GTK

GTK (formerly GIMP ToolKit and GTK+) is a free software cross-platform widget toolkit for creating graphical user interfaces (GUIs). It is licensed under the terms of the GNU Lesser General Public License, allowing both free and proprietary software to use it. It is one of the most popular toolkits for the Wayland and X11 windowing systems.

The GTK team releases new versions on a regular basis. GTK 4 and GTK 3 are maintained, while GTK 2 is end-of-life. GTK1 is independently maintained by the CinePaint project.

The GTK library contains a set of graphical control elements (widgets); version 3.22.16 contains 186 active and 36 deprecated widgets. GTK is an object-oriented widget toolkit written in the programming language C; it uses GObject (that is, the GLib object system) for object orientation. While GTK is mainly used with windowing systems based on X11 and Wayland, it works on other platforms, including Microsoft Windows (interfaced with the Windows API), and macOS (interfaced with Quartz). There is also an HTML5 back-end named Broadway.

GTK can be configured to change the look of the widgets drawn; this is done using different display engines. Several display engines exist which try to emulate the look of the native widgets on the platform in use.

Starting with version 2.8, released in 2005, GTK began the transition to using Cairo to render most of its graphical control elements widgets. Since GTK version 3.0, all rendering is done using Cairo.

On January 26, 2018 at DevConf.cz, Matthias Clasen gave an overview of the current state of GTK 4 development, including a high-level explanation of how rendering and input worked in GTK 3, what changes are being made in GTK 4 (>3.90), and why. On February 6, 2019 it was announced that GTK 4 will drop the “+” from the project's name.

GDK acts as a wrapper around the low-level functions provided by the underlying windowing and graphics systems.

GSK is the rendering and scene graph API for GTK. GSK lies between the graphical control elements (widgets) and the rendering. GSK was finally merged into GTK version 3.90 released March 2017.

GtkBuilder allows user interfaces to be designed without writing code. The interface is described in an Extensible Markup Language (XML) file which is written by hand or generated by a GUI designer, which is then loaded at runtime and the objects created automatically. The description of the user interface is independent from the programming language being used.

Language bindings are available for using GTK from languages other than C, including C++, Genie, JavaScript, Perl, Python, Vala, and others.

GTK supports various backends, which provides different ways to display GTK applications depending on the system and environment. Examples of GTK backends are:

• Wayland – Used with the Wayland display server on Linux systems, it is a modern replacement for X11.
• X11 – The default on Linux systems using the X.Org display server.
• Win32 – For running GTK applications on Windows.
• Quartz – For macOS support.
• Broadway – Allows GTK applications to run in web browsers using HTML5 and WebSocket.

There are several GUI designers for GTK. Here is a selection of GTK GUI designers:

• Cambalache - a successor to Glade, supports GTK 4.
• Glade - supports GtkBuilder, which is a GTK built-in GUI description format. (not actively maintained)
• Gazpacho - GUI builder for the GTK toolkit written in Python
• Crow Designer - relies on its own GuiXml format and GuiLoader library.
• Stetic - part of MonoDevelop, oriented toward Gtk#.
• Gambas (since version 2.0 atop BASIC)
• Xojo
• Lazarus (on Linux defaults to interfacing with GTK 2)

The GTK Inspector is a built-in interactive debugging tool in GTK, allowing developers to inspect and modify UI elements, test CSS changes, and analyze widget structure in real time. It can be enabled using the Control + Shift + I or Control + Shift + D shortcuts, or by setting the GTK_DEBUG=interactive environment variable. It was introduced with GTK version 3.14.

• Interactive debugging
• Real-time CSS testing and modifications
• Widget magnification for detailed inspection
• UI structure analysis and object property examination
• Customizable display settings via environment variables
• Detailed object inspection (type, state, properties, CSS, actions, etc.)
• Global application information display
• CSS rule debugging
• Rendering pipeline recording and inspection

GTK is mainly developed by The GNOME Project, which also develops the GNOME Development Platform and the GNOME Desktop Environment. GTK is mainly written in C. Many language bindings are available. GTK development is loosely managed.

GNOME developers and users gather at an annual GNOME Users And Developers European Conference GUADEC meeting to discuss GNOME's current state and future direction. GNOME incorporates standards and programs from freedesktop.org to better interoperate with other desktops.

Many GNOME applications have been ported to GTK 4, which was released in December 2020, however some still use GTK+ 3 (GIMP being a major one).

The master branch of GTK utilizes Meson for its build automation. GTK (and GNOME, GLib, etc.) formerly utilized the GNU Build System (named Autotools) as the build automation system of choice. Since August 14, 2017, the Autotools build system files have been dropped.

The most common criticism of GTK is the lack of backward-compatibility in major updates, most notably in the application programming interface (API) and theming. The result is that application developers or theme developers have to rewrite parts of their code to make it work with a newer version of GTK.

The compatibility breaks between minor releases during the GTK 3.x development cycle was explained by Benjamin Otte as due to strong pressures to innovate, such as providing the features modern users expect and supporting the increasingly influential Wayland display server protocol. With the release of GTK 4, the pressure from the need to innovate will have been released and the balance between stability and innovation will tip toward stability. Similarly, recent changes to theming are specifically intended to improve and stabilise that part of the API, meaning some investment now should be rewarded later.

