.
• However, to use the solution of the related question, the attribute tv and method sendMessage must be static, which is not what I want.
• So is there any way that I can pass the object (like this in java) into C++ method Java_com_example_davdmnn_MainActivity_stringFromJNI, or any other solutions?

Answer 1

Thanks to @Michael, jobject /* this */ is actually the java object that calls the C++ function. I modify it into jobject thiz and here is my final code:

• MainActivity.java

public class MainActivity extends AppCompatActivity {

    // Used to load the 'davdmnn' library on application startup.
    static {
        System.loadLibrary("davdmnn");
    }

    private ActivityMainBinding binding;
    private TextView trainLog;
    private Button btn;
    private Handler handler;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        binding = ActivityMainBinding.inflate(getLayoutInflater());
        setContentView(binding.getRoot());

        // Example of a call to a native method
        trainLog = binding.sampleText;

        handler = new Handler(){
            public void handleMessage(Message msg){
                trainLog.setText((String)msg.obj);
            }
        };

        String path = this.getFilesDir() + File.separator;
        Runnable mRunnable = new Runnable() {
            @Override
            public void run() {
                stringFromJNI(path);
            }
        };

        btn = binding.startButton;
        btn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                btn.setEnabled(false);
                new Thread(mRunnable).start();
            }
        });

    }

    /**
     * A native method that is implemented by the 'davdmnn' native library,
     * which is packaged with this application.
     */
    public native String stringFromJNI(String path);

    public void sendMessage(int msg) {
        Message msg = Message.obtain();
        msg.obj = Integer.toString(msg);
    
        handler.sendMessage(msg);
    }
}

• native-lib.cpp

extern "C" JNIEXPORT void JNICALL
Java_com_example_davdmnn_MainActivity_stringFromJNI(
        JNIEnv* env,
        jobject thiz, jstring path) {
    
    // find the java function
    jclass main_activity = (*env).FindClass("com/example/davdmnn/MainActivity");
    jmethodID mid = (*env).GetMethodID(main_activity, "sendMessage", "(I)V");
    for (int i = 0; i < 100; i++){
        // doing sth
        // update tv, like tv.setText(i)
        (*env).CallVoidMethod(thiz, mid, i);
    }

}