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

let

  inherit (lib)
    mapAttrsToList mkEnableOption mkIf mkMerge mkOption optional optionalString
    types;

  associationOptions = with types;
    attrsOf (coercedTo (either (listOf str) str)
      (x: lib.concatStringsSep ";" (lib.toList x)) str);

in {
  meta.maintainers = [ lib.maintainers.misterio77 ];

  options.xdg.portal = {
    enable = mkEnableOption
      "[XDG desktop integration](https://github.com/flatpak/xdg-desktop-portal)";

    extraPortals = mkOption {
      type = types.listOf types.package;
      default = [ ];
      description = ''
        List of additional portals that should be passed to the
        `xdg-desktop-portal.service`, via the `XDG_DESKTOP_PORTAL_DIR`
        variable.

        Portals allow interaction with system, like choosing files or taking
        screenshots. At minimum, a desktop portal implementation should be
        listed.
      '';
    };

    xdgOpenUsePortal = mkOption {
      type = types.bool;
      default = false;
      description = ''
        Sets environment variable `NIXOS_XDG_OPEN_USE_PORTAL` to `1`
        This will make `xdg-open` use the portal to open programs, which resolves bugs involving
        programs opening inside FHS envs or with unexpected env vars set from wrappers.
        See [#160923](https://github.com/NixOS/nixpkgs/issues/160923) for more info.
      '';
    };

    config = mkOption {
      type = types.attrsOf associationOptions;
      default = { };
      example = {
        x-cinnamon = { default = [ "xapp" "gtk" ]; };
        pantheon = {
          default = [ "pantheon" "gtk" ];
          "org.freedesktop.impl.portal.Secret" = [ "gnome-keyring" ];
        };
        common = { default = [ "gtk" ]; };
      };
      description = ''
        Sets which portal backend should be used to provide the implementation
        for the requested interface. For details check {manpage}`portals.conf(5)`.

        These will be written with the name `$desktop-portals.conf` for
        `xdg.portal.config.$desktop` and `portals.conf` for
        `xdg.portal.config.common` as an exception.

        These, together with `xdg.portal.configPackages`, will be joined into a
        directory and passed to `xdg-desktop-portal.service` through a
        `NIXOS_XDG_DESKTOP_PORTAL_CONFIG_DIR` variable.
      '';
    };

    configPackages = mkOption {
      type = types.listOf types.package;
      default = [ ];
      example = lib.literalExpression "[ pkgs.gnome.gnome-session ]";
      description = ''
        List of packages that provide XDG desktop portal configuration, usually in
        the form of `share/xdg-desktop-portal/$desktop-portals.conf`.

        Note that configs in `xdg.portal.config` will be preferred if set.
      '';
    };
  };

  config = let
    cfg = config.xdg.portal;

    joinedPortals = pkgs.buildEnv {
      name = "xdg-portals";
      paths = cfg.extraPortals;
      pathsToLink =
        [ "/share/xdg-desktop-portal/portals" "/share/applications" ];
    };

    portalConfigPath = n:
      "share/xdg-desktop-portal/${
        optionalString (n != "common") "${n}-"
      }portals.conf";
    mkPortalConfig = desktop: conf:
      pkgs.writeTextDir (portalConfigPath desktop)
      (lib.generators.toINI { } { preferred = conf; });

    joinedPortalConfigs = pkgs.buildEnv {
      name = "xdg-portal-configs";
      ignoreCollisions = true; # Let config override configPackages cfgs
      paths = (mapAttrsToList mkPortalConfig cfg.config) ++ cfg.configPackages;
      pathsToLink = [ "/share/xdg-desktop-portal" ];
    };
  in mkIf cfg.enable {
    warnings = optional (cfg.configPackages == [ ] && cfg.config == { }) ''
      xdg-desktop-portal 1.17 reworked how portal implementations are loaded, you
      should either set `xdg.portal.config` or `xdg.portal.configPackages`
      to specify which portal backend to use for the requested interface.

      https://github.com/flatpak/xdg-desktop-portal/blob/1.18.1/doc/portals.conf.rst.in

      If you simply want to keep the behaviour in < 1.17, which uses the first
      portal implementation found in lexicographical order, use the following:

      xdg.portal.config.common.default = "*";
    '';

    assertions = [
      (lib.hm.assertions.assertPlatform "xdg.portal" pkgs lib.platforms.linux)

      {
        assertion = cfg.extraPortals != [ ];
        message =
          "Setting xdg.portal.enable to true requires a portal implementation in xdg.portal.extraPortals such as xdg-desktop-portal-gtk or xdg-desktop-portal-kde.";
      }
    ];

    home = {
      sessionVariables =
        mkIf cfg.xdgOpenUsePortal { NIXOS_XDG_OPEN_USE_PORTAL = "1"; };

      # Make extraPortals systemd units available to the user
      packages = [ pkgs.xdg-desktop-portal ] ++ cfg.extraPortals;
    };

    systemd.user.services.xdg-desktop-portal = {
      Unit = {
        Description = "Portal service";
        PartOf = "graphical-session.target";
      };

      Service = {
        Environment = [
          "XDG_DESKTOP_PORTAL_DIR=${joinedPortals}/share/xdg-desktop-portal/portals"
        ] ++ (optional (cfg.configPackages != [ ])
          "NIXOS_XDG_DESKTOP_PORTAL_CONFIG_DIR=${joinedPortalConfigs}/share/xdg-desktop-portal");
        Type = "dbus";
        BusName = "org.freedesktop.portal.Desktop";
        ExecStart = "${pkgs.xdg-desktop-portal}/libexec/xdg-desktop-portal";
        Slice = "session.slice";
      };
    };
  };
}