Extra complexity cost is insane and probably not worth it. Basically you move all your crazy stubbing and mocking out of test code and into application code. Do not do this unless you really, really need this kind of dependency injection.
That's a straw man. I see how you made it up for a joke, but it's still confusing. "Don't overengineer" is not the same as "stuff all your code into button click handlers".
I like that the "tightly coupled" service class is 3 extremely simple LOC that should probably just be in a controller anyway, which is "fixed" by creating a class with 3X as many methods and at least twice as much logic to follow, plus a spec twice as long as the original class that tests nothing other than the fact that when you call a method with a set of arguments, that method receives that set of arguments.
I have a massive, and probably not entirely proportionate, negative reaction to constructor injection. It's in my mind the ultimate example of making your code more complex in service of your tests.
An approach that I've used recently (which is similar enough to parameter-style injection, is to use `class_attribute` at the top of a class to call out dependencies and explicitly provide a default value - so that consumers of your class don't need to know what exactly a WidgetFactory is supposed to be:
class Foo
class_attribute :some_service
self.some_service = SomeService.new
end
A nice advantage of this approach is that you can provide a different implementation of some_service on an individual instance of a class, which helps restore your code to a sane state when you're done with an individual test, like so:
describe Foo
subject(:foo) { Foo.new }
before { foo.some_service = double(SomeService).as_null_object }
end
On top of that, though, I think calling out dependencies really only makes sense when that dependency involves something external to the codebase itself, or is otherwise secondary to the main purpose of the class. So stub out a repository, or a logger to your hearts content - but there's little utility in stubbing out Widget in a WidgetBuilder class IMO.
The problem with dependency injection in this manner is that your test code is using a fake class that's injected while your production code is using a real class.
This is fine if all you want to do is unit test your classes. That's beside the point though -- I would posit if you're bolting dependency injection onto your interfaces just to make your tests faster, you're not doing it right. Instead, bring that dependency injection into the forefront: to the constructor. Yes, this means to make an object, you'll have to make another object first, both in your production and test code. But now your interface acts the same way in both places. If this sounds very Java-like, well... maybe Java got it right. (There was an article on the Twitter blog in the early days about this but I can't remember where it is now...)
Of course, this is going to introduce complexity and now you have to decide whether that complexity is worth it.
Even better:
expect("my_service.rb", "line X").to be("widget_factory.create")
Or yet better, simply:
; # realize that testing the exact implementation is bullshit