# HoboSupport - Module extensions

    doctest_require: '../../lib/hobo_support'
{.hidden}

## `Module#included_in_class_callbacks`

Bit involved, this one :-)

When a module is included in a class, it gets a callback on the `included` method, with the class passed as argument. However, if a module M2 is included in M1, and a class C includes M1, then M2 never gets to know about C. So, for example, M2 could not `alias_method_chain` a class method on C.

`included_in_class_callbacks` makes it easy to implement a notification from M1 to M2, so that M2 does have full access to the class. All you do is insert a call to `included_in_class_callbacks(base)` as the end of the module's `self.included` method.

(Note we're using the metaid extensions here too)

    >>
     module M2
       def self.included_in_class(klass)
         klass.metaclass_eval do
           def name_with_shouting; name_without_shouting.upcase; end
           alias_method_chain :name, :shouting
         end
       end
     end

     module M1
       def self.included(base)
         included_in_class_callbacks(base)
       end
       include M2
     end

     class C
       def self.name
         "my name is C"
       end
       include M1
     end

     C.name
    => "MY NAME IS C"


## `Module#inheriting_cattr_reader`

Declares a reader for an instance variable on the class. The attribute is looked up on the superclass if not defined on the receiving class. In other words, the superclass defines a default that subclasses can override. The declaration can also give a default, as shown here.

    >>
     class A
       inheriting_cattr_reader :nickname => "Andy"
     end

     class B < A; end

`B` has the same nickname as its superclass `A`.

    >> A.nickname
    => "Andy"
    >> B.nickname
    => "Andy"

Now we change the nickname of `B`. `A` retains it's existing nickname.

    >> class B; @nickname = "Bob"; end
    >> B.nickname
    => "Bob"
    >> A.nickname
    => "Andy"


## `Kernel#classy_module`

In Ruby we use modules to factor out features that are shared by more than one class. By including the module, a class gets the module's features, just as if they were defined inside the class. This mechanism is very simple if the shared features are all instance methods, but starts to get complicated when we want to share class-level features. For example, to create a module that adds class-methods to the including class, Ruby programmers often use the ClassMethods sub-module idiom:

    >>
     module M
       def self.included(base)
         base.send(:extend, ClassMethods)
       end

       module ClassMethods
         def foo; 123; end
       end
     end

     class C; include M; end

    >> C.foo
    => 123

It's a shame that such a basic capability isn't more declarative.

`classy_module` provides a mechanism for creating a module that, when included, will `class_eval` it's entire body (the block passed to `classy_module`). This means we can write shared class features exactly as we would write them if they were inside the class:

    >>
     MyClassyModule = classy_module do

       def self.foo; 123; end

     end

    >> MyClassyModule.class
    => Module
    >> class C2; include MyClassyModule; end
    >> C2.foo
    => 123