Ruby On Railsのコードリーディングをして設計の基礎を学んでみる Action View編

#なぜするのか

使っているライブラリやフレームワークのドキュメントを読んでも処理内容がよくわからなかったり、なんか怪しいと思った場合、どこにあるかわからない情報を探すより、確実に実行されるものを見た方が早い(一般的な話、なおWETなコードは除きます)事が多々有ります。
特にテストコードや、メタメタしてるコードを読むのは非常に勉強になります。

#Action View

RailsでWebアプリケーションを作るなら、ほぼ100%使うであろう[renderメソッドのコード](https://github.com/rails/rails/blob/5-0-1/actionview/lib/action_view/renderer/renderer.rb)根幹となる処理分岐は実はこんなに少ないコードで実装してあります。実際のrenderロジックは、他のクラスに集約して、自由に付け替え可能となるように実装されています。それぞれのクラスにrenderメソッドを実行させていることがわかります。
これらのクラスはabstructパターンで作られています。

```rb

#abstract_renderrer.rb


module ActionView
  # This class defines the interface for a renderer. Each class that
  # subclasses +AbstractRenderer+ is used by the base +Renderer+ class to
  # render a specific type of object.
  #
  # The base +Renderer+ class uses its +render+ method to delegate to the
  # renderers. These currently consist of
  #
  #   PartialRenderer - Used for rendering partials
  #   TemplateRenderer - Used for rendering other types of templates
  #   StreamingTemplateRenderer - Used for streaming
  #
  # Whenever the +render+ method is called on the base +Renderer+ class, a new
  # renderer object of the correct type is created, and the +render+ method on
  # that new object is called in turn. This abstracts the setup and rendering
  # into a separate classes for partials and templates.
  class AbstractRenderer #:nodoc:
    delegate :find_template, :find_file, :template_exists?, :any_templates?, :with_fallbacks, :with_layout_format, :formats, :to => :@lookup_context

    def initialize(lookup_context)
      @lookup_context = lookup_context
    end

    def render
      raise NotImplementedError
    end

    protected

    def extract_details(options)
      @lookup_context.registered_details.each_with_object({}) do |key, details|
        value = options[key]

        details[key] = Array(value) if value
      end
    end

    def instrument(name, **options)
      options[:identifier] ||= (@template && @template.identifier) || @path

      ActiveSupport::Notifications.instrument("render_#{name}.action_view", options) do |payload|
        yield payload
      end
    end

    def prepend_formats(formats)
      formats = Array(formats)
      return if formats.empty? || @lookup_context.html_fallback_for_js

      @lookup_context.formats = formats | @lookup_context.formats
    end
  end
end

```

ここで delegateという予約メソッドがありますね。

これは、delegeteを実行した時点でそのクラスに実装されていないメソッドやプロパティなどを、toでシンボルで指定した先の変数に実装を任せて、<b>とりあえずそのクラスのインスタンスからdelegateに指定された名前でメソッドの呼び出しをできるようにします</b>。

このメソッドが呼び出された時に、toで指定された変数内に格納されたオブジェクトが,delegateされたメソッドを実装していた場合、delegete元のクラスのインスタンスからそのメソッドを渡す事ができます。例えば以下のようになります。

```rb
class A
  def self.test
    "Hello"
  end
end

class B
  delegate :test,to: :A
end

B.new.test => "Hello"
```

以上のようにBクラスのインスタンスからAクラスのクラスメソッドが呼べたのが分かると思います。当然この時,Aクラスのクラスメソッドにtestが定義されていなければ、実行時にエラーになります。私はrubyではこのようにして、抽象化するということはこのコードから学びました。

次に、yieldを見てみましょう。yieldはpythonでもC#でもよく使うので、イテレータとして順番に値を返すのだろうと思いきや、rubyは仕様がちょっと違うんですよね。

rubyのyieldは関数自体の戻り値として値をイテレートするわけではなく、yieldした変数を、渡されたブロックにインジェクトして、ブロックの呼び出し自体を繰り返します。分かりやすい例でいうと

python

```py3
def my_iter():
    yield 1
    yield 2
    yield 3
 
it = my_iter()
for x in it:
    print(x)

