ref: https://gist.github.com/ahoward/5752290

web apps nearly always end up needing a plethora of little javascript helper methods: you know, auto-complete forms, populating defaults, validations, etc. these aren't API calls properly, just little tidbits of functionality needed to make the views work.

there is always a question of which controller to hang these methods off of. do you make a global helper controller for all this stuff? do you hang them off the controller in question? how to do share the backend of javascript helper methods across controllers?

step one

include the RPC module in your ApplicationController

  class ApplicationController < ::ActionController::Base
    include RPC
  end

step two

in controllers, declare rpc helper methods

  class PostsController < ApplicationController
    rpc(:defaults) do |params|
      post = Post.new(params)
      post.set_defaults!
      post.as_json
    end
  end

step three

setup js to be able to call rpc methods relative to the current controller

  // in file: app/views/layouts/application.html.erb
  //
  // notice the *relative* url rpc endpoint
  //

  // <script>

    jQuery(function(){

        var rpc = function(){
        // parse args
        //
          var args = Array.prototype.slice.call(arguments);
          var ajax;
          var method;

          if(args.length == 1 && typeof(args[0]) == 'object'){
            ajax = args.shift();
          } else {
            ajax = {};

            for(var i = 0; i < args.length; i++){
              var arg = args[i];

              switch (typeof(arg)) {
                case 'object':
                  ajax['data'] = arg;
                  break;
                case 'function':
                  ajax['success'] = arg;
                  break;
                case 'string':
                  method = arg;
                  break;
              };
            }
          };

        // normalize ajax options
        //
          ajax['url'] = ajax['url'] || rpc['url'];

          method = method || ajax['method'];
          delete ajax['method'];

          ajax['data'] = ajax['data'] || {};
          ajax['data']['method'] = method;

          ajax['async']          = true;
          ajax['type']           = 'GET';
          ajax['dataType']       = 'json';
          ajax['cache']          = false;
          ajax['contentType']    = 'application/json; charset = utf-8';

          var result = ajax;

        // ajax request - using callback or returning the result sync style
        //
          if(!ajax['success']){
            ajax['async'] = false;
            ajax['success'] = function(response){ result = response };
          };

          jQuery.ajax(ajax);

          return(result);
        };


        rpc['url'] = <%= raw url_for(:action => :rpc).to_json %>;


        window.rpc = rpc;



    });

  // </script>

step four

enable the rpc endpoints in config/routes.rb

  resources :posts do
    collection do
      get 'rpc'
    end
  end

step five

now you can use relative js rpc helpers at will in your views

  var title = $('#title');
  var slug = $('#slug');

  title.change(function(){
    var value = title.val();

    var data = {'title' : value};

    var success = function(defaults){
      slug.val(defaults['slug']);
    };

    rpc('defaults', data, success);
  });

this organization keeps your code clean by avoiding a proliferation of controllers and prevents the need to litter clean resourceful controllers with loads of fugly js helper actions

here is the code in it's entirety

  # file : ./lib/rpc.rb

  module RPC
    Actions =
      Hash.new{|hash, controller| hash[controller] = Hash.new}

    module ClassMethods
      def rpc(method = nil, &block)
        case
          when method.nil? && block.nil?
            Actions

          else
            controller = self
            method     = method.to_s

            Actions[controller][method] = block
        end
      end
    end

    module InstanceMethods
      def rpc
        method  = params.delete(:method) || params.delete(:call)

        if method
          controllers = self.class.ancestors.select{|controller| controller <= ::ApplicationController}

          controllers.each do |controller|
            action = Actions[controller][method]

            if action
              result = action.call(params)
              render(:json => result.to_json)
              return
            end
          end

          render(:nothing => true, :status => :not_found, :layout => false)
        else
          render(:json => RPC.index)
        end
      end
    end

    def RPC.index
      Hash.new.tap do |methods|
        Actions.each do |controller, actions|
          controller = controller.name.underscore
          methods[controller] = []

          actions.each do |method, block|
            methods[controller].push(method)
          end
        end
      end
    end

    def RPC.included(other)
      super
    ensure
      other.send(:extend, ClassMethods)
      other.send(:include, InstanceMethods)
    end
  end

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