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Ruby, script generador de script javascript para precarga de imágenes en tu html

El truco más sencillo para precargar las imágenes de tu web y así no ver el horrible efecto de mouse-over hueco es hacer un script javascript como este:

img1 = new Image();
img1.src="imagen.jpg";

Esto invocará la imagen y el navegador, sino es muy tonto, la dejará en la caché para cuando verdaderamente la necesite.

Bueno, pues es lo que quiero hacer para una web que tiene un montón de background-image definidos en su css.

Para ello me he hecho un script en ruby que me busca todas las imágenes en todos los css y me genera un script de javascript con todo el rollito de los preload.

class Preloator
  def self.generate_script( css_directory )
    # take all .css on directory
    css_file_names = []
    Dir.foreach( css_directory ) do |file|
      if file =~ /.*\.css$/
        css_file_names << css_directory + '/' + file
      end
    end
 
    # take all the images by looking at 'url'
    image_names = []
    css_file_names.each do |file_name|
      self.grep( 'url', file_name ).each do |line|
        image_names << line.scan( /.*\((.*)\)/ ).flatten.first
      end
    end
 
    # generate the script
    script = "<script>\n"
    image_names.each_with_index do |image_name, index|
      script += "    img#{index} = new Image();\n"
      script += "    img#{index}.src = \"#{image_name}\";\n"
    end
    script += "</script>"
  end
 
  def self.grep( pattern, filename )
    matches = []
    regexp = Regexp.new( pattern )
    File.open(filename) do |file|
      file.each do |line|
        matches << "#{filename} #{file.lineno.to_s}: #{line}" if regexp.match(line)
      end
    end
    matches
  end
end
 
puts Preloator.generate_script( ARGV[0] )

Esto genera una salida como esta:

$ ruby etc/preloator.rb public/stylesheets/
<script>
img0 = new Image();
img0.src="/imgs/fondo.jpg";
img1 = new Image();
img1.src="/imgs/fondo_carpeta.png";
img2 = new Image();
img2.src="/imgs/blog_sombra_abajo.png";
img3 = new Image();
</script>;

Seguramente quieras mejorarlo un poquito: meterlo en un .js o invocarlo cuando el document.load.

El script ahorra trabajo si, como en mi caso, tienes más de 20 imágenes en el css.. y escribir el javascript a mano sería un coñazo.

7 Comments to “Ruby, script generador de script javascript para precarga de imágenes en tu html”
  1. Damu Says:

    Lo de precargar imagenes lo he hecho alguna vez con una solucion de menor tecnologia, sin js, solo usando un div oculto con las imagenes dentro. ;-)

  2. fguillen Says:

    Sip.. también vale :), lo único que el javascript lo puedes ejecutar cuando toda la página ha sido cargada y así le permites al usuario verla e interactuar.. Si cargas las imágenes dentro del HTML puede que el navegador se quede tonto hasta que acabe.

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