after_do 0.3.0 released

I just released version 0.3.0 of my little aspect oriented programming/adding callbacks to methods library after_do! You can find an introduction here.

So what is in 0.3.0? Basically 2 things:

  • after_do now works properly with modules, meaning you can attach callbacks to the methods of a module and objects of classes including those methods will call them!
  • Fixed bugs around inheritance where it could happen that a block might get called too often or not at all

At the same time this release was able to delete code and remove complexity while improving functionality. How is that possible? Well thanks to block scoping it is!

One problem I always had is to figure out callbacks of which class to execute in combination with inheritance. You know – self is always the current object and callbacks might be defined in super classes. I wanted to have a way to know which class the currently called method is defined in, not what the class of the current object is. Luckily there is one point where I know that – the moment when I add the callbacks (since they are added on that exact class/module). So we just need to save it:

callback_klazz = self
define_method method do |*args|
  callback_klazz.send(:_after_do_execute_callbacks, :before, method, self, *args)
  return_value = send(alias_name, *args)
  callback_klazz.send(:_after_do_execute_callbacks, :after, method, self, *args)
  return_value
end

Simple yet powerful.

Enjoy the 0.3 release of after_do 🙂

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Introducing after_do: after/before method callbacks in Ruby

I want to introduce you to a little gem I built and use in some of my projects: after_do. What it does is pretty simple: you can attach callback blocks before/after methods are executed. And it looks like this:


MyClass.after :some_method do whatever_you_want end
# or/and
MyClass.before :some_method do pure_magic end

As I don’t fancy monkeypatching you will have to extend classes that you want to use after_do on with the AfterDo module. E.g. for the code above to work:

MyClass.extend AfterDo

after_do has no external runtime dependencies and the code is around 160 lines (blank lines and documentation included) with lots of small methods. So simplecov reports there are a little above 70 relevant lines code (it ignores blank lines, docs etc.).

It works and is tested with current releases of all major ruby interpreters, e.g. MRI (1.9.3 and 2.0), JRuby and rubinius.

The github repo has some more documentation about use cases etc. – I won’t go into all of it here.

Why would I want to do that?

For me this catches the essence of Aspect Oriented Programming – doing something before or after a method is executed to fight cross-cutting concerns. What are these cross-cutting concerns? Glad you asked! They are aspects of your application that you can’t confine to a single class but are rather spread over multiple classes.

One of the most common examples is logging: Logging is done in many classes and many methods. As a result the real purpose of a method is cluttered with logging statements and it’s hard to get an overview of all the logging statements in your application at once.
Another example would be statistics: You might want to keep track of successful purchases, failed logins etc… the code to do so goes in the method that handle these cases but really doesn’t contribute much to its actual purpose. And I’ve seen people use global variables to make statistics gathering available everywhere. Yikes.

With aspect oriented programming you could have all of those in a single file and as a result not clutter the original methods at all.

Access to parameters and the object

For a lot of purposes it’s nice to have access to the parameters of a method call and the object itself – after_do gives you just that:

MyClass.after :two_arg_method do |arg1, arg2, obj|
  something(arg1, arg2, obj)
end

With this you can log events like a succesful purchase:

# Assuming CheckoutProcess#complete gets user as an argument
CheckoutProcess.after :complete do |user, checkout|
  @logger.log "#{user.name} checked out #{checkout.id}"
end

Removing repetition

Another use case is removing repetition from within a class. E.g. if you want to call the save method of an object after several different methods you can do the following:

class CoolClass
  extend AfterDo
  # lots of methods
  after :m1, :m2, :m3 do |*args, object| object.save end
end

This might remind you a bit of before_action/filter in Rails controllers.

How does it work?

When you attach a callback to a method with after_do what it basically does is it creates a copy of that method and then redefines the method to basically look like this (pseudo code):


execute_before_callbacks
return_value = original_method
execute_after_callbacks
return_value

Why build something like this?

I was working on a side project after reading Objects on Rails and wanted to try to separate the persistence concern from the actual Object domain logic. For the fun of it and remove repetition along the way. My initial research didn’t come up with a good maintained tiny library to serve my purpose. So I wrote one myself, named it after_do et voila there is the solution to my initial problem:

persistor  = FilePersistor.new
Activity.extend AfterDo
Activity.after :start, :pause, :finish, :resurrect,
               :do_today, :do_another_day do |activity|
  persistor.save activity
end

Nice, isn’t it?

Is this a good idea?

Always depends on what you are doing with it. As many things out there it has its use cases but can easily be misused.

Advantages

  • Get cross cutting concerns packed together in one file – don’t have them scattered all over your code base obfuscating what the real responsibility of that class is
  • Don’t repeat yourself, define what is happening when in one file
  • I feel like it helps the Single Responsibility principle, as it enables classes to focus on what their main responsibility is and not deal with other stuff

Drawbacks

  • You lose clarity. With callbacks after a method it is not immediately visible what happens when a method is called as some behavior might be defined elsewhere.
  • You could use this to modify the behavior of classes everywhere. Don’t. Use it for what it is meant to be used for – a concern that is not the primary concern of the class you are adding the callback to but that class is still involved with.

A use case I feel this is particularly made for is redrawing. That’s what we use it for over at shoes4. E.g. we have multiple objects and different actions on these objects may trigger a redraw (such changing the position of a circle). This concern could be littered and repeated all over the code base. Or nicely packed into one file where you don’t repeat yourself for similar redrawing scenarios and you see all the redraws at one glance. Furthermore it makes it easier to do things like “Just do one redraw every 1/30s” (not yet implemented though).

Ultimately, you be the judge. I’d be happy if you were to go ahead and give after_do a try, say what you think about it, report bugs and feature requests.

Shoes and ruby gems

As I just stumbled upon this and I need this: So how do you use ruby gems with shoes? Shoes is kind of a ruby interpreter on it’s own, so you got no access to your normal ruby gems there. It’s actually pretty easy as described in an old saved blogpost by why.

What you need to do is something like this:

Shoes.setup do
 gem 'twitter'
 gem 'oauth'
 gem 'launchy'
end

require 'launchy'

In the Shoes.setup block you set up all the gems you need. Then you can require them in this file, or in any other file which is required by this file. As you can see, I only require launchy here, the rest is required in other files of my project.

What happens when you next launch the application is the following: Shoes goes on to install the gems:


So and there you got it, ready to play with shoes and some ruby gems!

edit: Of course this only applies to red shoes, as green shoes is a gem itself you may use other gems with it exactly like you are used to.

Teach Me How to Make A Ruby Gem (by @steveklabnik)

Just a quick solution so I don’t (and everybody else as well) doesn’t lose those valuable links just given to me by Steve Klabnik. So how do you make ruby gem? There are so many of them and now I know why, it’s insanely easy to create them and then make them available to everyone!

Just go and watch Steve’s video (slides) or read his blogpost. He also introduces some best practices for creating gems. It doesn’t take a lot of time, but you will gain a great amount of knowledge, I assure you that!

So long, I’m off writing code for mendicant university 🙂