Ruby in a Nutshell

Hooks

Ruby notifies you when a certain event happens, as shown in Table 2-2.

Table 2-2. Events and their hook methods

Event	Hook method	Of
Defining an instance method	method_added	Class
Defining a singleton method	singleton_method_added	Object
Make subclass	inherited	Superclass

These methods are called hooks. Ruby calls hook methods when the specific event occurs (at runtime). The default behavior of these methods is to do nothing. You have to override the method if you want to do something on a certain event:

class Foo
  def Foo::inherited(sub)
    printf "you made subclass of Foo, named %s\n", sub.name
  end
end
class Bar<Foo  # prints "you made subclass of Foo, named Bar"
end

There are other types of hook methods used by the mix-in feature. They are called by include and extend to do the actual mixing-in, as shown in Table 2-3. You can use these as hooks, but you have to call super when you override them.

Table 2-3. Mix-In hook methods

Event	Hook method	Of	From
Mixing in a module	append_features	Mix-in module	Module#include
Extending a object	extend_object	Mix-in module	Object#extend

Ruby 1.7 and later provide more hooks. See Chapter 6 for more information on future versions.