Prettier solution to circular dependency

[Helder Ribeiro]

This is an aesthetic problem: I have a method in a module, and I want
to include it in three different classes (they're all in the same
file). This method refers itself to one of those classes. So i have to
include the module into the classes, but for the method to be parsed,
one of those classes needs to be already defined.

module ElementFactory
def element_factory(what)
(...)
Element.new
end
end

class Element
include ElementFactory
(...)
end

class Document
include ElementFactory
(...)
end

class ElementCollections
include ElementFactory
(...)
end


So for this class Element there's a circular dependency and, as ruby
does single-pass parsing, this can't work. What I've done so far is to
replace the explicit reference to Element in the module instance
method by a call to Object.const_getElement) so that when it's
called, Element will already be defined. But this looks a bit ugly.
isn't there a better way to solve this?

The circular dependency hints at not having the module at all and just
putting the instance method element_factory() into Element itself but
there are those other two classes that include it, so I can't do that.

Thanks a lot for any ideas!


Cheers,

Helder


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[Joel VanderWerf]

This is an aesthetic problem: I have a method in a module, and I want
to include it in three different classes (they're all in the same
file). This method refers itself to one of those classes. So i have to
include the module into the classes, but for the method to be parsed,
one of those classes needs to be already defined.

You can define the Element class at the top, and then reopen it later to
include the module and define your methods. Just make sure that if
Element is a subclass of something, the _first_ "class Element" must
have the " < SuperClass". But in this case all you have to do is add
these lines:

class Element
end

 

[Caleb Clausen]

Your little snippet worked just fine for me.... ruby lets you use a
class before it has been defined. The constant lookup is done at
runtime.

 

[Daniel DeLorme]

This is an aesthetic problem: I have a method in a module, and I want
to include it in three different classes (they're all in the same
file). This method refers itself to one of those classes. So i have to
include the module into the classes, but for the method to be parsed,
one of those classes needs to be already defined.

You're still thinking in terms of static typing. In this case you
*don't* have a circular dependency. You can load ElementFactory first
and it doesn't matter if Element is not yet defined, as long as you
don't invoke the method 'element_factory'

Daniel

 

[Bas van Gils]

Having said that I'd still consider the cyclic dependency (on the
logical level) bad design. We do not know more about those classes but
my impression is that this can probably improved upon.

I'm new to the mailinglist .. so I'm not sure if this has been mentioned or
not. There's a cool site which discusses the implementation of (the gang of
four) patterns in Ruby. In this case you may want to check out

http://www.rubypatterns.org/doku.php/gang_of_four_patterns:factory_method

Hope this helps

Bas

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