ruby mini component system

components.rb

require'gosu';include Gosu

class Component
	class << self
		# @field should be set for each base class,
		# entities that include this component or a derivative
		# respond to this as a message
		def field() @field or superclass.field end

		def iface() (@interface or []) + (superclass.iface rescue []) end
		def interface method, &b
			(@interface ||= []) << method
			define_method method, &b
		end
	end
end

class Pos < Component
	@field = :pos
	attr_accessor :x,:y
	def initialize x,y
		self.x,self.y = x,y
	end

	def * b
		self.class.new(x * b, y * b)
	end
end

class Velocity < Pos
	@field = :vel
	interface :update do |ent, dt|
		ent.pos.x += x * dt
		ent.pos.y += y * dt
	end
end
class Friction < Component
	@field = :fric
	attr_accessor :f
	def initialize f = 0.93
		self.f = f 
	end

	interface :update do |ent, dt|
		ent.vel *= f ** dt
	end
end


class Renderable < Component
	@field = :rendr
	interface :draw do |entity, window| end
end
class Circle < Renderable
	attr_accessor :radius
	def initialize radius; self.radius = radius end

	def draw entity, window
		window.draw_circle entity.pos.x, entity.pos.y, radius, 
			Gosu::Color::GREEN, 0
		#require'pry';binding.pry
	end
end

def Entity(*components, &b)
	## make a new struct with the required fields
	c = Struct.new(*components.map{|c| c.field}.compact)
	c.class_eval(
		components.each_with_object({}){ |co, h|
			unless (i = co.iface).empty?
				i.each do |func|
					(h[func] ||= []) << co.field
				end
			end
		}.map { |m, fields|
			"def #{m}(*args) \n  #{
				fields.map { |f| "#{f}.#{m} self, *args" }.join("\n  ")
			}\nend\n"
		}.join#.tap{|x| puts x}
	)
	c.send :define_method, :initialize, &b if b
	c
end


class Gosu::Window
	def draw_circle cx, cy, radius, color, z, step = 30
		(0 .. 360).step(step) do |a|
			a2 = a + step
			draw_line \
				cx + Gosu::offset_x(a, radius),
				cy + Gosu::offset_y(a, radius),
				color, 
				cx + Gosu::offset_x(a2, radius),
				cy + Gosu::offset_y(a2, radius),
				color,
				z
		end
	end
end



Bob = Entity(Pos, Renderable, Friction, Velocity) do |x, y, r|
	#should write something to automate this
	self.pos = Pos.new(x,y)
	self.vel = Velocity.new(0, 30)
	self.rendr = Circle.new(r)
	self.fric  = Friction.new(0.95)
end

class GameWindow < Window
	def initialize
		super 640,480,false
		self.caption = "foo"
		@last = Gosu::milliseconds
		@ents = []
		@ents << Bob.new(100,100,3.0)
	end

	def button_down id
		close if id == Gosu::KbEscape
	end

	def update
		cur = Gosu::milliseconds
		dt, @last = (cur - @last)/1000.0, cur
		@ents.each do |e| e.update dt end 
	end
	def draw
		@ents.each do |e| e.draw self end
	end
end

GameWindow.new.show