Xfce

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Xfce is a lightweight desktop environment based on GTK+. It includes a window manager, a file manager, desktop and panel.

Enabling

To use xfce set service.xserver.desktopManager.xfce.enable to true. For example:

Breeze-text-x-plain.png
/etc/nixos/configuration.nix
{ config, pkgs, callPackage, ... }: {
  ...
  # if you use pulseaudio
  nixpkgs.config.pulseaudio = true;

  services.xserver = {
    enable = true;
    desktopManager = {
      xterm.enable = false;
      xfce.enable = true;
    };
    displayManager.defaultSession = "xfce";
  };
  ...
}

Using as a desktop manager and not a window manager

You can use xfce purely as a desktop manager, leaving window management to another window manager like i3 for example. In this scenario, xfce's role is to answer to media keys, prompt when plugging a new monitor and so on.

Example config:

Breeze-text-x-plain.png
/etc/nixos/configuration.nix
{ config, pkgs, callPackage, ... }: {
  ...
  services.xserver = {
    enable = true;   
    desktopManager = {
      default = "xfce";
      xterm.enable = false;
      xfce = {
        enable = true;
        noDesktop = true;
        enableXfwm = false;
      };
    };
    windowManager.i3.enable = true;
    displayManager.defaultSession = "xfce+i3";
  };
  ...
}

On first login, make sure to choose the session xfce+i3 in your display manager. If you choose xfce you will end up in xfce without panels nor window manager, which is unusable.

Note that xfce manages your session instead of i3: exiting i3 will blank your screen but not terminate your session. In your i3 config, replace i3-msg exit with xfce4-session-logout.

With xmonad as the window manager

One of the possibilities is to use xmonad as a window manager in a Xfce desktop environment.

Without xfce desktop

The previously described configuration is extended with the part that configures xmonad:

Breeze-text-x-plain.png
/etc/nixos/configuration.nix
{ config, pkgs, callPackage, ... }: {
  ...
  services.xserver = {
    enable = true;   
    desktopManager = {
      xterm.enable = false;
      xfce = {
        enable = true;
        noDesktop = true;
        enableXfwm = false;
      };
    };
    windowManager = {
      xmonad = {
        enable = true;
        enableContribAndExtras = true;
        extraPackages = haskellPackages : [
          haskellPackages.xmonad-contrib
          haskellPackages.xmonad-extras
          haskellPackages.xmonad
        ];
      };
    };
    displayManager.defaultSession = "xfce+xmonad";
  };
  ...
}


Xmonad's contrib package comes with a config to integrate seamlessly into Xfce, like connecting workspaces to xfce's top panel's preview of workspaces. To enable this config, put the following into the user's xmonad config file:

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~/.xmonad/xmonad.hs
  import XMonad
  import XMonad.Config.Xfce
  main = xmonad xfceConfig
         { terminal = "xfce4-terminal"
         , modMask = mod4Mask -- optional: use Win key instead of Alt as MODi key
         }

Since Xfce uses Alt for a lot of keybindings, using the Win key for xmonad hotkeys may be preferred.

After choosing the xfce+xmonad session in your display manager, you will be taken to a clean screen, where you can open a terminal with MOD+Shift+Enter or launch an application with MOD(+SHIFT)+p.

With xfce desktop

If you instead prefer to have panels (like the top panel) in addition to the main clean display area managed by xmonad, you can remove the xfce.noDesktop = true; option from the configuration.

After switching to your new configuration, reboot and clean your sessions with rm -rf ~/.cache/sessions/* before logging in to a graphical session.

After logging in you will be greeted by xfce's desktop which interferes with xmonad. To solve this issue you have to remove the xfdesktop process from being started in the session. Open the session manager in the application launcher with MOD(+SHIFT)+p and then typing in "Session and Startup". Go to tab "Session" and set the restart style of xfdesktop to "Never". Kill the process with "Quit program", then "Save session." After this, xfce4 and xmonad will work together nicely.

Java-based GUI applications

Java-based applications may not work properly with xmonad. The applications main window may stay blank or gray on startup. This is a known issue with some versions of Java, where xmonad is not recognized as a "non-reparenting" window manager. There are multiple solutions to this problem as described on xmonad's FAQ page.

One alternative is to fake xmonad's window manager name, after running the EMWH initialization. This particular approach works well when running xmonad alongside the xfce-based desktop (described above).

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~/.xmonad/xmonad.hs
    import XMonad
    import XMonad.Config.Xfce
    import XMonad.Hooks.EwmhDesktops
    import XMonad.Hooks.SetWMName

    main = xmonad xfceConfig
            { terminal = "xfce4-terminal"
            , modMask = mod4Mask -- Use Win as MOD key
            , startupHook = ewmhDesktopsStartup >> setWMName "LG3D" -- for some reason the double greater sign is escaped here due to wiki formatting, replace this with proper greater signs!
            }


Additional resources

Note that, unlike suggested in additional resources, the xmonad packages should not be installed in the environment (neither as systemPackages nor user packages), since that leads to errors when (re)compiling xmonad's config file.

Additional resources:

Haskell Wiki: Installing xmonad on NixOS

Haskell Wiki: Using xmonad in Xfce

Haskell Wiki: Xmonad default key bindings

Haskell Wiki FAQ: Problems with Java applications

Troubleshooting

Pulseaudio

If you use pulse audio, set nixpkgs.config.pulseaudio = true as shown above. Otherwise, you may experience glitches like being able to mute the sound card but not unmute it.