一度起動したあと、「システム環境設定」→「セキュリティとプライバシー」→「アクセシビリティ」で macruby を許可する必要あり

keyscreen.rb

#!macruby

framework "Cocoa"

class MainView < NSView
	def init
		super
		@log = ""
	end

	def drawRect(rect)
		super
		NSColor.clearColor.set
		NSRectFill(bounds)

		font = NSFont.boldSystemFontOfSize(24)

		shadow = NSShadow.alloc.init
		shadow.setShadowColor(NSColor.blackColor)
		shadow.setShadowBlurRadius(2)
		shadow.setShadowOffset([0, 0])
		attrs = NSMutableDictionary.alloc.initWithDictionary({
			NSForegroundColorAttributeName => NSColor.whiteColor,
			NSFontAttributeName            => font,
			NSShadowAttributeName          => shadow,
		})

		y = 0
		@log.split(/\n/).reverse.each do |line|
			storage   = NSTextStorage.alloc.initWithString(line, attributes: attrs)
			manager   = NSLayoutManager.alloc.init
			container = NSTextContainer.alloc.init

			manager.addTextContainer(container)
			storage.addLayoutManager(manager)

			range = manager.glyphRangeForTextContainer(container)
			10.times do
				manager.drawGlyphsForGlyphRange(range, atPoint: [0, y])
			end
			rect = manager.boundingRectForGlyphRange([0, manager.numberOfGlyphs], inTextContainer: container)
			y+= rect.size.height
		end
	end

	def <<(log)
		@log << log
		@log = @log.split(/\n+/, -1).last(5).join("\n")
	end

end

class AppDelegate
	attr_accessor :window
	def applicationDidFinishLaunching(a_notification)
		prevKeyed = 0
		NSEvent.addGlobalMonitorForEventsMatchingMask(NSKeyDownMask, handler: lambda {|e|
			return unless e.type == NSKeyDown

			mod = ""
			if e.modifierFlags & NSShiftKeyMask != 0
				mod += "⇧"
			end

			if e.modifierFlags & NSControlKeyMask != 0
				mod += "⌃"
			end

			if e.modifierFlags & NSAlternateKeyMask != 0
				mod += "⌥ "
			end

			if e.modifierFlags & NSCommandKeyMask != 0
				mod += "⌘"
			end

			if e.modifierFlags & (NSControlKeyMask | NSAlternateKeyMask | NSCommandKeyMask) == 0
				char = readable(e.characters)
				if Time.now.to_i - prevKeyed > 1
					@view << "\n#{char}"
				else
					@view << char
				end
			else
				@view << "\n#{mod}#{readable(e.charactersIgnoringModifiers).upcase}\n"
			end
			@view.needsDisplay = true

			prevKeyed = Time.now.to_i
		})
		rect = [0, 0, 800, 500]
		@window = NSWindow.alloc.initWithContentRect(rect, styleMask: NSBorderlessWindowMask, backing: NSBackingStoreBuffered, defer: 0)
		@window.opaque = false
		@window.hasShadow = false
		@window.level = 1000
		@window.movableByWindowBackground = true
		# @window.ignoresMouseEvents = true
		@window.makeKeyAndOrderFront(nil)
		@window.orderFrontRegardless

		@view = MainView.alloc.initWithFrame(rect)
		@view.init
		@view << "Initialized"

		@window.contentView = @view
	end

	def readable(char)
		p char
		char.gsub(/[\r\e\x7f]/, {
			"\r"   => "↵\n",
			"\e"   => "⎋",
			"\x7f" => "⌫",
		})
	end
end

app = NSApplication.sharedApplication
app.delegate = AppDelegate.new
app.run