FMI see http://leipzig.onruby.de/topics/ffi-ein-nachmittag-mit-systembibliotheken

ffi_CGDisplayBounds_explained.rb

#!/usr/bin/env ruby

###################################################
## Display screen dimensions in macOS using ruby ##
##################################################
# found https://developer.apple.com/reference/coregraphics/1456395-cgdisplaybounds?language=objc
# --> use CoreGraphics framework
# --> CGRect CGDisplayBounds(CGDirectDisplayID display);

#############################################
## How to bind (objective)C code in ruby ? ##
#############################################
# found https://github.com/ffi/ffi/wiki
# --> gem install ffi

require 'ffi'

module Display
  extend FFI::Library

  ######################################
  ## How to find CoreGraphics dynlib? ##
  ######################################
  # --> it's in "/System/Library/Frameworks" ;-)

  ffi_lib '/System/Library/Frameworks/CoreGraphics.framework/CoreGraphics'

  ###########################################################
  ## How to find and bind (objective)C functions in ruby ? ##
  ###########################################################
  # 1. $ nm
  # --> nm -gU '/System/Library/Frameworks/CoreGraphics.framework/CoreGraphics'
  # --> not really helpful :-(
  # 2. xml
  # --> "/System/Library/Frameworks/CoreGraphics.framework/Versions/A/Resources/BridgeSupport/CoreGraphics.bridgesupport"
  # <function name='CGMainDisplayID'>
  # <retval type='I'/>
  # </function>

  # What is 'I' ?
  # 1. https://github.com/ffi/ffi/wiki/Types / xml / dtd ...
  # --> not really helpful :-(
  # 2. https://github.com/patrickhno/cocoa/blob/master/lib/cocoa/objc/method_def.rb#L5
  # It's :uint

  ###################################
  ## Use cocoa in the first place? ##
  ###################################
  # --> tried, turns out there's a bug when using CGDirectDisplayID :-(
  # require 'cocoa'
  # Cocoa.CGMainDisplayID
  # --> ArgumentError: wrong number of arguments (given 1, expected 2)
  # Cocoa.CGMainDisplayID(0)
  # --> ArgumentError: wrong number of arguments (given 1, expected 0)
  # However, when falling back to default CGDirectDisplayID, Cocoa.CGDisplayBounds(0)
  # Cocoa.CGDisplayBounds(0)
  # --> <Cocoa::CGRect:0x007fa118b39078>
  # I don'd want to use unmaintained gems anyway (https://github.com/patrickhno/cocoa/commits/master)


  # What was 'I' again...?

  # CGRect CGDisplayBounds(CGDirectDisplayID display);
  # Define a new type from :uint called :CGDirectDisplayID
  typedef :uint, :CGDirectDisplayID

  ###################################################
  ## How to bind (objective)C functions using ffi? ##
  ###################################################
  # attach_function
  # --> http://www.rubydoc.info/github/ffi/ffi/FFI/Library:attach_function
  # --> https://github.com/ffi/ffi/blob/master/lib/ffi/library.rb#L204


  # <function name='CGMainDisplayID'>
  # <retval type='I'/>
  # </function>
  attach_function :CGMainDisplayID, [], :CGDirectDisplayID



  # Display.CGMainDisplayID
  # --> 69732928
  # weird number, but it seems to work


  # CGRect CGDisplayBounds(CGDirectDisplayID display);
  ###########################
  ## How to define CGRect? ##
  ###########################
  # --> <struct name="CGRect"
  #       type="{CGRect="origin"{CGPoint="x"f"y"f}"size"{CGSize="width"f"height"f}}"
  #       type64="{CGRect="origin"{CGPoint="x"d"y"d}"size"{CGSize="width"d"height"d}}"/>

  ######################
  ## What's all this? ##
  ######################
  # --> "name" is prefixed in quotes
  # --> {} defined a struct(ure)
  # --> f is :float
  # --> d is :double

  # CGPoint and CGSize need to be defined first

  ##################################
  ## How to define those structs? ##
  ##################################
  # --> use FFI::Struct
  # https://github.com/ffi/ffi/wiki/Basic-Usage#passing-structs-as-value-parameters-to-functions
  # http://www.rubydoc.info/github/ffi/ffi/FFI/Struct


  # <struct name="CGPoint" type="{CGPoint="x"f"y"f}" type64="{CGPoint="x"d"y"d}"/>
  class CGPoint < FFI::Struct
    layout :x, :double, :y, :double
  end

  # <struct name="CGSize" type="{CGSize="width"f"height"f}" type64="{CGSize="width"d"height"d}"/>
  class CGSize < FFI::Struct
    layout :width, :double, :height, :double
  end

  # <struct name="CGRect" type="{CGRect="origin"{CGPoint="x"f"y"f}"size"{CGSize="width"f"height"f}}" type64="{CGRect="origin"{CGPoint="x"d"y"d}"size"{CGSize="width"d"height"d}}"/>
  class CGRect < FFI::Struct
    layout :origin, CGPoint, :size, CGSize
  end


  # finally bind CGDisplayBounds

  # <function name='CGDisplayBounds'>
  # <arg type='I'/>
  # <retval type='{CGRect={CGPoint=ff}{CGSize=ff}}' type64='{CGRect={CGPoint=dd}{CGSize=dd}}'/>
  # </function>
  attach_function :CGDisplayBounds, [:CGDirectDisplayID], CGRect.by_value

  # Done!
end

displayID = Display.CGMainDisplayID
bounds = Display.CGDisplayBounds(displayID)
puts "width: #{'%d' % bounds[:size][:width]}, height: #{'%d' % bounds[:size][:height]}"


# There are helper methods like CGRectGetHeight, so you could spare the definition of CGSize.