I don't see the activity until the process ends. What to change?

Question

Android

I'm making an application for cleaning the gallery from unnecessary images. There are several rules for pixel-by-pixel image analysis in the analysis - it takes a long time, during which nothing is shown on the screen (dark screen), the application has to be monitored by logs, although I would like to make the image analysis process displayed on the screen of course. At least the number of the image that is currently being analyzed.
That is, after selecting a file through a standard file manager and displaying the final result of image analysis in a folder, a lot of time passes, during which it is not visible what is happening if the application is not logged.
Part of the code with the choice of a file for analysis was taken from the Internet, so I don’t quite understand what needs to be changed so that the activity is displayed during the entire image analysis process.

public class MainActivity extends AppCompatActivity {

    private static final int ACTIVITY_CHOOSE_FILE = 3;

    TextView sourcePath;
    TextView destPath;
    TextView imageNumber;
    ImageView imageOnProcess;

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

        Button chooseFolder = (Button) findViewById(R.id.choose_folder);
        sourcePath = (TextView) findViewById(R.id.source_path);
        destPath = (TextView) findViewById(R.id.destination_path);
        imageNumber = (TextView) findViewById(R.id.imagenumber);
        imageOnProcess = (ImageView) findViewById(R.id.imageOnProcess);

        chooseFolder.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {

                Intent chooseFile;
                Intent intent;
                chooseFile = new Intent(Intent.ACTION_GET_CONTENT);
                chooseFile.setType("file/*");
                intent = Intent.createChooser(chooseFile, "Choose a file");
                startActivityForResult(intent, ACTIVITY_CHOOSE_FILE);

            }
        });
    }


    protected void onActivityResult(int requestCode, int resultCode, Intent data) {
        if (resultCode != RESULT_OK) return;

        if (requestCode == ACTIVITY_CHOOSE_FILE) {
            Uri uri = data.getData();
            String firstFilePath = getRealPathFromURI(uri);

            //МНОГО КОДА ПОПИКСЕЛЬНОГО АНАЛИЗА ИЗОБРАЖЕНИЯ И ПЕРЕМЕЩЕНИЕ В НУЖНУЮ ПАПКУ

        }
    }

    public String getRealPathFromURI(Uri contentUri) {
        String[] proj = {MediaStore.Images.Media.DATA};
        Cursor cursor = getContentResolver().query(contentUri, proj, null, null, null);
        if (cursor == null) return null;
        int column_index = cursor.getColumnIndexOrThrow(MediaStore.Images.Media.DATA);
        cursor.moveToFirst();
        return cursor.getString(column_index);
    }
}