{ config, lib, pkgs, ... }:

with lib;

let

  cfg = config.xsession;

in

{
  meta.maintainers = [ maintainers.rycee ];

  options = {
    xsession = {
      enable = mkEnableOption "X Session";

      scriptPath = mkOption {
        type = types.str;
        default = ".xsession";
        example = ".xsession-hm";
        description = ''
          Path, relative <envar>HOME</envar>, where Home Manager
          should write the X session script.
        '';
      };

      windowManager.command = mkOption {
        type = types.str;
        example = literalExample ''
          let
            xmonad = pkgs.xmonad-with-packages.override {
              packages = self: [ self.xmonad-contrib self.taffybar ];
            };
          in
            "''${xmonad}/bin/xmonad";
        '';
        description = ''
          Window manager start command.
        '';
      };

      preferStatusNotifierItems = mkOption {
        type = types.bool;
        default = false;
        example = true;
        description = ''
          Whether tray applets should prefer using the Status Notifier
          Items (SNI) protocol, commonly called App Indicators. Note,
          not all tray applets or status bars support SNI.
        '';
      };

      profileExtra = mkOption {
        type = types.lines;
        default = "";
        description = "Extra shell commands to run before session start.";
      };

      initExtra = mkOption {
        type = types.lines;
        default = "";
        description = "Extra shell commands to run during initialization.";
      };
    };
  };

  config = mkIf cfg.enable {
    systemd.user = {
      services = mkIf (config.home.keyboard != null) {
        setxkbmap =  {
          Unit = {
            Description = "Set up keyboard in X";
            After = [ "graphical-session-pre.target" ];
            PartOf = [ "graphical-session.target" ];
          };

          Install = {
            WantedBy = [ "graphical-session.target" ];
          };

          Service = {
            Type = "oneshot";
            RemainAfterExit = true;
            ExecStart =
              let
                args = concatStringsSep " " (
                  [
                    "-layout '${config.home.keyboard.layout}'"
                    "-variant '${config.home.keyboard.variant}'"
                  ] ++
                  (map (v: "-option '${v}'") config.home.keyboard.options)
                );
              in
                "${pkgs.xorg.setxkbmap}/bin/setxkbmap ${args}";
          };
        };
      };

      # A basic graphical session target for Home Manager.
      targets.hm-graphical-session = {
        Unit = {
          Description = "Home Manager X session";
          Requires = [ "graphical-session-pre.target" ];
          BindsTo = [ "graphical-session.target" ];
        };
      };
    };

    home.file.".xprofile".text = ''
        . "${config.home.profileDirectory}/etc/profile.d/hm-session-vars.sh"

        if [[ -e "$HOME/.profile" ]]; then
          . "$HOME/.profile"
        fi

        # If there are any running services from a previous session.
        # Need to run this in xprofile because the NixOS xsession
        # script starts up graphical-session.target.
        systemctl --user stop graphical-session.target graphical-session-pre.target

        systemctl --user import-environment DBUS_SESSION_BUS_ADDRESS
        systemctl --user import-environment DISPLAY
        systemctl --user import-environment SSH_AUTH_SOCK
        systemctl --user import-environment XAUTHORITY
        systemctl --user import-environment XDG_DATA_DIRS
        systemctl --user import-environment XDG_RUNTIME_DIR
        systemctl --user import-environment XDG_SESSION_ID

        ${cfg.profileExtra}

        export HM_XPROFILE_SOURCED=1
    '';

    home.file.${cfg.scriptPath} = {
      executable = true;
      text = ''
        if [[ ! -v HM_XPROFILE_SOURCED ]]; then
          . ~/.xprofile
        fi
        unset HM_XPROFILE_SOURCED

        systemctl --user start hm-graphical-session.target

        ${cfg.initExtra}

        ${cfg.windowManager.command}

        systemctl --user stop graphical-session.target
        systemctl --user stop graphical-session-pre.target

        # Wait until the units actually stop.
        while [[ -n "$(systemctl --user --no-legend --state=deactivating list-units)" ]]; do
          sleep 0.5
        done
      '';
    };
  };
}