How are Rails instance variables passed to views?

Question

In my Rails app, I have a controller like this:

class MyController < ApplicationController
  def show
    @blog_post = BlogPost.find params[:id]
  end
end

In my view I can simply do this:

<%= @blog_post.title %>

I'm uncomfortable with magic. How is this achieved?

Answer 1

When the view is being rendered, instance variables and their values are picked up from the controller and passed to the view initializer which sets them to the view instance. This is done using these ruby methods:

instance_variables - gets names of instance variables (documentation) instance_variable_get(variable_name) - gets value of an instance variable (documentation) instance_variable_set(variable_name, variable_value) - sets value of an instance variable (documentation)

Here is the Rails code:

Collecting controller instance variables (github):

def view_assigns
  hash = {}
  variables  = instance_variables
  variables -= protected_instance_variables
  variables -= DEFAULT_PROTECTED_INSTANCE_VARIABLES
  variables.each { |name| hash[name[1..-1]] = instance_variable_get(name) }
  hash
end

Passing them to the view (github):

def view_context
  view_context_class.new(view_renderer, view_assigns, self)
end

Setting them in the view (github):

def assign(new_assigns) # :nodoc:
  @_assigns = new_assigns.each { |key, value| instance_variable_set("@#{key}", value) }
end