Application Helpers

Output Helpers

Output helpers are a collection of important methods for managing, capturing and displaying output in various ways and is used frequently to support higher-level helper functions. There are three output helpers worth mentioning: content_for, capture_html, and concat_content

The content_for functionality supports capturing content and then rendering this into a different place such as within a layout. One such popular example is including assets onto the layout from a template:

# app/views/site/index.erb
# ...
<% content_for :assets do %>
  <%= stylesheet_link_tag 'index', 'custom' %>
<% end %>
# ...

Added to a template, this will capture the includes from the block and allow them to be yielded into the layout:

# app/views/layout.erb
  <%= stylesheet_link_tag 'style' %>
  <%= yield_content :assets %>

This will automatically insert the contents of the block (in this case a stylesheet include) into the location the content is yielded within the layout.

You can also check if a content_for block exists for a given key using content_for?:

# app/views/layout.erb
<% if content_for?(:assets) %>
  <div><%= yield_content :assets %></div>
<% end %>

The capture_html and the concat_content methods allow content to be manipulated and stored for use in building additional helpers accepting blocks or displaying information in a template. One example is the use of these in constructing a simplified form_tag helper which accepts a block.

# form_tag '/register' do ... end
def form_tag(url, options={}, &block)
  # ... truncated ...
  inner_form_html = capture_html(&block)
  concat_content '<form>' + inner_form_html + '</form>'

This will capture the template body passed into the form_tag block and then append the content to the template through the use of concat_content. Note have been built to work for both haml and erb templates using the same syntax.

List of Output Helpers

  • content_for(key, &block)
    • Capture a block of content to be rendered at a later time.
    • Existence can be checked using the content_for?(key) method.
    • content_for(:head) { ...content... }
    • Also supports arguments passed to the content block
    • content_for(:head) { |param1, param2| ...content... }
  • yield_content(key, *args)
    • Render the captured content blocks for a given key.
    • yield_content :head
    • Also supports arguments yielded to the content block
    • yield_content :head, param1, param2
  • capture_html(*args, &block)
    • Captures the html from a block of template code for erb or haml
    • capture_html(&block) => "...html..."
  • concat_content(text="")
    • Outputs the given text to the templates buffer directly in erb or haml
    • concat_content("This will be output to the template buffer in erb or haml")

last updated: 2016-09-04

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