method triggers: instead, before, after

dseverin () — Sat, 03 Mar 2007 10:50:20 GMT

Redefine method calls, in manner similar to SQL triggers and PostgreSQL rules - i.e. execute code block instead, before or after call to original method. rules can be appended and removed.



class Object



  def self.__rules__

  # container for defined rules, each item is:

  #   [class, event_name, method_name, alias_for_original_method, caller, comment]

    @@rules ||= []

  end



  def self.__create_rule_instead( method, comment = '', &block) # creates and returns new rule

    b_id = "%04x" % block.object_id

    old_method_name = :"__previous_#{method}_#{b_id}"

    alias_method old_method_name, method

    define_method method, &block

    __rules__ << rule = [self, "INSTEAD", method, old_method_name,caller[0], comment ]

    rule

  end



  def self.__create_rule_before( method, comment = '', &block)

    args = instance_method(method).arity == 0 ? '' : '(*args)'

    b_id = "%04x" % block.object_id

    old_method_name = :"__previous_#{method}_#{b_id}"

    alias_method old_method_name, method

    define_method :"__before_#{method}_#{b_id}", &block

    class_eval <<-EOT

      def #{method}#{args}

        __before_#{method}_#{b_id}#{args}

        __previous_#{method}_#{b_id}#{args}

      end

    EOT

    __rules__ << rule = [self, "BEFORE", method, old_method_name, caller[0], comment]

    rule

  end



  def self.__create_rule_after( method, comment = '', &block)

    args = instance_method(method).arity == 0 ? '' : '(*args)'

    b_id = "%04x" % block.object_id

    old_method_name = :"__previous_#{method}_#{b_id}"

    alias_method old_method_name, method

    define_method :"__after_#{method}_#{b_id}", &block

    class_eval <<-EOT

      def #{method}#{args}

        res = __previous_#{method}_#{b_id}#{args}

        __after_#{method}_#{b_id}#{args}

        res

      end

    EOT

    __rules__ << rule = [self, "AFTER", method, old_method_name,caller[0], comment ]

    rule

  end



  def self.__remove_rule( rule ) # has some bugs when rules on subclasses are defined :(

    idx = __rules__.index(rule)

    if idx

      # look for next rule for the same method

      idx += 1

      while idx < __rules__.size

        break if  __rules__[idx][2] == rule[2] && __rules__[idx][0] == rule[0]

        idx+=1

      end

      if idx < __rules__.size

        next_rule = __rules__[idx]

        next_rule[0].send :remove_method, next_rule[3]

        next_rule[0].send :alias_method, next_rule[3], rule[3]

      else

        # that was last

        rule[0].send :remove_method, rule[2]

        rule[0].send :alias_method, rule[2], rule[3]

      end

      __rules__.delete(rule)

    end

  end



end

Example:



class Model

  def save

    puts "save"

  end

  def reload(flag)

    "reloaded"

  end

end



r1 = Model.__create_rule_instead(:reload) {|flag| flag ? "FRESH" : "STALE" }



obj = Model.new



puts "RELOAD:"+obj.reload(true)

puts "RELOAD:"+obj.reload(false)



Object.__remove_rule(r1)

puts "RELOAD:"+obj.reload(false)



Model.__create_rule_before(:save) { puts "BEFORE SAVE" }

r2 = Model.__create_rule_before(:save) { puts "YET BEFORE SAVE" }

Model.__create_rule_after(:save)  { puts "AFTER SAVE" }

r3 = Model.__create_rule_after(:save)  { puts "YET AFTER SAVE" }

obj.save

Object.__remove_rule(r2)

puts "----------"

obj.save

Object.__remove_rule(r3)

puts "----------"

obj.save

produces:



RELOAD:FRESH

RELOAD:STALE

RELOAD:reloaded

YET BEFORE SAVE

BEFORE SAVE

save

AFTER SAVE

YET AFTER SAVE

----------

BEFORE SAVE

save

AFTER SAVE

YET AFTER SAVE

----------

BEFORE SAVE

save

AFTER SAVE

DZone Snippets: metaprogramming code

method triggers: instead, before, after