How ViewComponent Works

At application boot

When ViewComponent::Base is inherited from, ViewComponent::Compiler.compile compiles a component’s template(s) into instance method(s) on the component:

class MyComponent < ViewComponent::Base
end

<%= Time.now %>

Compiles to:

class MyComponent < ViewComponent::Base
  def _call_my_component
    @output_buffer.append = (Time.now)
    @output_buffer
  end
end

At component instantiation

Component initializer runs if defined. ViewComponent does not perform any actions.

At render

When a ViewComponent is passed to render, Rails calls render_in, passing the current ActionView::Context. ActionView::Context is used to provide access to the current controller, request, and helpers, which is why they cannot be referenced until a component is rendered.

First, ViewComponent calls the before_render method, which gives ViewComponents the opportunity to execute logic before render time but after initialization.

Second, ViewComponent calls render?, returning early with an empty string if render? returns false.

ViewComponent then renders the component template.