```

Ruby

```rb
def my_iter
  yield 1
  yield 2
  yield 3 
end

my_iter{|it|
  puts it
}

```
以上のようにpythonのyieldは値を順番に返していくのに対し、rubyのyieldは呼ばれたタイミングでそのブロックにyieldした値をインジェクトしています。ただ、以下は全く無意味なコードですが

```rb
def for_each(list)
  list.each do|n|
    yield n
  end
end

for_each([1,2,3]){|it|
  puts it
}
```

とすると、メモリの動きまで調べたことないので、部処理までが一緒かどうかは分かりませんが、書き方が違うだけで本質的には同様な処理を実装できることが分かるのではないでしょうか。(ただrubyのyieldは中断と再開処理ができない？知ってる方いらっしゃれば教えて下さい。)

railsではpayloadの分だけ外部に処理を投げているようです。

そしておそらく、皆さんがrails開発をしていて一番気になるのが、optionはどれだけあるのか、どういう形式でデータを突っ込めるかだと思うのですが、例えばpartial_renderer.rbを例にして見てみます。

[partial_renderer.rb](https://github.com/rails/rails/blob/master/actionview/lib/action_view/renderer/partial_renderer.rb)

```rb
require "concurrent/map"
require "action_view/renderer/partial_renderer/collection_caching"

module ActionView
  class PartialIteration
    # The number of iterations that will be done by the partial.
    attr_reader :size

    # The current iteration of the partial.
    attr_reader :index

    def initialize(size)
      @size  = size
      @index = 0
    end

    # Check if this is the first iteration of the partial.
    def first?
      index == 0
    end

    # Check if this is the last iteration of the partial.
    def last?
      index == size - 1
    end

    def iterate! # :nodoc:
      @index += 1
    end
  end

  # = Action View Partials
  #
  # There's also a convenience method for rendering sub templates within the current controller that depends on a
  # single object (we call this kind of sub templates for partials). It relies on the fact that partials should
  # follow the naming convention of being prefixed with an underscore -- as to separate them from regular
  # templates that could be rendered on their own.
  #
  # In a template for Advertiser#account:
  #
  #  <%= render partial: "account" %>
  #
  # This would render "advertiser/_account.html.erb".
  #
  # In another template for Advertiser#buy, we could have:
  #
  #   <%= render partial: "account", locals: { account: @buyer } %>
  #
  #   <% @advertisements.each do |ad| %>
  #     <%= render partial: "ad", locals: { ad: ad } %>
  #   <% end %>
  #
  # This would first render "advertiser/_account.html.erb" with @buyer passed in as the local variable +account+, then
  # render "advertiser/_ad.html.erb" and pass the local variable +ad+ to the template for display.
  #
  # == The :as and :object options
  #
  # By default <tt>ActionView::PartialRenderer</tt> doesn't have any local variables.
  # The <tt>:object</tt> option can be used to pass an object to the partial. For instance:
  #
  #   <%= render partial: "account", object: @buyer %>
  #
  # would provide the <tt>@buyer</tt> object to the partial, available under the local variable +account+ and is
  # equivalent to:
  #
  #   <%= render partial: "account", locals: { account: @buyer } %>
  #
  # With the <tt>:as</tt> option we can specify a different name for said local variable. For example, if we
  # wanted it to be +user+ instead of +account+ we'd do:
  #
  #   <%= render partial: "account", object: @buyer, as: 'user' %>
  #
  # This is equivalent to
  #
  #   <%= render partial: "account", locals: { user: @buyer } %>
  #
  # == \Rendering a collection of partials
  #
  # The example of partial use describes a familiar pattern where a template needs to iterate over an array and
  # render a sub template for each of the elements. This pattern has been implemented as a single method that
  # accepts an array and renders a partial by the same name as the elements contained within. So the three-lined
  # example in "Using partials" can be rewritten with a single line:
  #
  #   <%= render partial: "ad", collection: @advertisements %>
  #
  # This will render "advertiser/_ad.html.erb" and pass the local variable +ad+ to the template for display. An
  # iteration object will automatically be made available to the template with a name of the form
  # +partial_name_iteration+. The iteration object has knowledge about which index the current object has in
  # the collection and the total size of the collection. The iteration object also has two convenience methods,
  # +first?