rails--rails/activesupport/lib/active_support/dependencies.rb

require File.dirname(__FILE__) + '/module_attribute_accessors'

module Dependencies
  extend self

  @@loaded = [ ]
  mattr_accessor :loaded

  @@mechanism = :load
  mattr_accessor :mechanism
  
  def load?
    mechanism == :load
  end
  
  def depend_on(file_name, swallow_load_errors = false)
    if !loaded.include?(file_name)
      loaded << file_name

      begin
        require_or_load(file_name)
      rescue LoadError
        raise unless swallow_load_errors
      rescue Object => e
        raise ScriptError, "#{e.message}"
      end
    end
  end

  def associate_with(file_name)
    depend_on(file_name, true)
  end
  
  def clear
    self.loaded = [ ]
  end
  
  def require_or_load(file_name)
    load? ? load("#{file_name}.rb") : require(file_name)
  end
  
  def remove_subclasses_for(*classes)
    classes.each { |klass| klass.remove_subclasses }
  end
  
  # LoadingModules implement namespace-safe dynamic loading.
  # They support automatic loading via const_missing, allowing contained items to be automatically
  # loaded when required. No extra syntax is required, as expressions such as Controller::Admin::UserController
  # load the relavent files automatically.
  #
  # Ruby-style modules are supported, as a folder named 'submodule' will load 'submodule.rb' when available.
  class LoadingModule < Module
    attr_reader :path
    attr_reader :root
    
    def self.root(*load_paths)
      RootLoadingModule.new(*load_paths)
    end
            
    def initialize(root, path=[])
      @path = path.clone.freeze
      @root = root
    end
    
    def load_paths() self.root.load_paths end
    
    # Load missing constants if possible.
    def const_missing(name)
      const_load!(name) ? const_get(name) : super(name)
    end
  
    # Load the controller class or a parent module.
    def const_load!(name, file_name = nil)
      path = self.path + [file_name || name]

      load_paths.each do |load_path|
        fs_path = load_path.filesystem_path(path)
        next unless fs_path

        if File.directory?(fs_path)
          self.const_set name, LoadingModule.new(self.root, self.path + [name])
          break
        elsif File.file?(fs_path)
          self.root.load_file!(fs_path)
          break
        end
      end

      return self.const_defined?(name)
    end
    
    # Is this name present or loadable?
    # This method is used by Routes to find valid controllers.
    def const_available?(name)
      self.const_defined?(name) || load_paths.any? {|lp| lp.filesystem_path(path + [name])}
    end
  end
  
  class RootLoadingModule < LoadingModule
    attr_reader :load_paths

    def initialize(*paths)
      @load_paths = paths.flatten.collect {|p| p.kind_of?(ConstantLoadPath) ? p : ConstantLoadPath.new(p)}
    end

    def root() self end

    def path() [] end
    
    # Load the source file at the given file path
    def load_file!(file_path)
      begin root.module_eval(IO.read(file_path), file_path, 1)
      rescue Object => exception
        exception.blame_file! file_path
        raise
      end
    end

    # Erase all items in this module
    def clear!
      constants.each do |name|
        Object.send(:remove_const, name) if Object.const_defined?(name) && self.path.empty?
        self.send(:remove_const, name)
      end
    end
  end
  
  # This object defines a path from which Constants can be loaded.
  class ConstantLoadPath
    # Create a new load path with the filesystem path
    def initialize(root) @root = root end
    
    # Return nil if the path does not exist, or the path to a directory
    # if the path leads to a module, or the path to a file if it leads to an object.
    def filesystem_path(path, allow_module=true)
      fs_path = [@root]
      fs_path += path[0..-2].collect {|name| const_name_to_module_name name}

      if allow_module
        result = File.join(fs_path, const_name_to_module_name(path.last))
        return result if File.directory? result # Return the module path if one exists
      end

      result = File.join(fs_path, const_name_to_file_name(path.last))

      return File.file?(result) ? result : nil
    end
    
    def const_name_to_file_name(name)
      name.to_s.underscore + '.rb'
    end

    def const_name_to_module_name(name)
      name.to_s.underscore
    end
  end
end

Object.send(:define_method, :require_or_load)     { |file_name| Dependencies.require_or_load(file_name) } unless Object.respond_to?(:require_or_load)
Object.send(:define_method, :require_dependency)  { |file_name| Dependencies.depend_on(file_name) } unless Object.respond_to?(:require_dependency)
Object.send(:define_method, :require_association) { |file_name| Dependencies.associate_with(file_name) } unless Object.respond_to?(:require_association)

class Object #:nodoc:
  class << self
    # Use const_missing to autoload associations so we don't have to
    # require_association when using single-table inheritance.
    def const_missing(class_id)
      if Object.const_defined?(:Controllers) and Object::Controllers.const_available?(class_id)
        return Object::Controllers.const_get(class_id)
      end

      begin
        require_or_load(class_id.to_s.demodulize.underscore)
        if Object.const_defined?(class_id) then return Object.const_get(class_id) else raise LoadError end
      rescue LoadError
        raise NameError, "uninitialized constant #{class_id}"
      end
    end
  end

  def load(file, *extras)
    begin super(file, *extras)
    rescue Object => exception
      exception.blame_file! file
      raise
    end
  end

  def require(file, *extras)
    begin super(file, *extras)
    rescue Object => exception
      exception.blame_file! file
      raise
    end
  end
end

# Add file-blaming to exceptions
class Exception
  def blame_file!(file)
    (@blamed_files ||= []).unshift file
  end

  def blamed_files
    @blamed_files ||= []
  end

  def describe_blame
    return nil if blamed_files.empty?
    "This error occured while loading the following files:\n   #{blamed_files.join "\n   "}"
  end
end