filechooser is an Emacs package that enables the use of Emacs to select files in applications like Firefox by providing an xdg-desktop-portal backend implementing the filechooser interface. It uses minibuffer to select a single file, so the experience is familiar one for commands like find-file. By default selecting multiple files uses a bespoke command which loops and completes one filename in minibuffer at a time. An alternative dired based selection is also available.

At this point filechooser has some serious limitations:

1. It requires a running Emacs instance with server also running. You can start the server by either running Emacs with the --daemon flag, or by typing M-x server-start RET in a non-daemon Emacs instance. If Emacs or server aren't running no file selection dialogues will appear and no error will be shown. More on this below.
2. If the systemd service xdg-desktop-portal.service starts before Emacs, it will be need to be restarted after the server has started to make filechooser work. A possible mitigation is to have

(with-eval-after-load 'server
  (start-process "restart-portal" nil "systemctl" "--user" "restart" "xdg-desktop-portal.service"))

in your config file (This likely has a race condition).

3. I don't know a way of reliably positioning Emacs windows so cooperation from compositor/window manager is required. For example, I use sway and I have,

for_window [app_id="emacs" title="filechooser-frame"] {
    floating enable
}

for_window [app_id="emacs" title="filechooser-miniframe"] {
    floating enable
}

so that the frame filechooser uses appears above tiling windows.

4. The choices combo boxes are not implemented. I don't what would be a good interface for them (suggestions welcome) and in my (limited) experience I have not encountered them.

5. In my opinion minor compared to these is the fact the filechooser dialogues are not modal, i.e. you can still freely interact with the window that sent the file selection request.

6. Although OpenFile and SaveFile methods have been tested by me, SaveFiles method is completely untested since I don't know a way to trigger it.

If despite these limitations you want to test the package you can use one of the following options:

1. Place filechooser.el file in the load path and generate the autoloads file for it.

2. Configure xdg-desktop-portal to use Emacs based filechooser. For recent versions this can be done by adding

org.freedesktop.impl.portal.FileChooser=emacs

to ~/.config/xdg-desktop-portal/portals.conf. For older versions add your $XDG_CURRENT_DESKTOP to the emacs.portal file and copy it to /usr/share/xdg-desktop-portal/portals/.

3. Copy the org.gnu.Emacs.FileChooser.service file /usr/share/dbus-1/services/.

4. Restart xdg-desktop-portal by using systemctl --user restart xdg-desktop-portal.service.

filechooser.el is available on GNU ELPA and can be installed using M-x package-install. This will take care of step 1 in the instructions for manual install but the other step will have to be taken manually.

Here's how to install this package on NixOS.

configuration.nix:

{ pkgs, ... }:

{
  xdg.portal = {
    enable = true;
    xdgOpenUsePortal = true;

    extraPortals = [
        # Add any fallback portals here
        pkgs.xdg-desktop-portal-gtk

        # Won't be needed with more recent xdg-desktop-portal versions
        (import ./emacs-portal.nix { inherit pkgs; })
      ];
      config.common = {
        default = [ "gtk" ];

        "org.freedesktop.impl.portal.FileChooser" = "emacs";
    };
  };
}

emacs-portal.nix:

{ pkgs }:
with pkgs;

stdenv.mkDerivation {
  pname = "Emacs portal";
  version = "git";

  src = fetchGit {
      name = "filechooser";
      url = "https://codeberg.org/rahguzar/filechooser";
      rev = "866304ab4244865108e12499f6e3be63e4981f92";
    };

  installPhase = ''
    mkdir -p $out/share/dbus-1/services/;
    cp org.gnu.Emacs.FileChooser.service $out/share/dbus-1/services/;
    mkdir -p $out/share/xdg-desktop-portal/portals/;
    cp emacs.portal $out/share/xdg-desktop-portal/portals/;
    '';
}

Start an application that uses xdg-desktop-portal. An example which is my main use case is to launch GTK_USE_PORTAL=1 firefox and initiate a file selection dialogue. Unless you have changed the value of `filechooser-use-popup-frame, a minibuffer only Emacs frame will hopefully appear at this point. You can select the file (or files depending on the request) by the same means you do usually in minibuffer.

The only new feature is that you can use different filters to determine which files are presented for completion. Some of these filters are defined by the request, but you can also define your own in filechooser-filters. To toggle a filter use C-f, this will present all filters grouped according to whether they are active or inactive. Select one to toggle it. Only those files which pass through an active filter are presented for completion.

If you are selecting multiple files M-TAB toggles selection of the current file without exiting the file selection dialogue. RET exits while toggling the selection of current file. If you want to exit with currently selected files without toggling the currently highlighted candidate use M-RET. M-a selects all of current candidates.

By default, filechooser uses a new frame for presenting the file selection dialogue. This can be disabled by setting filechooser-use-popup-frame to nil.

By default completing-read based interfaces are used for all operations. These interfaces are governed by the value of filechooser-choose-file, filechooser-choose-files and filechooser-choose-directory. The value of each of the these variables is a function that is called to do completion, see their doc strings for what is expected from these functions.

The only builtin alternative to default values is filechooser-with-dired. You can use,

(setq filechooser-choose-files #'filechooser-with-dired)

to change to a dired based selection for multiple files. In this mode the file chooser frame will have a dired buffer selected. To select files navigate to the desired directory. Mark the files to be selected and press C-c C-c to finish. C-c C-k aborts the selection process. Filters are available in this mode too and can be changed using C-f.

If you want to use some commands during file selection you can them on filechooser-mininuffer-map. For binding a command only during multiple file selection use filechooser-multiple-selection-map.

If you vertico as your completion UI, the following might be more intuitive for selecting multiple files,

(with-eval-after-load 'filechooser
  (defun +filechooser-multiple-vertico-tab ()
    (interactive)
    (vertico-insert)
    (unless (file-directory-p (minibuffer-contents))
      (filechooser-multiple-continue)))
  (define-key filechooser-multiple-selection-map
              (kbd "TAB") #'+filechooser-multiple-vertico-tab))

This rebinds TAB so what if you press it with a directory highlighted, it enters the directory, while if you press with a file highlighted, the selection of that file is toggled and you can continue selecting more files.

If you have some suggestions for improvement, please file a bug report or even better a start a pull request. Especially welcome are implementations for:

1. Choices combo boxes.
2. An auto start mechanism that starts an emacsclient and use it if Emacs or server are not running.

Originally I wanted to write a portal which acted as a global minor mode i.e a user would run M-x filechooser-mode and Emacs would take over file selection dialogues. User would run M-x filechooser-mode again and Emacs would no longer meddle with these dialogues. However this seems not be possible. xdg-desktop-portal does not allow user specific portal definitions, as a result to place the emacs.portal file in the correct location root access is required. Once that file is placed and xdg-desktop-portal restarted, only Emacs can be used for filechooser interface. If Emacs doesn't claim the dbus service required, no file selection dialogues will appear and no error will be raised either (at least firefox fails completely silently) since there is no fallback mechanism to use a working service if the first one couldn't be started.