adding a mouse pointer to your gosu window through monkey-patching

In a previous post I mentioned how I was displaying a mouse pointer in a Gosu window. You can read about it here. What if you don’t wanna do that each time you need to show a mouse pointer?

Monkey patching is a technique some people don’t wanna use. I think there are cases when it can be very useful. This is one of those cases. Here’s the code that makes it possible:

#require "gosu"

# this monkey patches a gosu window so that the draw will
# also draw a mouse pointer :)
module Gosu
	class Window
		# add this method to set the mouse pointer
		def pointer=(pointer_image)
			# see if we already defined the mouse_pointer accessor
			if self.methods.include? "mouse_pointer="
				# if it's a string, it means it's the path to the image
				# to be used as the pointer
				if pointer_image.class.to_s == "String"
					# create a gosu image from it
					self.mouse_pointer = Gosu::Image.new(self,pointer_image,false)
					# we assume it's an already build gosu image
					self.mouse_pointer = pointer_image
				# the mouse_pointer accessor isn't defined. let's add it
				# transform the parameter into an image ( if it's a string )
				pointer_image = Gosu::Image.new(self,pointer_image,false) if pointer_image.class.to_s == "String"
				# modify ! :)
				class << self
					# add the accessor
					attr_accessor :mouse_pointer
					# save the draw method by the name old_draw
					alias :old_draw :draw

					def draw
						# invoke old draw to get stuff painted on the screen
						# draw the mouse pointer
						@mouse_pointer.draw(mouse_x,mouse_y,0) if !@mouse_pointer.nil?
				# finally, assign it so that it will actually paint something
				self.mouse_pointer = pointer_image

This adds the method pointer= to the Gosu::Window class. Therefore, every class that inherits from Gosu::Window will have the method too. Here’s an example of how to use that:

require "gosu"
require "mouse_hack" # nice name right?

class MyWindow < Gosu::Window
   # do whatever you want here

my_w = MyWindow.new
my_w.pointer = "pointer.png"


my_w.pointer = Gosu::Image.new(my_w,"pointer.png",false)


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