+ and +last?+. In the case of the example above, the template would be fed +ad_iteration+.
  # For backwards compatibility the +partial_name_counter+ is still present and is mapped to the iteration's
  # +index+ method.
  #
  # The <tt>:as</tt> option may be used when rendering partials.
  #
  # You can specify a partial to be rendered between elements via the <tt>:spacer_template</tt> option.
  # The following example will render <tt>advertiser/_ad_divider.html.erb</tt> between each ad partial:
  #
  #   <%= render partial: "ad", collection: @advertisements, spacer_template: "ad_divider" %>
  #
  # If the given <tt>:collection</tt> is +nil+ or empty, <tt>render</tt> will return nil. This will allow you
  # to specify a text which will displayed instead by using this form:
  #
  #   <%= render(partial: "ad", collection: @advertisements) || "There's no ad to be displayed" %>
  #
  # NOTE: Due to backwards compatibility concerns, the collection can't be one of hashes. Normally you'd also
  # just keep domain objects, like Active Records, in there.
  #
  # == \Rendering shared partials
  #
  # Two controllers can share a set of partials and render them like this:
  #
  #   <%= render partial: "advertisement/ad", locals: { ad: @advertisement } %>
  #
  # This will render the partial "advertisement/_ad.html.erb" regardless of which controller this is being called from.
  #
  # == \Rendering objects that respond to `to_partial_path`
  #
  # Instead of explicitly naming the location of a partial, you can also let PartialRenderer do the work
  # and pick the proper path by checking `to_partial_path` method.
  #
  #  # @account.to_partial_path returns 'accounts/account', so it can be used to replace:
  #  # <%= render partial: "accounts/account", locals: { account: @account} %>
  #  <%= render partial: @account %>
  #
  #  # @posts is an array of Post instances, so every post record returns 'posts/post' on `to_partial_path`,
  #  # that's why we can replace:
  #  # <%= render partial: "posts/post", collection: @posts %>
  #  <%= render partial: @posts %>
  #
  # == \Rendering the default case
  #
  # If you're not going to be using any of the options like collections or layouts, you can also use the short-hand
  # defaults of render to render partials. Examples:
  #
  #  # Instead of <%= render partial: "account" %>
  #  <%= render "account" %>
  #
  #  # Instead of <%= render partial: "account", locals: { account: @buyer } %>
  #  <%= render "account", account: @buyer %>
  #
  #  # @account.to_partial_path returns 'accounts/account', so it can be used to replace:
  #  # <%= render partial: "accounts/account", locals: { account: @account} %>
  #  <%= render @account %>
  #
  #  # @posts is an array of Post instances, so every post record returns 'posts/post' on `to_partial_path`,
  #  # that's why we can replace:
  #  # <%= render partial: "posts/post", collection: @posts %>
  #  <%= render @posts %>
  #
  # == \Rendering partials with layouts
  #
  # Partials can have their own layouts applied to them. These layouts are different than the ones that are
  # specified globally for the entire action, but they work in a similar fashion. Imagine a list with two types
  # of users:
  #
  #   <%# app/views/users/index.html.erb %>
  #   Here's the administrator:
  #   <%= render partial: "user", layout: "administrator", locals: { user: administrator } %>
  #
  #   Here's the editor:
  #   <%= render partial: "user", layout: "editor", locals: { user: editor } %>
  #
  #   <%# app/views/users/_user.html.erb %>
  #   Name: <%= user.name %>
  #
  #   <%# app/views/users/_administrator.html.erb %>
  #   <div id="administrator">
  #     Budget: $<%= user.budget %>
  #     <%= yield %>
  #   </div>
  #
  #   <%# app/views/users/_editor.html.erb %>
  #   <div id="editor">
  #     Deadline: <%= user.deadline %>
  #     <%= yield %>
  #   </div>
  #
  # ...this will return:
  #
  #   Here's the administrator:
  #   <div id="administrator">
  #     Budget: $<%= user.budget %>
  #     Name: <%= user.name %>
  #   </div>
  #
  #   Here's the editor:
  #   <div id="editor">
  #     Deadline: <%= user.deadline %>
  #     Name: <%= user.name %>
  #   </div>
  #
  # If a collection is given, the layout will be rendered once for each item in
  # the collection. For example, these two snippets have the same output:
  #
  #   <%# app/views/users/_user.html.erb %>
  #   Name: <%= user.name %>
  #
  #   <%# app/views/users/index.html.erb %>
  #   <%# This does not use layouts %>
  #   <ul>
  #     <% users.each do |user| -%>
  #       <li>
  #         <%= render partial: "user", locals: { user: user } %>
  #       </li>
  #     <% end -%>
  #   </ul>
  #
  #   <%# app/views/users/_li_layout.html.erb %>
  #   <li>
  #     <%= yield %>
  #   </li>
  #
  #   <%# app/views/users/index.html.erb %>
  #   <ul>
  #     <%= render partial: "user", layout: "li_layout", collection: users %>
  #   </ul>
  #
  # Given two users whose names are Alice and Bob, these snippets return:
  #
  #   <ul>
  #     <li>
  #       Name: Alice
  #     </li>
  #     <li>
  #       Name: Bob
  #     </li>
  #   </ul>
  #
  # The current object being rendered, as well as the object_counter, will be
  # available as local variables inside the layout template under the same names
  # as available in the partial.
  #
  # You can also apply a layout to a block within any template:
  #
  #   <%# app/views/users/_chief.html.erb %>
  #   <%= render(layout: "administrator", locals: { user: chief }) do %>
  #     Title: <%= chief.title %>
  #   <% end %>
  #
  # ...this will return:
  #
  #   <div id="administrator">
  #     Budget: $<%= user.budget %>
  #     Title: <%= chief.name %>
  #   </div>
  #
  # As you can see, the <tt>:locals</tt> hash is shared between both the partial and its layout.
  #
  # If you pass arguments to "yield" then this will be passed to the block. One way to use this is to pass
  # an array to layout and treat it as an enumerable.
  #
  #   <%# app/views/users/_user.html.erb %>
  #   <div class="user">
  #     Budget: $<%= user.budget %>
  #     <%= yield user %>
  #   </div>
  #
  #   <%# app/views/users/index.html.erb %>
  #   <%= render layout: @users do |user| %>
  #     Title: <%= user.title %>
  #   <% end %>
  #
  # This will render the layout for each user and yield to the block, passing the user, each time.
  #
  # You can also yield multiple times in one layout and use block arguments to differentiate the sections.
  #
  #   <%# app/views/users/_user.html.erb %>
  #   <div class="user">
  #     <%= yield user, :header %>
  #     Budget: $<%= user.budget %>
  #     <%= yield user, :footer %>
  #   </div>
  #
  #   <%# app/views/users/index.html.erb %>
  #   <%= render layout: @users do |user, section| %>
  #     <%- case section when :header -%>
  #       Title: <%= user.title %>
  #     <%- when :footer -%>
  #       Deadline: <%= user.deadline %>
  #     <%- end -%>
  #   <% end %>
  class PartialRenderer < AbstractRenderer
    include CollectionCaching

    PREFIXED_PARTIAL_NAMES = Concurrent::Map.new do |h, k|
      h[k] = Concurrent::Map.new
    end

    def initialize(*)
      super
      @context_prefix = @lookup_context.prefixes.first
    end

    def render(context, options, block)
      setup(context, options, block)
      @template = find_partial

      @lookup_context.rendered_format ||= begin
        if @template && @template.formats.present?
          @template.formats.first
        else
          formats.first
        end
      end

      if @collection
        render_collection
      else
        render_partial
      end
    end

    private

      def render_collection
        instrument(:collection, count: @collection.size) do |payload|
          return nil if @collection.blank?

          if @options.key?(:spacer_template)
            spacer = find_template(@options[:spacer_template], @locals.keys).render(@view, @locals)
          end

          cache_collection_render(payload) do
            @template ? collection_with_template : collection_without_template
          end.join(spacer).html_safe
        end
      end

      def render_partial
        instrument(:partial) do |payload|
