How to wait for async open file action?

[jose1711]

The goal is to have a script open a local file, then manipulate the layers. Issue I am facing here is that I cannot find a way to tell script "wait until the file is loaded and only then continue execute the remaining tasks". As a result layer count is not updated in the code below:

Jython:

from javax.swing import JOptionPane
from org.openstreetmap.josm.gui import MainApplication
from org.openstreetmap.josm.actions import OpenFileAction
import java.io.File as File
from java.lang import Thread

# JOSM is started, no layer is opened
layerManager = MainApplication.getLayerManager()
all_layers = layerManager.getLayers()
JOptionPane.showMessageDialog(MainApplication.getMainFrame(), 'Layer count: {}'.format(len(all_layers)))
# Layer count: 0

# async open of a single file, we now (should) have 1 layer opened
# but we need this task to finish first
open_file_action = OpenFileAction.openFiles([File('/path/to/local/file.osm')])

'''
# this blocks forever:
while True:
    if open_file_action.isDone():
        break
    Thread.sleep(300);

# this blocks forever too:
open_file_action.get()
'''

# this works but a user must click [OK] to continue
JOptionPane.showMessageDialog(MainApplication.getMainFrame(), 'wait for open')

all_layers = layerManager.getLayers()
JOptionPane.showMessageDialog(MainApplication.getMainFrame(), 'Layer count: {}'.format(len(all_layers)))
# Layer count: 1

JS (courtesy of @zdila)

import josm from 'josm' 

const OpenFileAction = Java.type('org.openstreetmap.josm.actions.OpenFileAction');
const File = Java.type('java.io.File');

const future = OpenFileAction.openFiles([new File("/path/to/local/file.osm")]);

josm.console.println("LOOP ");

// no setTimeout available so let's try it hardcore
for (let i = 0; i < 100000; i++) {
  if (future.isDone()) {
    break;
  }
  
  josm.console.println("Layers: " + josm.layers.length + " " + i);
}

josm.console.println("DONE");

[jose1711]

It is rather interesting that almost the same code works fine inside gradle's tests. Could it be that JOSM's GUI interferes?