A simple last in - first out queue for RubyMotion. I feel like it should be harder than this...

lifo_queue.rb

class LifoQueue

  def initialize(name, depth)
    raise ArgumentError, "depth must be a number greater than zero" unless Fixnum === depth and depth > 0
    super

    @depth = depth
    @stack = []
    @mutex = Mutex.new
    @queue = Dispatch::Queue.send :new, name
  end

  def async(&block)
    @mutex.synchronize do
      if @stack.size >= @depth - 1
        @stack.shift
      end

      patch_state(block, :waiting)

      @stack.push(block)
    end

    call_next
  end

  def call_next
    @queue.async do
      next_proc = @mutex.synchronize do
        @stack.pop
      end

      if next_proc
        next_proc.state = :calling
        next_proc.call
        next_proc.state = :done

        call_next
      end
    end
  end

  def patch_state(obj, initial_state)
    obj.instance_variable_set('@_state', initial_state)
    def obj.state
      @_state
    end
    def obj.state=(state)
      @_state = state
    end
  end
end