          view, locals, block = @view, @locals, @block
          object, as = @object, @variable

          if !block && (layout = @options[:layout])
            layout = find_template(layout.to_s, @template_keys)
          end

          object = locals[as] if object.nil? # Respect object when object is false
          locals[as] = object if @has_object

          content = @template.render(view, locals) do |*name|
            view._layout_for(*name, &block)
          end

          content = layout.render(view, locals) { content } if layout
          payload[:cache_hit] = view.cache_hit
          content
        end
      end

      # Sets up instance variables needed for rendering a partial. This method
      # finds the options and details and extracts them. The method also contains
      # logic that handles the type of object passed in as the partial.
      #
      # If +options[:partial]+ is a string, then the +@path+ instance variable is
      # set to that string. Otherwise, the +options[:partial]+ object must
      # respond to +to_partial_path+ in order to setup the path.
      def setup(context, options, block)
        @view = context
        @options = options
        @block   = block

        @locals  = options[:locals] || {}
        @details = extract_details(options)

        prepend_formats(options[:formats])

        partial = options[:partial]

        if String === partial
          @has_object = options.key?(:object)
          @object     = options[:object]
          @collection = collection_from_options
          @path       = partial
        else
          @has_object = true
          @object = partial
          @collection = collection_from_object || collection_from_options

          if @collection
            paths = @collection_data = @collection.map { |o| partial_path(o) }
            @path = paths.uniq.one? ? paths.first : nil
          else
            @path = partial_path
          end
        end

        if as = options[:as]
          raise_invalid_option_as(as) unless /\A[a-z_]\w*\z/.match?(as.to_s)
          as = as.to_sym
        end

        if @path
          @variable, @variable_counter, @variable_iteration = retrieve_variable(@path, as)
          @template_keys = retrieve_template_keys
        else
          paths.map! { |path| retrieve_variable(path, as).unshift(path) }
        end

        self
      end

      def collection_from_options
        if @options.key?(:collection)
          collection = @options[:collection]
          collection ? collection.to_a : []
        end
      end

      def collection_from_object
        @object.to_ary if @object.respond_to?(:to_ary)
      end

      def find_partial
        find_template(@path, @template_keys) if @path
      end

      def find_template(path, locals)
        prefixes = path.include?(?/) ? [] : @lookup_context.prefixes
        @lookup_context.find_template(path, prefixes, true, locals, @details)
      end

      def collection_with_template
        view, locals, template = @view, @locals, @template
        as, counter, iteration = @variable, @variable_counter, @variable_iteration

        if layout = @options[:layout]
          layout = find_template(layout, @template_keys)
        end

        partial_iteration = PartialIteration.new(@collection.size)
        locals[iteration] = partial_iteration

        @collection.map do |object|
          locals[as]        = object
          locals[counter]   = partial_iteration.index

          content = template.render(view, locals)
          content = layout.render(view, locals) { content } if layout
          partial_iteration.iterate!
          content
        end
      end

      def collection_without_template
        view, locals, collection_data = @view, @locals, @collection_data
        cache = {}
        keys  = @locals.keys

        partial_iteration = PartialIteration.new(@collection.size)

        @collection.map do |object|
          index = partial_iteration.index
          path, as, counter, iteration = collection_data[index]

          locals[as]        = object
          locals[counter]   = index
