Write this Part:

//wait until picture on Phone
    File xaifile = new File(Environment.getExternalStorageDirectory() + "/image_from_server.jpg");
    while (true){
        if (xaifile.exists()){
            break;
        }
    }
    replacePicture(xaifile);

in onPostExecute

Currently You are Blocking Main Thread.

i have following problem: I have a mobile App which, on button click (“yesbutton”) fetches data from server in an Async Task (which works fine). While that task is running, I want my app to display a Layout (framelayout), which I set to invisible at creation and which contains a textview and a Circle Progressbar. So i put the line where i set it visible into my onPreExecute method in my Asnyc task, because i read that this method will be executed on the UI Thread.

But this framelayout is only showing after my whole code has run, but not while executing.

Here is my code, I removed a chunk from it where i dont think the problem is, to make it more readable for you.

public class KeepImageActivity extends AppCompatActivity {

    private static final int PORT_NO = 1234;
    private String pathname;
    private File imgFile;
    private Button yesbutton;
    private Button nobutton;
    private ImageView imageView;
    private FrameLayout frameLayout;


    @SuppressLint("SourceLockedOrientationActivity")
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setRequestedOrientation(ActivityInfo.SCREEN_ORIENTATION_PORTRAIT);
        setContentView(R.layout.activity_keep_image);

        Intent receivedIntent = getIntent();
        pathname = receivedIntent.getStringExtra(CameraActivity.EXTRA_MESSAGE);
        imgFile = new File(pathname);

        imageView = findViewById(R.id.imageViewID);
        yesbutton = findViewById(R.id.yesbuttonID);
        nobutton = findViewById(R.id.discardbuttonID);

        nobutton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                onDiscard(pathname);
            }
        });
        yesbutton.setOnClickListener(new View.OnClickListener() {
            @RequiresApi(api = Build.VERSION_CODES.O)
            @Override
            public void onClick(View view) {
                onYes();
            }
        });

        frameLayout = findViewById(R.id.framelayout);
        frameLayout.setVisibility(View.GONE);
        showImage();
    }

    //method to show a picture in an ImageView
    private void showImage() {
        imgFile = new File(pathname);
        Bitmap bitmap = BitmapFactory.decodeFile(imgFile.getAbsolutePath());
        imageView.setRotation(90);
        imageView.setImageBitmap(bitmap);
    }

    private void onDiscard(String pathname) {
        //....not relevant
}


    @RequiresApi(api = Build.VERSION_CODES.O)
    private void onYes() {

        //doInBackground just fetches data and writes to jpg
        class ConnectTask extends AsyncTask<Void, Void, Void> {
            @Override
            protected Void doInBackground(Void... voids) {
                //doing stuff (connection to server and downloading image ("image_from_server"))
            }

            @Override
            //here my frameLayout is not being set to Visible, at least not DURING execution.
            protected void onPreExecute() {
                frameLayout.setVisibility(View.VISIBLE);

            }

            @Override
            protected void onPostExecute(Void aVoid) {

            }

            @Override
            protected void onProgressUpdate(Void... values) {
            }
        }



        ConnectTask connect =  new ConnectTask();
        Void[] param = null;

        //execute Async task
        connect.execute(param);


        //wait until picture on Phone
        File xaifile = new File(Environment.getExternalStorageDirectory() + "/image_from_server.jpg");
        while (true){
            if (xaifile.exists()){
                break;
            }
        }
        replacePicture(xaifile);
    }


    //here i replace the picture/bitmap in the ImageView with another one (that just came from server)
    private void replacePicture(File xaifile) {
        Bitmap bitmap = BitmapFactory.decodeFile(Environment.getExternalStorageDirectory() + "/image_from_server.jpg");
        imageView.setRotation(90);
        imageView.setImageBitmap(bitmap);

        //finished loading, I commented this line to see if my Layout would show afterwards. It does.
        //frameLayout.setVisibility(View.INVISIBLE);

        //delete both files, not needed anymore
        xaifile.delete();
        imgFile.delete();
    }

//AND IT SEEMS LIKE HERE IS THE MOMENT THAT THE UPDATE TO THE UI IS SHOWN. WHY??
}