Resize an OSX display with RubyCocoa.

screensize.rb

require 'osx/cocoa'
include OSX

class Screen
  class << self
    def resize(width, height, &block)
      new.resize(width, height, &block)
    end
  end

  def initialize(screen = CGMainDisplayID())
    @screen = screen
    @mode   = CGDisplayCopyDisplayMode(@screen)
  end

  def resize(width, height, &block)
    configure(best_mode(width, height))
    block.call
  ensure
    configure(@mode)
  end

  private

  def best_mode(width, height)
    # TODO Try to figure out color depth from CGDisplayModeCopyPixelEncoding?
    CGDisplayCopyAllDisplayModes(@screen, nil).detect do |mode|
      CGDisplayModeGetWidth(mode) == width &&
      CGDisplayModeGetHeight(mode) == height &&
      CGDisplayModeIsUsableForDesktopGUI(mode)
    end
  end

  def configure(mode)
    result, config = CGBeginDisplayConfiguration()
    if result == KCGErrorSuccess
      CGConfigureDisplayWithDisplayMode(config, @screen, mode, nil)
      CGCompleteDisplayConfiguration(config, KCGConfigureForSession)
    else
      raise "I have no clue what just happened: #{result}"
    end
  end
end