Presenters and Decorators: A Code Tour

Presenter and Decorators are design approaches that can be used in Rails applications outside of the standard Models, Views and Controllers. These approaches are becoming more and more popular as teams search for new ways to identify and manage the complexity within their applications.

In this session Mike Moore will defined the Presenter and Decorator approaches using simple and clear terminology. Common design problems in Rails applications will be shown using real-life code examples and refactored toward Presenters and Decorators. Code will be improved and strengthened by identifying and respecting the dependencies within large applications.

Transcript

1. Presenters
   and Decorators
   A Code Tour
   

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3. Mike Moore
   

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4. @blowmage
   

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5. Disclaimer
   

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6. Presenters
   should ease
   PAIN
   

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7. Components...
   

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8. <%= render_component(@component) %>
   app/views/dashboard/index.html.erb
   

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9. def render_component(component)
   render :partial => "shared/component",
   :locals => {:component => component}
   end
   app/controllers/application_controller.rb
   

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10. <%= render_component(component) %>
    app/views/shared/_component.html.erb
    

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11. module ComponentHelper
    def render_component(component)
    return "" if component.nil?
    if component.respond_to?(:render)
    raw component.render(self)
    else
    raw component.to_s
    end
    end
    end
    app/helpers/component_helper.rb
    

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12. class DashboardController < ApplicationController
    before_filter :require_user
    def index
    @component = DashboardComponentBuilder.build(current_user)
    end
    end
    app/controller/dashboard_controller.rb
    

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13. class DashboardComponentBuilder
    def self.build(user)
    my_posts = user.posts.recent
    DashboardComponent.new(
    :post_items =>
    PostItemComponentBuilder.build_list(my_posts),
    :author_stats_component =>
    AuthorAnalyticsComponentBuilder.build(user)
    )
    end
    end
    lib/dashboard_component_builder.rb
    

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14. class PostItemComponentBuilder < ComponentBuilder
    def self.build(post)
    post_path = UrlGenerator.post_path(post)
    PostItemComponent.new(
    :title_link => Link.new(post.name, post_path),
    :description => post.description,
    :views => post.view_count,
    :post_thumbnail =>
    PostThumbnailComponentBuilder.build(post),
    :comments => post.comments.published.count,
    :edit_link => Link.new("Edit Post",
    UrlGenerator.edit_post_path(post)),
    :status_class => build_status_class(post),
    :status_text => build_status_text(post),
    :delete_button =>
    DeleteButtonComponentBuilder.build(post)
    )
    end
    #...
    end
    lib/post_item_component_builder.rb
    

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15. class ComponentBuilder
    def self.build_list(items,*args)
    return [] if items.nil? or items.empty?
    if self.respond_to?(:build)
    items.map do |item|
    self.build(item,*args)
    end.compact
    else
    raise "ComponentBuilder must implement .build"
    end
    end
    end
    app/components/component_builder.rb
    

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16. class PostItemComponentBuilder < ComponentBuilder
    def self.build(post)
    post_path = UrlGenerator.post_path(post)
    PostItemComponent.new(
    :title_link => Link.new(post.name, post_path),
    :description => post.description,
    :views => post.view_count,
    :post_thumbnail =>
    PostThumbnailComponentBuilder.build(post),
    :comments => post.comments.published.count,
    :edit_link => Link.new("Edit Post",
    UrlGenerator.edit_post_path(post)),
    :status_class => build_status_class(post),
    :status_text => build_status_text(post),
    :delete_button =>
    DeleteButtonComponentBuilder.build(post)
    )
    end
    #...
    end
    lib/post_item_component_builder.rb
    

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17. PostItemComponent = PartialComponent.define(
    "posts/post_item",
    :title_link,
    :description,
    :views,
    :comments,
    :delete_button,
    :edit_link,
    :post_thumbnail,
    :status_class,
    :status_text)
    app/components/post_item_component.rb
    

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18. class PartialComponent
    def self.define(partial, *var_names)
    component_class = ConstructorStruct.new(
    *[var_names + [:partial]].flatten)
    component_class.send(:define_method, :locals) do
    locals = {}
    var_names.each do |k|
    locals[k] = instance_variable_get("@#{k}")
    end
    locals
    end
    component_class.send(:define_method, :render) do |template|
    template.send(:render,
    :partial => @partial || partial,
    :locals => locals)
    end
    component_class
    end
    end
    app/components/partial_component.rb
    

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19. PostItemComponent = PartialComponent.define(
    "posts/post_item",
    :title_link,
    :description,
    :views,
    :comments,
    :delete_button,
    :edit_link,
    :post_thumbnail,
    :status_class,
    :status_text)
    app/components/post_item_component.rb
    

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20. .post_item{ :class => status_class }
    = render_component(post_thumbnail)
    .text
    %h5
    - if status_text
    %span= status_text
    = render_link(title_link)
    .description= description
    .stats_and_actions
    .item_stats
    %p.views.stat_display
    %strong.stat_count= views
    Views
    %p.comments.stat_display
    %strong.stat_count= comments
    Comments
    .item_actions
    .edit
    %a.edit{:href => edit_link.url}
    %span Edit
    .delete
    = render_component(delete_button)
    app/views/posts/_post_item.html.haml
    

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21. Welcome to
    the team!
    

