How Mac OS X support works in Emacsen

This page is about internals. A list of implementations is at EmacsForMacOS.

A short glossary

Mac OS X is buzzword-intensive. Here’s a short glossary:

Basics: the event loop

The event loop collects events from the OS’s GUI subsystem, processes some immediately (eg, those having to do with redisplay when a window is exposed by motion of an obscuring window), and queues of the rest for execution by LISP.

Both XEmacs and GNU Emacs have deep roots in the X window system, and this has had a strong influence on design of the Emacs event loops. The typical Carbon loop is more similar to X / XT than Cocoa’s is, therefore the integration code for Carbon (and W32 for that matter) is more similar to that in the X GUI. Cocoa’s loop integration, though different, is fairly straightforward, but some issues with multiplexing between GUI and TTY interaction and external processes remain to be ironed out.

Basics: the drawing strategy

Emacs (re)display implements a cascade structure in which high-level text display update operations are translated down to piecewise updates in whichever “terminal” (text-based, X windows, etc.) is being used. In GNU Emacs 21, this system was rewritten by Gerd Moellmann to enhance functionality while increasing the granularity of the operations implemented by each terminal. The result is that the typical port ends up implementing very low-level functions such as “render these characters at these x-y positions”, and “blank this rectangle with background color Z”. This approach, taken by the AndrewChoi / YamamotoMitsuharu Carbon port, makes implementation of new GUIs (somewhat) straightforward and provides interface uniformity across platforms, but can lead to complex, unnatural code, and inability to utilize the strengths of the underlying API.

At least three alternatives have been tried. In the first, the orignal port of the X-based GNU Emacs for NeXTstep, the GUI module acted as a TTY device, connecting over a socket and interpreting the text-based formatting commands. This approach was unable to implement the full richness of the X interface, and the port was abandoned in favor of a more standard approach. Later on, PittsJarvis, working with XEmacs 21.5.10 for MacOS X, intercepted calls directly from LISP (site of primary control in emacs), avoiding the cascade structure entirely. This resulted in a solid implementation, but work has unfortunately been suspended following the author passing away in 2004. Ovidiu Predescu subsequently tried a related approach using XEmacs for MacOS X, in which all functions outside actual management of the text buffers themselves are handled by the object-oriented Cocoa text system. This sounded promising, but has not progressed, perhaps because the buffer and GUI management code in (X)Emacs is insufficiently separated.

Finally, the successor to the original NeXTstep port, currently active in a Cocoa incarnation, takes an intermediate approach in which stateful object components are used to implement the procedurally-oriented redisplay calls. This leads to a bit of friction at the boundary, but succeeds to the extent that the code in the Cocoa port is around half the size of the standard ports.