Ruby before filter

before_filter.rb

module MethodInterception
  def before_filter(*meths, &blk)
    return @wrap_next_method = true if meths.empty?
    meths.delete_if { |meth| wrap(meth, &blk) if method_defined?(meth) }
    @intercepted_methods += meths
  end

  private

  def wrap(meth, &blk)
    old_meth = instance_method(meth)
    define_method(meth) do |*args, &block|
      # before filter execution
      blk.call(*args) if blk
      # original method call
      old_meth.bind(self).call(*args, &block)
    end
  end

  def method_added(meth)
    return super unless @intercepted_methods.include?(meth) || @wrap_next_method
    return super if @recursing == meth

    @recursing = meth # protect against infinite recursion
    wrap(meth)
    @recursing = nil
    @wrap_next_method = false

    super
  end

  def self.extended(klass)
    klass.instance_variable_set(:@intercepted_methods, [])
    klass.instance_variable_set(:@recursing, false)
    klass.instance_variable_set(:@wrap_next_method, false)
  end
end

example.rb

class HelloWord
  extend MethodInterception
  
  before_filter(:say_hello) do |*args|
    puts "Before the message '#{args[0]}'"
  end
  
  def say(message)
    puts message
  end
end

# HelloWord.new.say('hello:')
# => Before the message 'hello!'
# => hello!