ActiveSupport::LazyLoadHooks

lazy_load_hooks 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)
Methods
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Instance Public methods
execute_hook(base, options, block)

Source: show | on GitHub 

# File activesupport/lib/active_support/lazy_load_hooks.rb, line 41
def execute_hook(base, options, block)
  if options[:yield]
    block.call(base)
  else
    base.instance_eval(&block)
  end
end
on_load(name, options = {}, &block)

Declares a block that will be executed when a Rails component is fully loaded.

Source: show | on GitHub 

# File activesupport/lib/active_support/lazy_load_hooks.rb, line 33
def on_load(name, options = {}, &block)
  @loaded[name].each do |base|
    execute_hook(base, options, block)
  end

  @load_hooks[name] << [block, options]
end
run_load_hooks(name, base = Object)

Source: show | on GitHub 

# File activesupport/lib/active_support/lazy_load_hooks.rb, line 49
def run_load_hooks(name, base = Object)
  @loaded[name] << base
  @load_hooks[name].each do |hook, options|
    execute_hook(base, options, hook)
  end
end