A native macOS app written in Ruby using FFI to call libobjc, etc.

macos_app.rb

#!/usr/bin/env ruby
# A macOS GUI app written in Ruby, calling libobjc directly via Fiddle.
# No gems. No Objective-C. Just Ruby and the runtime.

require "fiddle"

OBJC   = Fiddle.dlopen("/usr/lib/libobjc.A.dylib")
APPKIT = Fiddle.dlopen("/System/Library/Frameworks/AppKit.framework/AppKit")

# Core runtime functions
ObjcGetClass      = Fiddle::Function.new(OBJC["objc_getClass"], [Fiddle::TYPE_VOIDP], Fiddle::TYPE_VOIDP)
SelRegisterName   = Fiddle::Function.new(OBJC["sel_registerName"], [Fiddle::TYPE_VOIDP], Fiddle::TYPE_VOIDP)
AllocClassPair    = Fiddle::Function.new(OBJC["objc_allocateClassPair"], [Fiddle::TYPE_VOIDP, Fiddle::TYPE_VOIDP, Fiddle::TYPE_SIZE_T], Fiddle::TYPE_VOIDP)
RegisterClassPair = Fiddle::Function.new(OBJC["objc_registerClassPair"], [Fiddle::TYPE_VOIDP], Fiddle::TYPE_VOID)
ClassAddMethod    = Fiddle::Function.new(OBJC["class_addMethod"], [Fiddle::TYPE_VOIDP, Fiddle::TYPE_VOIDP, Fiddle::TYPE_VOIDP, Fiddle::TYPE_VOIDP], Fiddle::TYPE_INT)

MSG_SEND = OBJC["objc_msgSend"]

def cls(name) = ObjcGetClass.call(name)
def sel(name) = SelRegisterName.call(name)

# Build a correctly-typed objc_msgSend call each time.
# types: array of Fiddle type constants for the method arguments (not including self/SEL).
# ret: Fiddle type constant for the return value.
def msg(obj, sel_name, *args, types: [], ret: Fiddle::TYPE_VOIDP)
  sig = [Fiddle::TYPE_VOIDP, Fiddle::TYPE_VOIDP] + types
  fn = Fiddle::Function.new(MSG_SEND, sig, ret)
  fn.call(obj, sel(sel_name), *args)
end

def nsstr(s)
  msg(cls("NSString"), "stringWithUTF8String:", s, types: [Fiddle::TYPE_VOIDP])
end

def alloc_init(class_name)
  msg(msg(cls(class_name), "alloc"), "init")
end

# Convenience type constants
T_PTR    = Fiddle::TYPE_VOIDP
T_LONG   = Fiddle::TYPE_LONG
T_ULONG  = Fiddle::TYPE_ULONG
T_DOUBLE = Fiddle::TYPE_DOUBLE
T_BOOL   = Fiddle::TYPE_INT
T_VOID   = Fiddle::TYPE_VOID

# ---- App setup ----

app = msg(cls("NSApplication"), "sharedApplication")
msg(app, "setActivationPolicy:", 0, types: [T_LONG], ret: T_VOID)

# ---- Menu bar ----

menubar = alloc_init("NSMenu")
app_menu_item = alloc_init("NSMenuItem")
msg(menubar, "addItem:", app_menu_item, types: [T_PTR], ret: T_VOID)

app_menu = alloc_init("NSMenu")
quit_item = msg(
  msg(cls("NSMenuItem"), "alloc"),
  "initWithTitle:action:keyEquivalent:",
  nsstr("Quit"), sel("terminate:"), nsstr("q"),
  types: [T_PTR, T_PTR, T_PTR]
)
msg(app_menu, "addItem:", quit_item, types: [T_PTR], ret: T_VOID)
msg(app_menu_item, "setSubmenu:", app_menu, types: [T_PTR], ret: T_VOID)
msg(app, "setMainMenu:", menubar, types: [T_PTR], ret: T_VOID)

# ---- Delegate (so closing the window quits the app) ----

# Create a callback for applicationShouldTerminateAfterLastWindowClosed:
should_terminate = Fiddle::Closure::BlockCaller.new(
  T_BOOL, [T_PTR, T_PTR, T_PTR]
) { |_self, _cmd, _sender| 1 }

dc = AllocClassPair.call(cls("NSObject"), "AppDel", 0)
ClassAddMethod.call(dc, sel("applicationShouldTerminateAfterLastWindowClosed:"), should_terminate, "B@:@")
RegisterClassPair.call(dc)

delegate = msg(msg(dc, "alloc"), "init")
msg(app, "setDelegate:", delegate, types: [T_PTR], ret: T_VOID)

# ---- Window ----
# initWithContentRect:styleMask:backing:defer:
# CGRect is 4 doubles passed in registers on ARM64.

window = msg(
  msg(cls("NSWindow"), "alloc"),
  "initWithContentRect:styleMask:backing:defer:",
  200.0, 200.0, 500.0, 300.0,  # CGRect as 4 doubles
  15,                            # style mask (titled|closable|miniaturizable|resizable)
  2,                             # backing (buffered)
  0,                             # defer (NO)
  types: [T_DOUBLE, T_DOUBLE, T_DOUBLE, T_DOUBLE, T_ULONG, T_ULONG, T_BOOL]
)
msg(window, "setTitle:", nsstr("Hello from Ruby + libobjc"), types: [T_PTR], ret: T_VOID)

# ---- Text label ----

label = msg(
  msg(cls("NSTextField"), "alloc"),
  "initWithFrame:",
  50.0, 100.0, 400.0, 80.0,
  types: [T_DOUBLE, T_DOUBLE, T_DOUBLE, T_DOUBLE]
)
msg(label, "setStringValue:", nsstr("Hello, World!\nFrom Ruby, via Fiddle, into libobjc.\nNo gems. No Objective-C."), types: [T_PTR], ret: T_VOID)
msg(label, "setBezeled:",         0, types: [T_BOOL], ret: T_VOID)
msg(label, "setDrawsBackground:", 0, types: [T_BOOL], ret: T_VOID)
msg(label, "setEditable:",        0, types: [T_BOOL], ret: T_VOID)
msg(label, "setSelectable:",      0, types: [T_BOOL], ret: T_VOID)

font = msg(cls("NSFont"), "systemFontOfSize:", 24.0, types: [T_DOUBLE])
msg(label, "setFont:", font, types: [T_PTR], ret: T_VOID)

content_view = msg(window, "contentView")
msg(content_view, "addSubview:", label, types: [T_PTR], ret: T_VOID)

# ---- Show and run ----

msg(window, "makeKeyAndOrderFront:", 0, types: [T_PTR], ret: T_VOID)
msg(app, "activateIgnoringOtherApps:", 1, types: [T_BOOL], ret: T_VOID)
msg(app, "run", ret: T_VOID)