Rails image_set_tag_3x helper for srcset.

images_helper.rb

module ImagesHelper
  # Example usage:
  #
  #   image_set_tag_3x "foo_1x.png", alt: "foo"
  #
  # Will assume there is a 2x and 3x version and provide those automagically.
  #
  # Based on https://gist.github.com/mrreynolds/4fc71c8d09646567111f
  def image_set_tag_3x(source, options = {})
    srcset = [ 2, 3 ].map { |num|
      name = source.sub("_1x.", "_#{num}x.")
      "#{path_to_image(name)} #{num}x"
    }.join(", ")

    image_tag(source, options.merge(srcset: srcset))
  end
end

images_helper_spec.rb

# This is a Rails-less unit test.

require "spec_helper"
require "helpers/images_helper"

describe ImagesHelper, "#image_set_tag_3x" do
  let(:helper) { Class.new { include ImagesHelper }.new }

  it "generates an image tag" do
    allow(helper).to receive(:path_to_image) { |x| "path/to/#{x}" }

    expect(helper).to receive(:image_tag).with(
      "foo_1x.png",
      srcset: "path/to/foo_2x.png 2x, path/to/foo_3x.png 3x",
      alt: "foo",
    )

    helper.image_set_tag_3x("foo_1x.png", alt: "foo")
  end
end