Using handlebars.rb (handlebars.js) from Rails 4

server-side-handlebars-with-rails-4.rb

################################################################
# config/initializers/handlebars.rb:
################################################################

template_path = File.join(Rails.root.to_s, 'app/views')
helpers = {
  t: lambda do |context, key|
    I18n.t(key)
  end,
  path: lambda do |context, path_name|
    Seashore::Gangway::LocalUrlHelper.localized_path_for(path_name)
  end,
  css_classes: lambda do |context|
    # NOTE: arrays in context are V8:Array instances, see: https://github.com/cowboyd/therubyracer/wiki/Accessing-V8-Objects-From-Ruby
    css_classes = context['css_classes'].try(:to_a)
    hide_from_classes = context['hide_from'].try(:to_a).try(:map) { |user_type| "hide-from-#{user_type}" }
    [css_classes, hide_from_classes].compact.flatten.join(" ")
  end,
  join: lambda do |context, vals, sep|
    vals && vals.join(sep || " ")
  end,
  ifEq: lambda do |context, val1, val2, block|
    if val1 == val2
      block.fn(context)
    else
      block.inverse(context)
    end
  end
}
options = {
  cache: !Rails.env.development?,
  template_path: template_path,
  partials_path: File.join(template_path, 'partials'),
  helpers: helpers
}
Rails.configuration.handlebars_context = HandlebarsContext.new(options)

################################################################
# app/models/handlebars_context.rb:
################################################################

require 'handlebars'

class HandlebarsContext
  PREFIX = '_'
  EXT = '.hbs'

  attr_reader :handlebars, :cached_templates, :cached_partials, :options

  def initialize(options = {})
    options.assert_valid_keys(:cache, :template_path, :partials_path, :helpers)
    default_options = {template_path: '.', partials_path: './partials', helpers: {}}
    @options = default_options.merge(options)
    @cached_templates = {}
    @cached_partials = {}
    @handlebars = Handlebars::Context.new
    @handlebars.partial_missing do |name|
      lambda do |this, context, options|
        path = partial_path(name)
        if partial = lookup_partial(path)
          render_compiled(partial, context)
        else
          puts "[ERROR] HandlebarsContext could not find partial #{name} at path #{path}"
        end
      end
    end
    @options[:helpers].each do |name, block|
      @handlebars.register_helper(name, &block)
    end
  end

  def render(path, data)
    template = lookup_template(template_path(path))
    if template
      render_compiled(template, data)
    else
      puts "[ERROR] HandlebarsContext#render could not find template at path #{path}"
    end
  end

  def lookup_template(path)
    return @cached_templates[path] if @cached_templates[path] && options[:cache]
    if File.exists?(path)
      template_str = IO.read(path)
      @cached_templates[path] = handlebars.compile(template_str)
    else
      nil
    end
  end

  def lookup_partial(path)
    return @cached_partials[path] if @cached_partials[path] && options[:cache]
    if File.exists?(path)
      template_str = IO.read(path)
      @cached_partials[path] = handlebars.compile(template_str)
    else
      nil
    end
  end

  private

  def render_compiled(template, data)
    template ? template.call(data) : nil
  rescue Exception => e
    puts "[ERROR] HandlebarsContext#render exception: #{e.message} #{e.backtrace.join("\n")}"
    ""
  end

  def template_path(path)
    filename = PREFIX + File.basename(path) + EXT
    File.join(options[:template_path], File.dirname(path), filename)
  end

  def partial_path(name)
    filename = PREFIX + name + EXT
    File.join(options[:partials_path], filename)
  end
end

################################################################
# application_helper.rb:
################################################################

def render_hbs(path, data)
  Rails.configuration.handlebars_context.render(path, data).try(:html_safe)
end