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22. Won’t somebody
    please think of
    the new
    developers?!?!
    

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23. View ⬌ Bikeshed
    

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24. Typical
    Summary
    Example...
    

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25. <% published_count = current_user.posts.published.count +
    current_user.videos.published.count %>
    <% if published_count > 0 %>
    
    Published: <%= published_count %>
    
    <% end %>
    <% draft_count = current_user.posts.unpublished.count +
    current_user.videos.unpublished.count %>
    <% if draft_count > 0 %>
    
    Drafted:
    <%= draft_count %>
    
    <% end %>
    app/shared/nav/_author_details.html.erb
    

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26. <% if current_user.admin? %>
    <% approval_count =
    current_user.organization.posts.unapproved.count %>
    <% if approval_count.to_i > 0 %>
    
    Needing Approval:
    <%= approval_count %>
    
    <% end %>
    <% end %>
    app/shared/nav/_author_details.html.erb (cont)
    

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27. Solutions?
    

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28. class User < ActiveRecord::Base
    def published_count
    self.posts.published.count +
    self.videos.published.count
    end
    def draft_count
    self.posts.unpublished.count +
    self.videos.unpublished.count
    end
    def approval_count
    if self.admin?
    self.organization.posts.unapproved.count
    else
    0
    end
    end
    # ...
    end
    app/models/user.rb
    

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29. class AuthorSummary
    def initialize(author)
    @author = author
    end
    def published_count
    @author.posts.published.count +
    @author.videos.published.count
    end
    def draft_count
    @author.posts.unpublished.count +
    @author.videos.unpublished.count
    end
    def approval_count
    if @author.admin?
    @author.organization.posts.unapproved.count
    else
    0
    end
    end
    end
    lib/author_summary.rb
    

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30. <% summary = AuthorSummary.new(current_user) %>
    <% if summary.published_count > 0 %>
    
    Published:
    <%= summary.published_count %>
    
    <% end %>
    <% if summary.draft_count > 0 %>
    
    Drafted:
    <%= summary.draft_count %>
    
    <% end %>
    <% if summary.approval_count > 0 %>
    
    Needing Approval:
    <%= summary.approval_count %>
    
    <% end %>
    app/shared/nav/_author_details.html.erb
    

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31. ActiveDecorator
    github.com/amatsuda/active_decorator
    

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32. module UserDecorator
    def published_count
    self.posts.published.count +
    self.videos.published.count
    end
    def draft_count
    self.posts.unpublished.count +
    self.videos.unpublished.count
    end
    def approval_count
    if self.admin?
    self.organization.posts.unapproved.count
    else
    0
    end
    end
    end
    app/decorators/user_decorator
    

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33. <% if current_user.published_count > 0 %>
    
    Published:
    <%= current_user.published_count %>
    
    <% end %>
    <% if current_user.draft_count > 0 %>
    
    Drafted:
    <%= current_user.draft_count %>
    
    <% end %>
    <% if current_user.approval_count > 0 %>
    
    Needing Approval:
    <%= current_user.approval_count %>
    
    <% end %>
    app/shared/nav/_author_details.html.erb
    

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34. It helped!
    

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35. Typical
    Serialization
    Example...
    

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36. class PostController < ApplicationController
    before_filter :require_user
    def show
    @post = current_organization.posts.find(params[:id])
    respond_to do |format|
    format.html
    format.json { render :json => @post }
    end
    end
    end
    app/controllers/posts_controller.rb
    

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37. object @post
    attributes :id, :title
    # look up author_name on the model, but use author in the JSON
    attributes :author_name => :author
    if current_user.admin?
    # only show views, popularity to admins
    attributes :view_count => :views,
    :popularity_index => :popularity
    end
    app/views/posts/show.json.rabl
    

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38. class PostController < ApplicationController
    before_filter :require_user
    def show
    @post = current_organization.posts.find(params[:id])
    respond_to do |format|
    format.html
    format.json
    end
    end
    end
    app/controllers/posts_controller.rb
    

