Ruby comes with a bunch of hook methods that allow you to manipulate classes, modules and objects on the fly.

Here is a list of the most important hook methods:

Module#included

Module#extended

Module#prepended

Class#inherited

Before to start

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included, extended and prepended modules

These hook methods are invoked whenever a module is included, extended or prepended in another module or class.

They work pretty similarly. So here we’re going to detail the included hook method

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This hook method allows you to add methods and attributes according to the class/module that includes the module where the hook method is defined.

This mechanism is actually used by the ActiveSupport::Concern module.

The implementation is pretty similar for the Module#extended and Module#prepended hook methods.

Class#inherited

This hook method is called whenever a subclass of the class that implements the hook method is created

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This hook method is pretty handy when you want to define a variable or a class_attribute at class definition for each children.

That’s exactly what the ActiveRecord::Core module does with the initialize_find_by_cache class method.

Conclusion

Hook methods can be pretty handy for class and module manipulation on the fly.

However, as they modify the module/class blueprint, it can provoke some undesired side effects like superclass method overriding, etc..

I’ll detail the BasicObject#method_missing hook method in another article.

Voilà !

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