Mixin module with class variables and class methods

[John Lane]

Hello,

I am trying to learn ruby and am experimenting with a Mixin.

I have the following test module: http://pastie.org/816001

On line 17, the class variable @@name gives an error:

uninitialized class variable @@name in TestMixin::ClassMethods

I understand it is because that's in a module within the TestMixin
module and that the @@name class variable is created in the including
class which is in a separate scope.

Can someone advise how this should be written so that a class method in
a mixin can access a class variable created by the mixin ?

Instance methods work fine.

Thanks in advance.

 

[Brian Candler]

Can someone advise how this should be written so that a class method in
a mixin can access a class variable created by the mixin ?

Class variables are a pain, for exactly this sort of reason. Instance
variables of a class are much easier to handle and understand.

module TestMixin

def self.included(base)
p "TextMixin included in #{base}"
base.class_eval {
@name = "foobar"
}
base.extend ClassMethods
end

module ClassMethods

def name
@name
end

def name=(x)
@name=(x)
end

def class_method(str)
$stderr.puts "this is a class method in TestMixin #{str}"
$stderr.puts "the class variable is #{name}"
end

end

def name
self.class.name
end

def name=(x)
self.class.name=(x)
end

def instance_method
$stderr.puts "this is an instance method in TextMixin"
$stderr.puts "the class variable is #{name}"
end

end

class Foo

include TestMixin
class_method "foo"
def initialize
instance_method
end
end

Foo.new

 

[John Lane]

Class variables are a pain, for exactly this sort of reason.

Yes, I'm getting that feeling! My example test was just a simplified
model to understand how it worked and, in the case of the example, I can
see instance variables provide a solution.

What I'm trying to do is end up with a module mixed into a number of
classes that will each call a mixed-in class method to add values to a
class variable that will be a hash. Other instance methods in the mixin
will then make use of the information in the class variable hash.

So I think I really need a class variable.

 

[Marnen Laibow-Koser]

Yes, I'm getting that feeling! My example test was just a simplified
model to understand how it worked and, in the case of the example, I can
see instance variables provide a solution.

What I'm trying to do is end up with a module mixed into a number of
classes that will each call a mixed-in class method to add values to a
class variable that will be a hash. Other instance methods in the mixin
will then make use of the information in the class variable hash.

So I think I really need a class variable.

But you're probably wrong. What you probably want is a class instance
variable -- a weird concept, to be sure, until you recall that in Ruby
classes are instances of class Class. So:

class MyClass
@@class_var = 'foo'
@class_ivar = 'bar'

def self.class_method
puts @@class_var
puts @class_ivar
end
end

puts MyClass.class_method # prints 'foo' and 'bar'

In other words, class @instance variables act just like @@class
variables but without the problems.

Best,
-- 
Marnen Laibow-Koser
http://www.marnen.org
(e-mail address removed)

 

[John Lane]

But you're probably wrong. What you probably want is a class instance
variable -- a weird concept, to be sure, until you recall that in Ruby
classes are instances of class Class.
snip

In other words, class @instance variables act just like @@class
variables but without the problems.

Ok so I get that point. I've now got working code, well almost. I mix
the module into multiple classes and each gets their own instance of the
variable, which is not what I need.

Here's the code: http://pastie.org/816654

There are three classes, One, Two and Three. Classes One and Two both
mix in the module. Class Three is a subclass of class Two and therefore
does not directly mix in the module.

The output i get demonstrates that each class gets its own instance of
the variable that I want to be shared across all three. Here is the
output:

TestMixin included in One
b added for class One : a b
c added for class One : a b c
TestMixin included in Two
d added for class Two : a d
e added for class Two : a d e
f added for class Three : f
g added for class Three : f g
the list for class One is a b c
the list for class Two is a d e
the list for class Three is f g

What I want to get is this:

TestMixin included in One
b added for class One : a b
c added for class One : a b c
TestMixin included in Two
d added for class Two : a b c d
e added for class Two : a b c d e
f added for class Three : a b c d e f
g added for class Three : a b c d e f g
the list for class One is a b c d e f g
the list for class Two is a b e d e f g
the list for class Three is a b c d e f g

 

[Brian Candler]

Ok so I get that point. I've now got working code, well almost. I mix
the module into multiple classes and each gets their own instance of the
variable, which is not what I need.

If everything which mixes in this module shares the same state, then you
can use a module instance variable.

If you want each subtree of related classes to share state, then one
option is to use a class instance variable and use the class hierarchy
to find where the value is held. Along the lines of:

def name
defined?(@name) ? @name : super
end

Another option is for each class to have its own @name instance
variable, but for them all to refer to the same object (a Hash in your
case); in self.included you initialize @name to the same as @name in the
parent class, if it exists.

But in your case class One and Two are unrelated but you want them to
share state anyway, so I'd go with a module instance variable. This
passes your test:

module TestMixin

@list = "a"

def self.list
@list
end

def self.included(base)
puts "TestMixin included in #{base}"

base.extend ClassMethods
end

module ClassMethods

def list
TestMixin.list
end

def class_method(str)
self.list << " #{str}"
puts "#{str} added for class #{self} : #{list}"
end

end

def list
TestMixin.list
end

def instance_method
puts "the list for class #{self.class} is #{list}"
end

end

class One

include TestMixin

class_method 'b'
class_method 'c'

end

class Two

include TestMixin

class_method 'd'
class_method 'e'

end

class Three < Two

class_method 'f'
class_method 'g'

end

one = One.new

two = Two.new

three = Three.new

one.instance_method

two.instance_method

three.instance_method

 

[John Lane]

Thank you Brian, that sure does what I need. I will now go away and try
it in a real application. Thank you all - there's some useful techniques
highlighted in these answers.