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39. class PostSerializer
    def initialize(post, user)
    @post, @user = post, user
    end
    def to_json
    if @user && @user.admin?
    full_detail
    else
    summary
    end
    end
    def full_detail
    summary.merge({ :views => @post.view_count,
    :popularity => @post.popularity_index })
    end
    def summary
    { :id => @post.id,
    :title => @post.title,
    :author => @post.author_name }
    end
    end
    lib/post_serializer.rb
    

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40. class PostController < ApplicationController
    before_filter :require_user
    def show
    @post = current_organization.posts.find(params[:id])
    respond_to do |format|
    format.html
    format.json { s = PostSerializer.new(@post, current_user)
    render :json => s.to_json }
    end
    end
    end
    app/controllers/posts_controller.rb
    

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41. ActiveModel::
    Serializers
    github.com/josevalim/
    active_model_serializers
    

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42. class PostSerializer < ActiveModel::Serializer
    attributes :id, :title
    # look up author_name on the model, but use author in the JSON
    attribute :author_name, :key => :author
    # @scoped is set to current_user by default
    if @scoped.admin?
    attribute :view_count, :key => :views
    attribute :popularity_index, :key => :popularity
    end
    end
    app/serializers/post_serializer.rb
    

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43. class PostController < ApplicationController
    before_filter :require_user
    def show
    @post = current_organization.posts.find(params[:id])
    respond_to do |format|
    format.html
    format.json { render :json => @post }
    end
    end
    end
    app/controllers/posts_controller.rb
    

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44. Pretty cool!
    

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45. But...
    

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46. Is that it?
    

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47. Presenter
    Definition?
    

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48. Design Patterns
    

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49. Decorator
    Design Pattern
    Attach additional
    responsibilities to an object
    dynamically. Decorators
    provide a flexible alternative
    to subclassing for extending
    functionality.
    

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50. Mediator
    Design Pattern
    Define an object that
    encapsulates how a set of
    objects interact. Mediator
    promotes loose coupling by
    keeping objects from referring
    to each other explicitly, and
    lets you vary their interaction
    independently.
    

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51. Presenter
    Spectrum
    

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52. Presenter Spectrum
    Models
    Views
    Decorators
    Presenters?
    

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53. Rails: Presenter
    Pattern
    blog.jayfields.com/2007/03/rails-
    presenter-pattern.html
    

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54. Presentation
    Model
    martinfowler.com/eaaDev/
    PresentationModel.html
    

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55. Presentation
    Model
    Presentation Model may
    interact with several domain
    objects, but Presentation
    Model is not a GUI friendly
    facade to a specific domain
    object...
    

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56. Presentation
    Model
    Instead it is easier to
    consider Presentation Model
    as an abstract of the view
    that is not dependent on a
    specific GUI framework...
    

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57. Presentation
    Model
    While several views can
    utilize the same Presentation
    Model, each view should
    require only one
    Presentation Model...
    

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58. Presentation
    Model
    In the case of composition a
    Presentation Model may
    contain one or many child
    Presentation Model
    instances, but each child
    control will also have only
    one Presentation Model.
    

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59. Definition?
    

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60. “Representation
    of the State of
    the View”
    

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61. Presenter ==
    Presentation
    Model?
    

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62. ViewModel ==
    Presentation
    Model
    

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63. A Slightly
    Better
    Example...
    

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64. <% if @course.available? &&
    @course.self_enrollment &&
    @course.open_enrollment &&
    (!@course_enrollment || !@course_enrollment.active?) &&
    !session["role_course_#{@course.id}"] %>
    <%= render :partial => "join_course", :object => @course %>
    <% elsif @course_enrollment &&
    @course_enrollment.self_enrolled &&
    @course_enrollment.active? &&
    (!session["role_course_#{@course.id}"]) %>
    <%= render :partial => "drop_course",
    :locals => { :course => @course,
    :enrollment => @course_enrollment } %>
    <% elsif temp_type = session["role_course_#{@course.id}"] %>
    <%= render :partial => "temp_access",
    :object => temp_type %>
    <% end %>
    app/views/course.html.erb
    

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65. <% if @view.show_join_course? %>
    <%= render :partial => "join_course",
    :object => @view.course %>
    <% elsif @view.show_drop_course? %>
    <%= render :partial => "drop_course",
    :locals => { :course => @view.course,
    :enrollment => @view.enrollment } %>
    <% elsif @view.show_temp_access? %>
    <%= render :partial => "temp_access",
    :object => @view.temp_access %>
    <% end %>
    app/views/course.html.erb
    

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66. class CourseShowPresenter
    attr_accessor :course, :enrollment, :user, :session
    def show_join_course?
    @course.available? &&
    @course.self_enrollment &&
    @course.open_enrollment &&
    (!@enrollment || !@enrollment.active?) &&
    !show_temp_access?
    end
    def show_drop_course?
    @enrollment &&
    @enrollment.self_enrolled &&
    @enrollment.active? &&
    !show_temp_access?
    end
    def show_temp_access?
    @session["role_course_#{@course.id}"]
    end
    alias :temp_access, :show_temp_access?
    end
    app/presenters/course_show_presenter.rb
    

