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