{ pkgs, pkgsStable, pkgsMaster, config, lib, ... } @args: with lib; let
	cfg = config.module.desktop.app;

	mangohud = import ./mangohud args;
	firefox  = import ./firefox args;
in {
	options = {
		module.desktop.app = {
			common.enable   = mkEnableOption "Common Apps.";
			gaming.enable   = mkEnableOption "Gaming Apps.";
			creative.enable = mkEnableOption "Creative Apps.";
			dev.enable      = mkEnableOption "Dev Apps.";
			extra.enable    = mkEnableOption "Extra Apps.";
		};
	};

	config = mkMerge [
		# Common apps.
		(mkIf cfg.common.enable { xdg.mime.defaultApplications = {
				# Use `file -i file.txt` to find file mime type.
				# Use `xdg-mime query default "text/plain"` to find default app.
				"application/pdf" = "org.gnome.Evince.desktop";
				"application/vnd.openxmlformats-officedocument.*" = "onlyoffice-desktopeditors.desktop";
				"audio/*" = "mpv.desktop";
				"image/*" = "org.gnome.Loupe.desktop";
				"text/*"  = "nvim.desktop";
				"video/*" = "mpv.desktop";
			};

			environment = {
				systemPackages = with pkgs; [
					appimage-run             # Tool to run .AppImage files in NixOS.
					evince                   # Document viewer.
					foot                     # Terminal emulator.
					gimp                     # Image manipulation program.
					gnome.adwaita-icon-theme # GTK icons.
					gnome.gnome-calculator   # Calculator.
					gnome.nautilus           # File manager.
					jellyfin-media-player    # Jellyfin client (self-hosted Netflix).
					loupe                    # Image viewer.
					obs-studio               # Streaming/recording app.
					onlyoffice-bin           # Office documents app suite.

					(mpv.override {scripts = [mpvScripts.mpris];}) # Media player.
				];

				variables = {
					# GTK apps compat.
					GTK_CSD = 0;

					# Terminal settings.
					TERM     = "xterm-256color";
					TERMINAL = config.setting.terminal.bin;

					# Disable Firefox profile switching on rebuild.
					MOZ_LEGACY_PROFILES = "1";
				};
			};

			programs.firefox = let
				mkExtension = install_url: {
					inherit install_url;
					installation_mode = "force_installed";
				};

				mkBookmark = name: url: { inherit name url; };
			in {
				enable  = true;
				package = pkgs.firefox-esr;
				languagePacks = [ "en-US" "ru" ];
				autoConfig = firefox.config;
				policies = {
					ManagedBookmarks = [
						{ toplevel_name = "Pin"; }
						(mkBookmark "Dashboard" "https://home.voronind.com")
						(mkBookmark "Music"     "https://music.yandex.ru")
						(mkBookmark "Telegram"  "https://web.telegram.org/a")
						(mkBookmark "Discord"   "https://discord.com")
						(mkBookmark "WorkMail"  "https://mail.fsight.ru")
						(mkBookmark "Git"       "https://git.voronind.com")
						(mkBookmark "WorkGit"   "https://git.fmp.fsight.world")
						(mkBookmark "WorkBoard" "https://support.fsight.ru")
						(mkBookmark "Hass"      "https://iot.voronind.com")
						(mkBookmark "Cloud"     "https://cloud.voronind.com")
					];
					ExtensionUpdate = true;
					ExtensionSettings = {
						# Block extension installation outside of this config.
						"*" = {
							install_sources   = [ "*" ];
							installation_mode = "blocked";
						};
						"addon@darkreader.org"                   = mkExtension "https://addons.mozilla.org/firefox/downloads/latest/darkreader/latest.xpi";
						"cliget@zaidabdulla.com"                 = mkExtension "https://addons.mozilla.org/firefox/downloads/latest/cliget/latest.xpi";
						"uBlock0@raymondhill.net"                = mkExtension "https://addons.mozilla.org/firefox/downloads/latest/ublock-origin/latest.xpi";
						"{446900e4-71c2-419f-a6a7-df9c091e268b}" = mkExtension "https://addons.mozilla.org/firefox/downloads/latest/bitwarden-password-manager/latest.xpi";
						"{d7742d87-e61d-4b78-b8a1-b469842139fa}" = mkExtension "https://addons.mozilla.org/firefox/downloads/latest/vimium-ff/latest.xpi";
						"{e7625f06-e252-479d-ac7a-db68aeaff2cb}" = mkExtension "https://addons.mozilla.org/firefox/downloads/latest/togglefonts/latest.xpi";
						"{a6c4a591-f1b2-4f03-b3ff-767e5bedf4e7}" = mkExtension "https://addons.mozilla.org/firefox/downloads/latest/user-agent-string-switcher/latest.xpi";
						# NOTE: This extension is helpful to find the required parameters for this config.
						# Or find them yourself inside the `about:support`.
						# "queryamoid@kaply.com"                   = mkExtension "https://github.com/mkaply/queryamoid/releases/download/v0.1/query_amo_addon_id-0.1-fx.xpi";
					};
					# NOTE: `firefox-esr` edition is required to change default search engine.
					SearchEngines = {
						Default = "Searx";
						Add = [
							{
								Alias              = "s";
								Description        = "Searx Search";
								IconURL            = "https://search.voronind.com/favicon.ico";
								Method             = "POST";
								Name               = "Searx";
								PostData           = "q={searchTerms}";
								# SuggestURLTemplate = "https://search.voronind.com/autocomplete?q={searchTerms}";
								URLTemplate        = "https://search.voronind.com/search?q=%{searchTerms}";
							}
						];
					};
				};
			};
		})

		# Gaming.
		(mkIf cfg.gaming.enable {
			hardware.graphics = let
				packages = with pkgs; [
					dxvk
					gamescope
					pkgs.mangohud
					vkd3d
				];
			in {
				extraPackages   = packages;
				extraPackages32 = packages;
			};

			programs.steam.enable = true;

			environment = {
				systemPackages = with pkgs; [
					steam-run
					bottles dxvk gamescope pkgs.mangohud vkd3d wine64 # Gaming!
				];

				variables = {
					MANGOHUD = "1";
					MANGOHUD_CONFIGFILE  = pkgs.writeText "MangoHudConfig" mangohud.config;
					MANGOHUD_PRESETSFILE = pkgs.writeText "MangoHudPreset" mangohud.presets;

					WINEFSYNC = "1";
				};
			};
		})

		# Creative.
		(mkIf cfg.creative.enable {
			environment.systemPackages = with pkgs; [
				aseprite    # Pixel Art draw app. WARNING: Always builds from source.
				blender-hip # Blender with HiP support.
				krita       # Draw!
			];
		})

		# Development.
		(mkIf cfg.dev.enable {
			environment.systemPackages = with pkgs; [
				android-studio
				jetbrains.idea-community
			];
		})

		# Extras.
		(mkIf cfg.extra.enable {
			environment.systemPackages = with pkgs; [
				anilibria-winmaclinux   # Anime!
				blanket                 # Sounds generator.
				calibre                 # Book library manager.
				gnome.gnome-font-viewer # Font viewer.
				tor-browser             # Privacy browser.
			];
		})
	];
}