Module: ActiveSupport::Callbacks::ClassMethods

Define sets of events in the object life cycle that support callbacks.

define_callbacks :validate
define_callbacks :initialize, :save, :destroy

Options

• :terminator - Determines when a before filter will halt the callback chain, preventing following before and around callbacks from being called and the event from being triggered. This should be a lambda to be executed. The current object and the result lambda of the callback will be provided to the terminator lambda.

  define_callbacks :validate, terminator: ->(target, result_lambda) { result_lambda.call == false }

  In this example, if any before validate callbacks returns false, any successive before and around callback is not executed.

  The default terminator halts the chain when a callback throws :abort.

• :skip_after_callbacks_if_terminated - Determines if after callbacks should be terminated by the :terminator option. By default after callbacks are executed no matter if callback chain was terminated or not. This option has no effect if :terminator option is set to nil.

• :scope - Indicates which methods should be executed when an object is used as a callback.

  class Audit
    def before(caller)
      puts 'Audit: before is called'
    end
  
    def before_save(caller)
      puts 'Audit: before_save is called'
    end
  end
  
  class Account
    include ActiveSupport::Callbacks
  
    define_callbacks :save
    set_callback :save, :before, Audit.new
  
    def save
      run_callbacks :save do
        puts 'save in main'
      end
    end
  end

  In the above case whenever you save an account the method Audit#before will be called. On the other hand

  define_callbacks :save, scope: [:kind, :name]

  would trigger Audit#before_save instead. That’s constructed by calling #{kind}_#{name} on the given instance. In this case “kind” is “before” and “name” is “save”. In this context :kind and :name have special meanings: :kind refers to the kind of callback (before/after/around) and :name refers to the method on which callbacks are being defined.

  A declaration like

  define_callbacks :save, scope: [:name]

  would call Audit#save.

Notes

names passed to define_callbacks must not end with !, ? or =.

Calling define_callbacks multiple times with the same names will overwrite previous callbacks registered with #set_callback.

Remove all set callbacks for the given event.

Install a callback for the given event.

set_callback :save, :before, :before_method
set_callback :save, :after,  :after_method, if: :condition
set_callback :save, :around, ->(r, block) { stuff; result = block.call; stuff }

The second argument indicates whether the callback is to be run :before, :after, or :around the event. If omitted, :before is assumed. This means the first example above can also be written as:

set_callback :save, :before_method

The callback can be specified as a symbol naming an instance method; as a proc, lambda, or block; or as an object that responds to a certain method determined by the :scope argument to #define_callbacks.

If a proc, lambda, or block is given, its body is evaluated in the context of the current object. It can also optionally accept the current object as an argument.

Before and around callbacks are called in the order that they are set; after callbacks are called in the reverse order.

Around callbacks can access the return value from the event, if it wasn’t halted, from the yield call.

Options

• :if - A symbol or an array of symbols, each naming an instance method or a proc; the callback will be called only when they all return a true value.

  If a proc is given, its body is evaluated in the context of the current object. It can also optionally accept the current object as an argument.

• :unless - A symbol or an array of symbols, each naming an instance method or a proc; the callback will be called only when they all return a false value.

  If a proc is given, its body is evaluated in the context of the current object. It can also optionally accept the current object as an argument.

• :prepend - If true, the callback will be prepended to the existing chain rather than appended.

Skip a previously set callback. Like #set_callback, :if or :unless options may be passed in order to control when the callback is skipped.

Note: this example uses PersonRecord and #saving_message, which you can see defined here

class Writer < PersonRecord
  attr_accessor :age
  skip_callback :save, :before, :saving_message, if: -> { age > 18 }
end

When if option returns true, callback is skipped.

writer = Writer.new
writer.age = 20
writer.save

Output:

- save
saved

When if option returns false, callback is NOT skipped.

young_writer = Writer.new
young_writer.age = 17
young_writer.save

Output:

saving...
- save
saved

An ArgumentError will be raised if the callback has not already been set (unless the :raise option is set to false).