builder.rb

# Temary
# ------------------------
# Explain about how works:
# > class method self ✅
# > method_missing ✅
# > blocks as parameter ✅
# > instance_eval ✅

# What are we going to build?

# markup = Html.build do
#   body do
#     section class: "flex flex-col" do
#       h1 "Dark is amazing", class: "title"
#       p "Martha mata a jonas", class: "text-center", id: "martha"
#     end
#   end
# end

class Html
  def initialize(&block)
    @content = []
    @ident = 0
    instance_eval(&block) if block_given?
  end

  def method_missing(method, text = nil, **options, &block)
    builder_tag(method, text, options, &block)
  end

  def get_html
    @content.join("\n")
  end

  def self.build(&block)
    builder = Html.new(&block)
    builder.get_html
  end

  private

  def builder_tag(tag, text, options, &block)
    @content << "#{" " * @ident }<#{tag} #{builder_options(options)}>"
    @ident += 2
    @content << "#{" " * @ident } " + text if text
    instance_eval(&block) if block_given?
    @ident -= 2
    @content << "#{" " * @ident }</#{tag}>"
  end

  def builder_options(options)
    options.collect { |key, value| "#{key}='#{value}'"}.join(" ")
  end

end

markup = Html.build do
  section do
    h1 "my title", class: "text-center", id: "my-id" # <h1>
    h2 "my second title"
  end
end

puts markup

Spoiler Warning