• Aurélien Gâteau started Gwenview as GTK application but switched to qt early in development.
• Dirk Hohndel, codeveloper of Subsurface and member of Intel's Open-Source Technology Center, criticized the GTK developers for being abrasive and ignoring most community requests.
• Hong Jen Yee, developer of LXDE (the GTK version of which was dropped and all efforts focused on the Qt port), expressed disdain for version 3 of the GTK toolkit's radical API changes and increased memory usage, and ported PCMan File Manager (PCManFM) to Qt. PCManFM is being developed with a GTK and with a Qt backend at the same time.
• The Audacious music player moved to Qt in version 3.6. The reasons stated by the developers for this include a transition to client-side window decorations, which they claim cause the application to look "GNOME-y and out of place."
• Wireshark has switched to Qt due to not having a good experience with GTK's cross-platform support.

Some notable applications that use GTK as a widget toolkit include:

• Ardour, a digital audio workstation (DAW)
• Deluge, a BitTorrent client
• Foliate, an ebook reader
• GIMP, a raster graphics editor
• GNOME Core Applications, a collection of applications as a standard bundle of the GNOME desktop environment
• GNOME Circle, a collection of applications created to work within the GNOME ecosystem
• GNOME Evolution, a personal information manager
• HandBrake, digital video transcoder
• Inkscape, a vector graphics editor
• LibreOffice, an office suite
• Lutris, a game manager
• Mozilla Firefox, a web browser
• Mozilla Thunderbird, a personal information manager
• Pitivi, a video editor
• PCSX-Reloaded, a video game console emulator
• REAPER, a digital audio workstation (DAW)
• Remmina, a remote desktop client
• Transmission, a Bit Torrent client

GTK programs can be run on desktop environments based on X11 and Wayland, or others including ones not made with GTK, provided the needed libraries are installed; this includes macOS if X11.app is installed. GTK can be also run on Microsoft Windows. It is used by some popular cross-platform applications like Pidgin and GIMP. wxWidgets, a cross-platform GUI toolkit, uses GTK on Linux by default. Other ports include DirectFB (for example used by the Debian installer).

Several desktop environments utilize GTK as the widget toolkit.

• GNOME, based on GTK, meaning that programs native to GNOME use GTK
• Budgie, built from scratch for the SolusOS successor, Solus Operating System
• Cinnamon, a fork of GNOME 3 which uses GTK version 3
• MATE, a fork of GNOME 2 which uses GTK 3 since version 1.18
• Xfce, based on GTK 3 since version 4.14
• Pantheon uses GTK 3 & 4, being developed by elementary OS
• Sugar, a desktop environment for youth primary education, which uses GTK, especially PyGTK
• Phosh, a mobile UI designed for PureOS
• LXDE (Lightweight X11 Desktop Environment) is based on GTK 2
• Unity, the former default desktop environment of Ubuntu

• Access Linux Platform (successor of the Palm OS PDA platform)
• Consort, the GNOME 3.4 Fallback Mode – fork from Solus
• GPE, the GPE Palmtop Environment
• ROX Desktop, a lightweight desktop, with features from the GUI of RISC OS

The following window managers use GTK:

For syntax highlighting there is GtkSourceView, "source code editing widget". GtkSourceView is maintained by GNOME separately from GTK as a library: gtksourceview. There are plans to rename to gsv.

GtkSpell is a library separate from GTK. GtkSpell depends on GTK and Enchant. Enchant is a wrapper for ispell, hunspell, etc., the actual spell checker engine/software. GtkSpell uses GTK's GtkTextView widget, to highlight misspelled words and offer replacement.

GTK was originally designed and used in the GNU Image Manipulation Program (GIMP) as a replacement of the Motif toolkit; at some point Peter Mattis became disenchanted with Motif and began to write his own GUI toolkit named the GIMP toolkit and had successfully replaced Motif by the 0.60 release of GIMP. Finally GTK was re-written to be object-oriented and was renamed GTK+. This was first used in the 0.99 release of GIMP. GTK was subsequently adopted for maintenance by the GNOME Foundation, which uses it in the GNOME desktop environment.

The GTK 2.0.0 release (2002) series introduced new features which include improved text rendering using Pango, a new theme engine, improved accessibility using the Accessibility Toolkit, transition to Unicode using UTF-8 strings, and a more flexible API. Starting with version 2.8, GTK 2 depends on the Cairo graphics library for rendering vector graphics.

GTK version 3.0.0 (2011) included revised input device handling, support for themes written with CSS-like syntax, and the ability to receive information about other opened GTK applications.

The '+' was dropped returning to simply 'GTK' in February 2019 during a Hackathon.

Release of the first GTK 4 version was in December 2020. At the 2018 edition of DevConf.cz, Matthias Clasen gave an overview of the then-current state of GTK 4 development, including a high-level explanation of how rendering and input worked in GTK 3, what changes were being made to GTK 4, and the reasons for those changes. Examples of things that have become possible with GTK 4 were given as well.

One of the main changes made during the GTK 4 development cycle (i.e. GTK 3.92, etc.) was the removal of user customization options (like individual keyboard shortcuts that could be set in GTK+ 2), and the delegation of functionality to ancillary objects instead of encoding it into the base classes provided by GTK. Other changes include:

• Event handling from signal handlers described by GtkWidget is delegated to event controllers.
• Rendering is delegated to GtkSnapshot objects.
• The layout mechanism is delegated from GtkWidget to GtkLayoutManager.

• Client-side decoration
• List of widget toolkits
• gtkmm – C++ bindings for GTK
• Enlightenment Foundation Libraries (EFL) – widget toolkit written for the Enlightenment window manager
• FLTK – a light, cross-platform, non-native widget toolkit
• Fox toolkit – a fast, open source, cross-platform widget toolkit
• IUP – a multi-platform toolkit for building native graphical user interfaces
• Ultimate++
• Visual Component Library (VCL)

• Official website