emacs/admin/notes/nextstep

This file summarizes primary aspects of the NS port architecture.  If
possible, it should be updated for changes.

Currently it summarizes the state as of:

  summer 2008 shortly after merging to trunk



Startup
-------

Init sequence:
  emacs.c: ns_alloc_autorelease_pool()		nsterm.m
  emacs.c: ns_init_paths()			nsterm.m
    - override EMACSLOADPATH, etc. so resources can be found in-bundle
  emacs.c: init_display()			dispnew.c
    - sets Vwindow_system (window-system) to 'ns
  emacs.c: loadup.el -> startup.el -> ns-initialize-window-system
    -> x-open-connection (nsfns.m)
      - ns-list-services
      -> nsterm.m: ns_term_init()
	- EmacsApp sharedApplication
	- read NS defaults (org.gnu.Emacs.plist)
	- init X-style color list
	- ns_create_terminal()
	- NSApp run (goes to applicationDidFinishLaunching which terminates
	  	     event loop -- see below)



Event Loop
----------

In an NS application, the event loop is normally managed by system and all
user code is event-driven.  [NSApp run] is called by user and never returns.

In Emacs, the event loop is managed by emacs itself.

The NS port mediates between these two styles by intercepting the NS event
dispatch at [NSApp sendEvent].  If a special event is detected, the event loop
is broken, and control returned to Emacs.  This special event is sent by
ns_send_appdefined, which is called under these circumstances:

 - if a user input event is received
 - when a timeout fires

NS event processing is instigated from Emacs through ns_select() and
ns_read_socket() in nsterm.m.  Parts of the codepaths leading to these
functions are:


 keyboard.c:read_avail_input()
     -> ns_read_socket (ns_send_appdefined) -> [NSApp run]

 process.c:wait_reading_process_output()
     -> ns_select -> gobble_input (global inNsSelect=1)
       -> ns_read_socket (ns_send_appdefined if !expected) -> [NSApp run]

 sysdep.c:sys_select() -> read_input_waiting()
     -> ns_read_socket (send_appdefined) -> [NSApp run]
 [this codepath may not be used]


Currently ctrl-g is not detected in as many circumstances as other emacsen.
It is not certain whether this is due to the means of event loop integration,
or errors of omission in the NS code.  This is an area for improvement.




Text Rendering and Font Handling
--------------------------------

nsfont.m implements the font driver, responsible for managing fonts and
rendering text.  Fonts are obtained through NSFontManager.  Rendering must be
done at a low level due to emacs' fine control over this process, therefore
there are different approaches under Cocoa and GNUstep.  Under GNUstep, the
original NeXT Display PostScript (DPS) APIs are available and used.  Under
Cocoa, these were removed and Quartz drawing functions replaced them.

In both cases, font glyphs are accessed through UTF8 character
representations.  It would be preferable to use Unicode indices, but prior
attempts at this have failed.

Multi-script fontsets are auto-created in nsfont_make_fontset_for_font() using
the facilities of NSTextStorage and NSLayoutManager.


Object Architecture
-------------------

Unlike the other GUIs, the NS interface is based on a high-level and
object-oriented API.  This creates some tension in the code because emacs
itself has been architected around the low-level Xlib and Xt APIs.  The NS
port tries to strike a balance between simplifying code on its side using OO
features, and keeping code as similar as possible to other ports to ease
maintenance.  The following are the main classes (see nsterm.h):

EmacsApp : NSApplication
  - event loop integration, interapp comms point for Finder (NSWorkspace) msgs,
    Services
  - one global instance (NSApp)
  - nsterm.m

EmacsView : NSView <TextInput>
  - handles rendering of text and fringe, interapp comms for drag/drop
  - instance for each frame
  - child of window's content view
  - nsterm.m

EmacsWindow : NSWindow
  - utility override for resize handling

EmacsScroller : NSScroller
  - instance for each emacs window, renders scrollbar
  - child of window's content view
  - nsterm.m

EmacsImage : NSImage
  - image rendering, toolbar icons, stippling, fringe bitmaps
  - instance for each image
  - nsimage.m

EmacsMenu : NSMenu
  - menu management
  - one tree of instances for menubar, one instance for each popup menu
  - nsmenu.m

EmacsToolbar : NSToolbar
  - toolbar management, one instance for each frame
  - nsmenu.m


EmacsDialogPanel : NSPanel
  - popup dialogs, one instance for each
  - nsmenu.m

EmacsTooltip : NSObject
  - tooltip popups, one instance for each
  - nsmenu.m

EmacsGlyphStorage : NSObject <NSGlyphStorage>
  - utility for text rendering
  - nsfont.m

EmacsPrefsController : NSObject
  - utility for preferences panel management, one global instance
  - nsterm.m
  - nextstep/Cocoa/Emacs.base/Contents/Resources/preferences.nib
  - nextstep/GNUstep/Emacs.base/Resources/preferences.gorm

EmacsSavePanel : NSSavePanel
EmacsOpenPanel : NSOpenPanel
  - utility override for panel notifications