After looking closer at the dependency injection proposal by Jamis Buck and talking to Dave Thomas about his hesitations with full-blown DI, I had all these thoughts spinning and flying around my head. The first representation of these thoughts are that requires should be made part of the domain language, so from the next release it'll be possible to scrap almost all your explicit require statements.
I've added three new words: model, service, and observer. The first two will just do a regular require, but will then also enable you to introspect the controller for its dependencies. The latter will also instantiate the observer, so it's active:
class MsgController < AbstractApplicationController
model :post, :comment, :attachment
service :notification_service
observer :comment_observer
end
MsgController.dependencies_on(:model) # => [ :post, :comment, :attachment ]
MsgController.dependencies_on(:service) # => [ :notification_service ]
MsgController.dependencies_on(:observer) # => [ :comment_observer ]
This it not only shorter and more expressive, it also opens up to a world of interesting automation and scaffolding when you can programatically inquire about the dependencies of a controller. A simple example would be that fixtures for functional testing the controller could automatically be managed and setup.
I've also went ahead and improved associations, so they now too automatically will attempt to require the class they need. So instead of repeating yourself with:
require 'comment'
class Post < ActiveRecord::Base
has_many :comments
end
You can now leave out the explicit require and just rely on Active Record to follow your association specification and do the require:
class Post < ActiveRecord::Base
has_many :comments
end
This should also spell an end to all those annoying errors you get when you forgot to require a class before starting to use it. Just like I was thrilled to be rid of the semi-colon at the end of each line when I ditched PHP for Ruby, this will further swift the burden of repetition over to the framework.
Do the Mocking
But this is not all! In companionship with Dave Thomas, I've relieved the primary pain that got me interested in dependency injection in the first place: How to inject mocks in hard to reach places? Specifically, how could I replace my FTPServer, PaymentGateway, and Notification services in Basecamp with mocks that didn't rely on external machines and that I could inquire about how they were called?
In the next version of Rails, that'll be incredibly easy. There's a new directory called test/mocks where you can drop in any class that you'd like to see replaced with a mock under the test environment. So if I want to mock out the PaymentGateway in Basecamp, I'll create the following file in test/mocks/payment_gateway.rb:
require 'models/payment_gateway'
class PaymentGateway
def commit
SuccessfulSubmission.new
end
end
I require the original payment gateway first, then I re-open the class (Ruby has open classes, you can reopen anything — even core classes!) and mock out the behavior that I want to change. In this case, it's the commit method that would otherwise start to talk to the Authorize.net server.
You could also forgo requiring the original class and just use a straight mock, but it's often much nicer to rely on as much "real" functionality as possible. Just mock out the few methods that would otherwise make testing hard to do.
Anyway, these latest breakthroughs does cast some doubt in my mind on whether full-blown dependency injection is needed in Rails. All of the additions above where created with an absolute minimal amount of code. They give zeroconf dependency injection for mocks and the conceptual complexity of Rails doesn't go up one bit.
I'll be sure to revisit Jamis' proposal and look for other advantages of integrating with Needle, though. These improvements, however, have cured my immediate needs for injection. I got my fix, so to speak.
UPDATE: If you want to try out these new features, the beta gems have them. Upgrade with gem update -s http://gems.rubyonrails.org.
Challenge by Vincent Foley on November 19, 16:19
I think I speak for every documenter when I say "GNYAH! Curse you David, curse you!" :)
Seriously, this is hot stuff, but I just hope that you still remember what you said at RubyConf about having a good trade-off between an ivory tower and quick and dirty PHP-like framework: we don't want the new comers to learn about Ruby, Rails, databases, Dependency Injection, the Danish Monarch System, the Canadian maple syrup industry and radasaq in order to be able to learn Rails.
I think I'm gonna put off writing my new application and keep thinking about its design (which is what I usually never do, so this is a good thing ;)) until the new Rails comes out. In the mean time, I will continue my documenting of Rails.
That's one of my prime concerns about including real DI in Rails. It would need to offer significant and substantial improvements over solutions like the one described above before it's worth the conceptual complexity.
I think these new changes will make it easier for the new comers to adopt Rails. Especially that requires will be handled almost automatically now. And that observers have been made easier and more obvious in their usage.
I'm excited too! And this road has a lot of additional simplifications down it that I can't wait to get in.
Oh, and important to mention too is that all this stuff was implemented this morning. Gotta love Less Software ;)
Less Software usually refers to almost but not quite More Software.
Challenge by Vinu on November 20, 2:22
Hi David,
I searched all the archives of your site and searched the Rails site too, and I came upon references of "Less software" but I coudn't really find any information on it. I take it that it's basically a way to develop software by doing the simplest thing that works, (analagous to XP principles). Is there anything else to it?
Anyways, I just recently discovered Rails and Ruby and am inspired by the productivity they bring. I am slowly trying to learn Rails (and Ruby as well!) and was just interested in your methodology of software development.
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