// First let the user choose a file, e.g. using the file chooser plugin.

showOpenPanel((result, files) {
  if (result != FileChooserResult.ok || files.isEmpty) {
    return;
  }
  
  // Now create a security scoped bookmark
  final secureBookmarks = SecureBookmarks();
  final bookmark = await secureBookmarks.bookmark(File(_file));
  
  // Now store the bookmark somewhere for later invocations
});

// resolve bookmark from persistet 'bookmark' string from earlier
final resolvedFile = await _secureBookmarks.resolveBookmark(_bookmark);
// resolved is now a File object, but before accessing it, call:
await startAccessingSecurityScopedResource(resolvedFile);

// now read/write the file

// and later give up access.
await stopAccessingSecurityScopedResource(resolvedFile);