native_example/app/src/main/java/com/example/realsense_native_example/MainActivity.java

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package com.example.realsense_native_example;

import android.Manifest;
import android.content.pm.PackageManager;
import android.os.Bundle;
import androidx.core.app.ActivityCompat;
import androidx.core.content.ContextCompat;
import androidx.appcompat.app.AppCompatActivity;
import android.widget.TextView;

import com.intel.realsense.librealsense.DeviceListener;
import com.intel.realsense.librealsense.RsContext;

public class MainActivity extends AppCompatActivity {
    private static final int MY_PERMISSIONS_REQUEST_CAMERA = 0;

    private RsContext mRsContext;

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

    @Override
    public void onRequestPermissionsResult(int requestCode, String permissions[], int[] grantResults) {
        if (ContextCompat.checkSelfPermission(this, Manifest.permission.CAMERA) != PackageManager.PERMISSION_GRANTED) {
            ActivityCompat.requestPermissions(this, new String[]{Manifest.permission.CAMERA}, MY_PERMISSIONS_REQUEST_CAMERA);
        }
    }

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        //RsContext.init must be called once in the application's lifetime before any interaction with physical RealSense devices.
        //For multi activities applications use the application context instead of the activity context
        RsContext.init(getApplicationContext());

        printMessage();

        //Register to notifications regarding RealSense devices attach/detach events via the DeviceListener.
        mRsContext = new RsContext();
        mRsContext.setDevicesChangedCallback(new DeviceListener() {
            @Override
            public void onDeviceAttach() {
                printMessage();
            }

            @Override
            public void onDeviceDetach() {
                printMessage();
            }
        });

        // Android 9 also requires camera permissions
        if (android.os.Build.VERSION.SDK_INT > android.os.Build.VERSION_CODES.O &&
                ContextCompat.checkSelfPermission(this, Manifest.permission.CAMERA) != PackageManager.PERMISSION_GRANTED) {
            ActivityCompat.requestPermissions(this, new String[]{Manifest.permission.CAMERA}, MY_PERMISSIONS_REQUEST_CAMERA);
        }
    }

    private void printMessage(){
        // Example of a call to native methods
        int cameraCount = nGetCamerasCountFromJNI();
        final String version = nGetLibrealsenseVersionFromJNI();
        final String cameraCountString;
        if(cameraCount == 0)
            cameraCountString = "No cameras are currently connected.";
        else
            cameraCountString = "Camera is connected";
        runOnUiThread(new Runnable() {
            @Override
            public void run() {
                TextView tv = (TextView) findViewById(R.id.sample_text);
                tv.setText("This app use librealsense: " + version + "\n" + cameraCountString);
            }
        });
    }

    private static native String nGetLibrealsenseVersionFromJNI();
    private static native int nGetCamerasCountFromJNI();
}