app_delegate.rb

class AppDelegate
  def application(application, didFinishLaunchingWithOptions:launchOptions)
    @window = UIWindow.alloc.initWithFrame(UIScreen.mainScreen.bounds)
    @window.rootViewController = ExampleViewController.alloc.init

    @window.makeKeyAndVisible
    true
  end
end

class ExampleViewController < UIViewController
  def init
    return self if self.initWithNibName(nil, bundle: nil)
  end

  def loadView
    self.view = ExampleView.alloc.init
  end
end

class View < UIView
  INITIALIZERS = {
    UIView => 'initWithFrame:',
    UIImageView => 'initWithImage:',
    UISegmentedControl => 'initWithItems:',
  }

  class << self
    def current_views
      @current_views ||= []
    end

    def attributes
      @attributes ||= []
    end

    def view_builders
      @view_builders ||= []
    end

    def subview(klass, *args, &attributes_builder)
      options = args.pop if args.last.is_a?(Hash)
      options = {} unless options

      if current_views.empty?
        view_builders << [klass, args, attributes_builder, options]
        return
      end

      build(klass, args, attributes_builder, options)
    end

    def method_missing(name, *args)
      method = "set#{camelize(name)}:"

      if current_views.empty?
        attributes.push [method, args.first]
        return
      end

      view = current_views.last
      super unless view.respond_to?(method)

      puts "::: Performing #{method} with #{args.first.inspect} on #{view.inspect}"
      # I would rather use send, but it crashes
      view.performSelectorOnMainThread(method, withObject: args.first, waitUntilDone: true)
    end

    private

    def build(klass, args, attributes_builder, options)
      args = args.map{ |arg| arg.respond_to?(:call) ? arg.call : arg }
      view = klass.alloc.send(INITIALIZERS[klass], *args)

      current_views << view
      attributes_builder.call if attributes_builder
      current_views.pop

      root_view, last_view = current_views.first, current_views.last
      root_view.views[options[:name]] = view if options[:name]
      last_view.addSubview view
      view
    end

    def camelize(term)
      string = term.to_s
      string = string.sub(/^[a-z\d]*/) { $&.capitalize }
      #string = string.sub(/^(?:(?=a)b(?=\b|[A-Z_])|\w)/) { $&.downcase }
      string.gsub(/(?:_|(\/))([a-z\d]*)/) { "#{$1}#{$2.capitalize}" }
    end
  end

  attr_reader :views

  def init
    if self.initWithFrame(CGRectZero)
      @views = {}

      self.class.attributes.each do |(method, arg)|
        self.performSelectorOnMainThread(method, withObject: arg, waitUntilDone: true)
      end

      self.class.current_views << self
      self.class.view_builders.each do |builder|
        view = self.class.build(*builder)
      end
      self.class.current_views.pop
    end

    self
  end
end

class ExampleView < View
  frame [[0, 0], [320, 460]]
  background_color UIColor.redColor

  subview UIView, [[10, 10], [300, 440]] do
    background_color UIColor.greenColor

    subview UIView, [[10, 10], [280, 420]] do
      background_color UIColor.blueColor

      subview UISegmentedControl, ["Example", "Test"] do
        frame [[40, 20], [200, 40]]
        tint_color UIColor.blackColor
      end
    end
  end
end