GNUstep

GNUstep is a free software implementation of the Cocoa (formerly OpenStep) Objective-C frameworks, widget toolkit, and application development tools for Unix-like operating systems and Microsoft Windows. It is part of the GNU Project.
GNUstep features a cross-platform, object-oriented IDE. Apart from the default Objective-C interface, GNUstep also has bindings for Java, Ruby, GNU Guile and Scheme. The GNUstep developers track some additions to Apple's Cocoa to remain compatible. The roots of the GNUstep application interface are the same as the roots of Cocoa: NeXTSTEP and OpenStep. GNUstep thus predates Cocoa, which emerged when Apple acquired NeXT's technology and incorporated it into the development of the original Mac OS X, while GNUstep was initially an effort by GNU developers to replicate the technically ambitious NeXTSTEP's programmer-friendly features.
History.
GNUstep began when Paul Kunz and others at Stanford Linear Accelerator Center wanted to port HippoDraw from NeXTSTEP to another platform. Instead of rewriting HippoDraw from scratch and reusing only the application design, they decided to rewrite the NeXTSTEP object layer on which the application depended. This was the first version of "libobjcX". It enabled them to port HippoDraw to Unix systems running the X Window System without changing a single line of their application source. After the OpenStep specification was released to the public in 1994, they decided to write a new "objcX" which would adhere to the new APIs. The software would become known as "GNUstep".
Software architecture.
Rendering.
GNUstep contains a set of graphical control elements 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. X11 or Wayland, and its graphical user interface has to be 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 functions. The package gnustep-back provides the following backends:
Paradigms.
GNUstep inherits some design principles proposed in OPENSTEP (GNUstep predates Cocoa, but Cocoa is based on OPENSTEP) as well as the Objective-C language.
Other interfaces.
In addition to the Objective-C interface, some small projects under the GNUstep umbrella implement other APIs from Apple:
, there are no projects that builds the Swift programming language against the GNUstep Objective-C environment.
Applications.
Here are some examples of applications written for or ported to GNUstep.
Class capabilities.
Foundation Kit.
The Foundation Kit provides basic classes such as wrapper classes and data structure classes.
Application Kit.
The Application Kit provides classes oriented around graphical user interface capabilities.