          locals[iteration] = partial_iteration

          template = (cache[path] ||= find_template(path, keys + [as, counter]))
          content = template.render(view, locals)
          partial_iteration.iterate!
          content
        end
      end

      # Obtains the path to where the object's partial is located. If the object
      # responds to +to_partial_path+, then +to_partial_path+ will be called and
      # will provide the path. If the object does not respond to +to_partial_path+,
      # then an +ArgumentError+ is raised.
      #
      # If +prefix_partial_path_with_controller_namespace+ is true, then this
      # method will prefix the partial paths with a namespace.
      def partial_path(object = @object)
        object = object.to_model if object.respond_to?(:to_model)

        path = if object.respond_to?(:to_partial_path)
          object.to_partial_path
        else
          raise ArgumentError.new("'#{object.inspect}' is not an ActiveModel-compatible object. It must implement :to_partial_path.")
        end

        if @view.prefix_partial_path_with_controller_namespace
          prefixed_partial_names[path] ||= merge_prefix_into_object_path(@context_prefix, path.dup)
        else
          path
        end
      end

      def prefixed_partial_names
        @prefixed_partial_names ||= PREFIXED_PARTIAL_NAMES[@context_prefix]
      end

      def merge_prefix_into_object_path(prefix, object_path)
        if prefix.include?(?/) && object_path.include?(?/)
          prefixes = []
          prefix_array = File.dirname(prefix).split("/")
          object_path_array = object_path.split("/")[0..-3] # skip model dir & partial

          prefix_array.each_with_index do |dir, index|
            break if dir == object_path_array[index]
            prefixes << dir
          end

          (prefixes << object_path).join("/")
        else
          object_path
        end
      end

      def retrieve_template_keys
        keys = @locals.keys
        keys << @variable if @has_object || @collection
        if @collection
          keys << @variable_counter
          keys << @variable_iteration
        end
        keys
      end

      def retrieve_variable(path, as)
        variable = as || begin
          base = path[-1] == "/".freeze ? "".freeze : File.basename(path)
          raise_invalid_identifier(path) unless base =~ /\A_?(.*?)(?:\.\w+)*\z/
          $1.to_sym
        end
        if @collection
          variable_counter = :"#{variable}_counter"
          variable_iteration = :"#{variable}_iteration"
        end
        [variable, variable_counter, variable_iteration]
      end

      IDENTIFIER_ERROR_MESSAGE = "The partial name (%s) is not a valid Ruby identifier; " +
                                 "make sure your partial name starts with underscore."

      OPTION_AS_ERROR_MESSAGE  = "The value (%s) of the option `as` is not a valid Ruby identifier; " +
                                 "make sure it starts with lowercase letter, " +
                                 "and is followed by any combination of letters, numbers and underscores."

      def raise_invalid_identifier(path)
        raise ArgumentError.new(IDENTIFIER_ERROR_MESSAGE % (path))
      end

      def raise_invalid_option_as(as)
        raise ArgumentError.new(OPTION_AS_ERROR_MESSAGE % (as))
      end
  end
end
```
実は、コミッターの方はすごく丁寧に実装にコメントを残してくれてることが多いので、ここで知らなかったことが解ることが多いです。ここでoptionsで検索かけてみると、partialだけでもオプションたくさんあることが解るのではないでしょうか。localeとかはよく使いますが、あまり馴染みのないものもありますね。delegateされたメソッドもちゃんと定義されています。

#masterブランチに追加されているTagHelperなるヘルパー
Railsの公式リリースの最新バージョンは5.0.0(と思いきや公式リポジトリでも5.0.1rc1,5.0.1rc2がつい三日ほど前にリリースされていました。)ですが、最新版のmasterブランチにはタグヘルパーなるヘルパーにチェーンメソッドが追加されています。
[tag_helper.rb](https://github.com/rails/rails/blob/master/actionview/lib/action_view/helpers/tag_helper.rb)
これは、例えば、

```erb
<%= tag.h1 タイトル%>
<%# =><h1>タイトル</h1>　%>
```

のようにchainでタグを追加していける機能のようですが、便利そうなので早くリリースされて欲しいですね。


#まとめ

ドキュメントやネットで情報探せば早く解決することもありますが、実際のコードの知見やそこで使われてるテクニックが見れてとてもいいですよ。また、難しそうな実装に見えて以外と簡単だったり、プロジェクトによっては、コードが綺麗に見えてぐちゃぐちゃだったりといろんな発見があると思います。技術力の伸び悩みを感じたらオープンソースでこの処理いいなと思うプロジェクトの真似をしてみるのがいいのではないでしょうかというお話でした。