Lazy Load Hooks

LazyLoadHooks allows Rails to lazily load a lot of components and thus making the app boot faster. Because of this feature now there is no need to require ActiveRecord::Base at boot time purely to apply configuration. Instead a hook is registered that applies configuration once ActiveRecord::Base is loaded. Here ActiveRecord::Base is used as example but this feature can be applied elsewhere too.

Here is an example where on_load method is called to register a hook.

initializer 'active_record.initialize_timezone' do
  ActiveSupport.on_load(:active_record) do
    self.time_zone_aware_attributes = true
    self.default_timezone = :utc
  end
end

When the entirety of ActiveRecord::Base has been evaluated then run_load_hooks is invoked. The very last line of ActiveRecord::Base is:

ActiveSupport.run_load_hooks(:active_record, ActiveRecord::Base)

run_load_hooks will then execute all the hooks that were registered with the on_load method. In the case of the above example, it will execute the block of code that is in the initializer.

Registering a hook that has already run results in that hook executing immediately. This allows hooks to be nested for code that relies on multiple lazily loaded components:

initializer "action_text.renderer" do
  ActiveSupport.on_load(:action_controller_base) do
    ActiveSupport.on_load(:action_text_content) do
      self.default_renderer = Class.new(ActionController::Base).renderer
    end
  end
end

Instance Public methods

on_load(name, options = {}, &block)

Declares a block that will be executed when a Rails component is fully loaded. If the component has already loaded, the block is executed immediately.

Options:

:yield - Yields the object that run_load_hooks to block.
:run_once - Given block will run only once.

📝 Source code

# File activesupport/lib/active_support/lazy_load_hooks.rb, line 60
    def on_load(name, options = {}, &block)
      @loaded[name].each do |base|
        execute_hook(name, base, options, block)
      end

      @load_hooks[name] << [block, options]
    end

🔎 See on GitHub

run_load_hooks(name, base = Object)

Executes all blocks registered to name via on_load, using base as the evaluation context.

ActiveSupport.run_load_hooks(:active_record, ActiveRecord::Base)

In the case of the above example, it will execute all hooks registered for :active_record within the class ActiveRecord::Base.