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 callbacks from being called and the event from being triggered. This should be a lambda to be executed. The current object and the return result of the callback will be called with the lambda.

  define_callbacks :validate, terminator: ->(target, result) { result == false }
  

  In this example, if any before validate callbacks returns false, other callbacks are not executed. Defaults to false, meaning no value halts the chain.

• :skip_after_callbacks_if_terminated - Determines if after callbacks should be terminated by the :terminator option. By default after callbacks executed no matter if callback chain was terminated or not. Option makes sense only when :terminator option is specified.

• :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.

# File activesupport/lib/active_support/callbacks.rb, line 761
def define_callbacks(*names)
  options = names.extract_options!
  if options.key?(:terminator) && String === options[:terminator]
    ActiveSupport::Deprecation.warn "String based terminators are deprecated, please use a lambda"
    value = options[:terminator]
    line = class_eval "lambda { |result| #{value} }", __FILE__, __LINE__
    options[:terminator] = lambda { |target, result| target.instance_exec(result, &line) }
  end

  names.each do |name|
    class_attribute "_#{name}_callbacks", instance_writer: false
    set_callbacks name, CallbackChain.new(name, options)
  end
end

Remove all set callbacks for the given event.

# File activesupport/lib/active_support/callbacks.rb, line 681
def reset_callbacks(name)
  callbacks = get_callbacks name

  ActiveSupport::DescendantsTracker.descendants(self).each do |target|
    chain = target.get_callbacks(name).dup
    callbacks.each { |c| chain.delete(c) }
    target.set_callbacks name, chain
  end

  self.set_callbacks name, callbacks.dup.clear
end

Install a callback for the given event.

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

The second arguments 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_meth

The callback can be specified as a symbol naming an instance method; as a proc, lambda, or block; as a string to be instance evaluated; 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 naming an instance method or a proc; the callback will be called only when it returns a true value.

• :unless - A symbol naming an instance method or a proc; the callback will be called only when it returns a false value.

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

# File activesupport/lib/active_support/callbacks.rb, line 642
def set_callback(name, *filter_list, &block)
  type, filters, options = normalize_callback_params(filter_list, block)
  self_chain = get_callbacks name
  mapped = filters.map do |filter|
    Callback.build(self_chain, filter, type, options)
  end

  __update_callbacks(name) do |target, chain|
    options[:prepend] ? chain.prepend(*mapped) : chain.append(*mapped)
    target.set_callbacks name, chain
  end
end

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

class Writer < Person
   skip_callback :validate, :before, :check_membership, if: -> { self.age > 18 }
end

# File activesupport/lib/active_support/callbacks.rb, line 662
def skip_callback(name, *filter_list, &block)
  type, filters, options = normalize_callback_params(filter_list, block)

  __update_callbacks(name) do |target, chain|
    filters.each do |filter|
      filter = chain.find {|c| c.matches?(type, filter) }

      if filter && options.any?
        new_filter = filter.merge(chain, options)
        chain.insert(chain.index(filter), new_filter)
      end

      chain.delete(filter)
    end
    target.set_callbacks name, chain
  end
end