How does one - without inheritance - override a class method and call the original from within the new method?

Question

I found one source which successfully overrode Time.strftime like this:

class Time
  alias :old_strftime :strftime
  def strftime
    #do something
    old_strftime
  end
end

The trouble is, strftime is an instance method. I need to override Time.now - a class method - in such away that any caller gets my new method, while the new method still calls the original .now method. I've looked at alias_method and have met with no success.

Answer 1

This is kinda hard to get your head around sometimes, but you need to open the "eigenclass" which is the singleton associated with a specific class object. the syntax for this is class << self do...end.

class Time
  alias :old_strftime :strftime

  def strftime
    puts "got here"
    old_strftime
  end
end

class Time
  class << self
    alias :old_now :now
    def now
      puts "got here too"
      old_now
    end
  end
end

t = Time.now
puts t.strftime

Answer 2

Class methods are just methods. I highly recommend against this, but you have two equivalent choices:

class Time
  class << self
    alias_method :old_time_now, :now

    def now
      my_now = old_time_now
      # new code
      my_now
    end
  end
end

class << Time
  alias_method :old_time_now, :now

  def now
    my_now = old_time_now
    # new code
    my_now
  end
end

Answer 3

If the you need to override it for testing purposes (the reason I normally want to override Time.now), Ruby mocking/stubbing frameworks will do this for you easily. For instance, with RSpec (which uses flexmock):

Time.stub!(:now).and_return(Time.mktime(1970,1,1))

By the way, I highly recommend avoiding the need to stub out Time.now by giving your classes an overrideable clock:

class Foo
  def initialize(clock=Time)
    @clock = clock
  end

  def do_something
    time = @clock.now
    # ...
  end
end

Answer 4

I've been trying to figure out how to override an instance method using modules.

module Mo
  def self.included(base)
    base.instance_eval do
      alias :old_time_now :now
      def now
        my_now = old_time_now
        puts 'overrided now'
        # new code
        my_now
      end
    end
  end
end
Time.send(:include, Mo) unless Time.include?(Mo)

> Time.now
overrided now
=> Mon Aug 02 23:12:31 -0500 2010