Editing GNUstep (section)

== Software architecture==
[[File:Free and open-source-software display servers and UI toolkits.svg|thumb|Software components of the Linux desktop include the [[display server]], [[widget toolkit|graphics control element libraries]], and [[graphical shell]]s.]]

The [[Foundation Kit]] provides basic classes such as wrapper classes and data structure classes. The [[Application Kit]] provides classes oriented around [[graphical user interface]] capabilities. GNUstep contains a set of [[graphical control element]]s written in the Objective-C programming language.

The [[graphical user interface]] (GUI) of [[GNUMail]] is composed of graphics control elements. GNUMail has to interact with the [[windowing system]], e.g. [[X Window System core protocol|X11]] or [[Wayland (display server protocol)|Wayland]], and its graphical user interface has to be [[Rendering (computer graphics)|rendered]]. GNUstep's backend provides a small set of functions used by the user interface library to interface to the actual [[windowing system]]. It also has a [[rendering engine]] which emulates common [[PostScript|Postscript]] functions. The package gnustep-back provides the following backends:

* cairo – default backend using the [[cairo (graphics)|Cairo]] 2D graphics library.
* winlib – default backend on Microsoft Windows systems. Cairo and [[Windows API]] variants.
* art – old (deprecated) backend on unix-like systems. Uses the vector-based PostScript-like 2D graphics library [[Libart]].
* [[xlib]] – old (deprecated) X11 backend.

GNUstep inherits some design principles proposed in OPENSTEP (GNUstep predates Cocoa, but Cocoa is based on OPENSTEP) as well as the [[Objective-C]] language.

* [[Model–view–controller]] paradigm
* [[Target–action]]
* [[Drag-and-drop]]
* [[Delegation (computer science)|Delegation]]
* [[Message passing|Message forwarding]] (through NSInvocation)

In addition to the Objective-C interface, some small projects under the GNUstep umbrella implement other APIs from Apple:
* The Boron library aims to implement the [[Carbon (API)|Carbon API]]. It is very incomplete.
* The CoreBase library is designed to be compatible with [[Core Foundation]]. It is not complete enough for the Base (Foundation Kit) component to simply be a wrapper around it.
* The QuartzCore library implements [[Core Animation]] APIs. The Opal library implements [[Quartz 2D]].
{{as of|2020|02}}, there are no projects that build the [[Swift programming language]] against the GNUstep Objective-C environment.