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67. class CourseShowPresenter
    attr_accessor :course, :enrollment, :user, :session
    def show_join_course?
    @course.can_user_join?(@user) &&
    !show_temp_access?
    end
    def show_drop_course?
    @course.can_user_drop?(@user) &&
    !show_temp_access?
    end
    def show_temp_access?
    @session["role_course_#{@course.id}"]
    end
    alias :temp_access, :show_temp_access?
    end
    app/presenters/course_show_presenter.rb
    

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68. <% if @view.show_join_course? %>
    <%= render :partial => "join_course",
    :object => @view.course %>
    <% elsif @view.show_drop_course? %>
    <%= render :partial => "drop_course",
    :locals => { :course => @view.course,
    :enrollment => @view.enrollment } %>
    <% elsif @view.show_temp_access? %>
    <%= render :partial => "temp_access",
    :object => @view.temp_access %>
    <% end %>
    app/views/course.html.erb
    

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69. A Good
    Example...
    

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72. <% if current_organization.feature?(:categories) %>
    <% if current_site.setting?(:categories) %>
    <% if user_logged_in? ||
    current_site.setting?(:show_all_content) %>
    categories = current_site.top_categories
    <% else %>
    categories = current_site.top_public_categories
    <% end %>
    <% if categories.any? %>
    
    
    Top Categories
    
    
    <%= render :partial => "category_item",
    :collection => categories %>
    
    
    <% end %>
    <% end %>
    <% end %>
    app/views/home/_sidebar.html.erb
    

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73. class HomepagePresenter
    attr_accessor :organization, :site, :user
    # Categories
    def show_categories?
    @organization.feature?(:categories) &&
    @site.setting?(:categories) &&
    self.categories.any?
    end
    def categories
    if @user.present? || @site.setting?(:show_all_content)
    @categories ||= @organization.top_categories
    else
    @categories ||= @organization.top_public_categories
    end
    end
    #...
    end
    app/presenters/homepage_presenter.rb
    

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74. <% if current_organization.feature?(:categories) %>
    <% if current_site.setting?(:categories) %>
    <% if user_logged_in? ||
    current_site.setting?(:show_all_content) %>
    categories = current_site.top_categories
    <% else %>
    categories = current_site.top_public_categories
    <% end %>
    <% if categories.any? %>
    
    
    Top Categories
    
    
    <%= render :partial => "category_item",
    :collection => categories %>
    
    
    <% end %>
    <% end %>
    <% end %>
    app/views/home/_sidebar.html.erb
    

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75. <% if @view.show_categories? %>
    
    
    Top Categories
    
    
    <%= render :partial => "category_item",
    :collection => @view.categories %>
    
    
    <% end %>
    app/views/home/_sidebar.html.erb
    

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76. class HomepagePresenter
    attr_accessor :organization, :site, :user
    # Categories
    def show_categories?
    @organization.feature?(:categories) &&
    @site.setting?(:categories) &&
    self.categories.any?
    end
    def categories
    if @user.present? || @site.setting?(:show_all_content)
    @categories ||= @organization.top_categories
    else
    @categories ||= @organization.top_public_categories
    end
    end
    #...
    end
    app/presenters/homepage_presenter.rb
    

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77. class HomepagePresenter
    #...
    def categories
    if show_all_content?
    @categories ||= @organization.top_categories
    else
    @categories ||= @organization.top_public_categories
    end
    end
    protected
    def show_all_content?
    @user.present? || @site.setting?(:show_all_content)
    end
    #...
    end
    app/presenters/homepage_presenter.rb
    

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78. # Presenter when user is logged in
    class HomepageUserPresenter
    #...
    end
    # Presenter when site shows only public content
    class HomepagePublicPresenter
    #...
    end
    # Presenter when site shows all content, all the time
    class HomepagePermissivePresenter
    #...
    end
    app/presenters/homepage_*_presenter.rb
    

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79. class HomepageController < ApplicationController
    def index
    @view = build_homepage_presenter
    respond_to do |format|
    format.html
    end
    end
    def build_homepage_presenter
    view = if user_logged_in?
    HomepageUserPresenter.new
    elsif current_site.setting?(:show_all_content)
    HomepagePermissivePresenter.new
    else
    HomepagePublicPresenter.new
    end
    view.organization = current_organization
    view.site = current_site
    view.user = current_user
    view
    end
    end
    app/controllers/homepage_controller.rb
    

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80. Warning!
    

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84. Thank you!
    

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85. HIRE ME!!!
    blowmage.com
    Questions?
    

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