How do I run code before and after a method in a sub class?

Question

My first thoughts are some thing like this:

class AbstractBuilder
  attr_reader :time_taken

  def build_with_timer
    started_at = Time.now
    build
    @time_taken = Time.now - started_at
  end

  def build
    raise 'Implement this method in a subclass' 
  end
end

class MyBuilder < AbstractBuilder
  def build
    sleep(5)
  end
end

builder = MyBuilder.new.build_with_timer
puts builder.time_taken

I would suspect there is a better way which offers better flexibility, for example ideally I'd like to call 'build' on an instance of MyBuilder instead of 'build_with_timer' and always have the execution time recorded.

I did consider using alias_method from initialize or even using a module mixin instead of class inheritance which would override the build method calling super in the middle (not sure if that would work). Before I go down the rabbit hole I thought I'd see if there is an established practice.

Answer 1

Sounds like you're looking for hooks into object lifecycle events. You'll have to build this into your base object and provide a little DSL -- I'm thinking you're after something like ActiveRecord Callbacks. Here's how we might modify your example to allow something like that:

class AbstractBuilder
  attr_reader :time_taken

  def construct! # i.e., build, and also call your hooks
    @@prebuild.each { |sym| self.send(sym) }
    build
    @@postbuild.each { |sym| self.send(sym) }
  end  

  def construct_with_timer
    started_at = Time.now
    construct!
    @time_taken = Time.now - started_at

    puts "!!! Build time: #@time_taken"
  end

  class << self
    def before_build(fn); @@prebuild ||= []; @@prebuild << fn; end
    def after_build(fn);  @@postbuild ||= []; @@postbuild << fn; end
  end
end

class MyBuilder < AbstractBuilder
  before_build :preprocess
  after_build  :postprocess

  def build; puts "BUILDING"; sleep(3); end
  def preprocess;  puts "Preparing to build..."; end
  def postprocess; puts "Done building. Thank you for waiting."; end
end

builder = MyBuilder.new
builder.construct_with_timer

# => Preparing to build...
# => BUILDING
# => Done building. Thank you for waiting.
# => !!! Build time: 3.000119

Answer 2

I'd play with alias_method:

module Timeable
  def time_methods *meths
    meths.each do |meth|
      alias_method "old_#{meth}", meth

      define_method meth do |*args|
        started_at = Time.now
        res = send "old_#{meth}", *args
        puts "Execution took %f seconds" % (Time.now - started_at)
        res
      end
    end
  end

end

class Foo
  def bar str
    puts str
  end
end

Foo.extend Timeable
Foo.time_methods :bar
Foo.new.bar('asd')
#=>asd
#=>Execution took 0.000050 seconds

Answer 3

I had a stab at a version to achieve what you want. This version doesn't require the subclass to have any extra code either.

class AbstractBuilder

  @@disable_override = false

  def before_method
    puts "before"
  end

  def after_method
    puts "after"
  end

  def self.method_added name
    unless @@disable_override
      if name == :build
        @@disable_override = true # to stop the new build method 
        self.send :alias_method, :sub_build, :build
        self.send :remove_method, :build
        self.send :define_method, :build do
          before_method
          sub_build
          after_method
        end
        @@disable_override = false
      else
        puts "defining other method #{name}"
      end
    end
  end

end

class MyBuilder < AbstractBuilder

  def build
    puts "starting build"
    sleep(5)
    puts "built."
  end

  def unnaffected_method
    # this method won't get redefined
  end

end

b = MyBuilder.new
b.build

Outputs

defining other method unnaffected_method
before
starting build
built.
after

Answer 4

This is a textbook-definition use case for Aspect-Oriented Programming. It generally offers a cleaner separation of concerns. In this arena, Ruby offers Aquarium and AspectR. However, you may not want to add another dependency to your project. As such, you might still consider using one of the other approaches.