aboutsummaryrefslogtreecommitdiffstats
path: root/activesupport/lib/active_support/callbacks.rb
blob: ddfa91a34292e1a9a66f4efb991099ab450b2556 (plain) (blame)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
require "active_support/concern"
require "active_support/descendants_tracker"
require "active_support/core_ext/array/extract_options"
require "active_support/core_ext/class/attribute"
require "active_support/core_ext/kernel/reporting"
require "active_support/core_ext/kernel/singleton_class"
require "active_support/core_ext/string/filters"
require "active_support/deprecation"
require "thread"

module ActiveSupport
  # Callbacks are code hooks that are run at key points in an object's life cycle.
  # The typical use case is to have a base class define a set of callbacks
  # relevant to the other functionality it supplies, so that subclasses can
  # install callbacks that enhance or modify the base functionality without
  # needing to override or redefine methods of the base class.
  #
  # Mixing in this module allows you to define the events in the object's
  # life cycle that will support callbacks (via +ClassMethods.define_callbacks+),
  # set the instance methods, procs, or callback objects to be called (via
  # +ClassMethods.set_callback+), and run the installed callbacks at the
  # appropriate times (via +run_callbacks+).
  #
  # Three kinds of callbacks are supported: before callbacks, run before a
  # certain event; after callbacks, run after the event; and around callbacks,
  # blocks that surround the event, triggering it when they yield. Callback code
  # can be contained in instance methods, procs or lambdas, or callback objects
  # that respond to certain predetermined methods. See +ClassMethods.set_callback+
  # for details.
  #
  #   class Record
  #     include ActiveSupport::Callbacks
  #     define_callbacks :save
  #
  #     def save
  #       run_callbacks :save do
  #         puts "- save"
  #       end
  #     end
  #   end
  #
  #   class PersonRecord < Record
  #     set_callback :save, :before, :saving_message
  #     def saving_message
  #       puts "saving..."
  #     end
  #
  #     set_callback :save, :after do |object|
  #       puts "saved"
  #     end
  #   end
  #
  #   person = PersonRecord.new
  #   person.save
  #
  # Output:
  #   saving...
  #   - save
  #   saved
  module Callbacks
    extend Concern

    included do
      extend ActiveSupport::DescendantsTracker
      class_attribute :__callbacks, instance_writer: false, default: {}
    end

    CALLBACK_FILTER_TYPES = [:before, :after, :around]

    # Runs the callbacks for the given event.
    #
    # Calls the before and around callbacks in the order they were set, yields
    # the block (if given one), and then runs the after callbacks in reverse
    # order.
    #
    # If the callback chain was halted, returns +false+. Otherwise returns the
    # result of the block, +nil+ if no callbacks have been set, or +true+
    # if callbacks have been set but no block is given.
    #
    #   run_callbacks :save do
    #     save
    #   end
    #
    #--
    #
    # As this method is used in many places, and often wraps large portions of
    # user code, it has an additional design goal of minimizing its impact on
    # the visible call stack. An exception from inside a :before or :after
    # callback can be as noisy as it likes -- but when control has passed
    # smoothly through and into the supplied block, we want as little evidence
    # as possible that we were here.
    def run_callbacks(kind)
      callbacks = __callbacks[kind.to_sym]

      if callbacks.empty?
        yield if block_given?
      else
        env = Filters::Environment.new(self, false, nil)
        next_sequence = callbacks.compile

        invoke_sequence = Proc.new do
          skipped = nil
          while true
            current = next_sequence
            current.invoke_before(env)
            if current.final?
              env.value = !env.halted && (!block_given? || yield)
            elsif current.skip?(env)
              (skipped ||= []) << current
              next_sequence = next_sequence.nested
              next
            else
              next_sequence = next_sequence.nested
              begin
                target, block, method, *arguments = current.expand_call_template(env, invoke_sequence)
                target.send(method, *arguments, &block)
              ensure
                next_sequence = current
              end
            end
            current.invoke_after(env)
            skipped.pop.invoke_after(env) while skipped && skipped.first
            break env.value
          end
        end

        # Common case: no 'around' callbacks defined
        if next_sequence.final?
          next_sequence.invoke_before(env)
          env.value = !env.halted && (!block_given? || yield)
          next_sequence.invoke_after(env)
          env.value
        else
          invoke_sequence.call
        end
      end
    end

    private

      # A hook invoked every time a before callback is halted.
      # This can be overridden in ActiveSupport::Callbacks implementors in order
      # to provide better debugging/logging.
      def halted_callback_hook(filter)
      end

      module Conditionals # :nodoc:
        class Value
          def initialize(&block)
            @block = block
          end
          def call(target, value); @block.call(value); end
        end
      end

      module Filters
        Environment = Struct.new(:target, :halted, :value)

        class Before
          def self.build(callback_sequence, user_callback, user_conditions, chain_config, filter)
            halted_lambda = chain_config[:terminator]

            if user_conditions.any?
              halting_and_conditional(callback_sequence, user_callback, user_conditions, halted_lambda, filter)
            else
              halting(callback_sequence, user_callback, halted_lambda, filter)
            end
          end

          def self.halting_and_conditional(callback_sequence, user_callback, user_conditions, halted_lambda, filter)
            callback_sequence.before do |env|
              target = env.target
              value  = env.value
              halted = env.halted

              if !halted && user_conditions.all? { |c| c.call(target, value) }
                result_lambda = -> { user_callback.call target, value }
                env.halted = halted_lambda.call(target, result_lambda)
                if env.halted
                  target.send :halted_callback_hook, filter
                end
              end

              env
            end
          end
          private_class_method :halting_and_conditional

          def self.halting(callback_sequence, user_callback, halted_lambda, filter)
            callback_sequence.before do |env|
              target = env.target
              value  = env.value
              halted = env.halted

              unless halted
                result_lambda = -> { user_callback.call target, value }
                env.halted = halted_lambda.call(target, result_lambda)

                if env.halted
                  target.send :halted_callback_hook, filter
                end
              end

              env
            end
          end
          private_class_method :halting
        end

        class After
          def self.build(callback_sequence, user_callback, user_conditions, chain_config)
            if chain_config[:skip_after_callbacks_if_terminated]
              if user_conditions.any?
                halting_and_conditional(callback_sequence, user_callback, user_conditions)
              else
                halting(callback_sequence, user_callback)
              end
            else
              if user_conditions.any?
                conditional callback_sequence, user_callback, user_conditions
              else
                simple callback_sequence, user_callback
              end
            end
          end

          def self.halting_and_conditional(callback_sequence, user_callback, user_conditions)
            callback_sequence.after do |env|
              target = env.target
              value  = env.value
              halted = env.halted

              if !halted && user_conditions.all? { |c| c.call(target, value) }
                user_callback.call target, value
              end

              env
            end
          end
          private_class_method :halting_and_conditional

          def self.halting(callback_sequence, user_callback)
            callback_sequence.after do |env|
              unless env.halted
                user_callback.call env.target, env.value
              end

              env
            end
          end
          private_class_method :halting

          def self.conditional(callback_sequence, user_callback, user_conditions)
            callback_sequence.after do |env|
              target = env.target
              value  = env.value

              if user_conditions.all? { |c| c.call(target, value) }
                user_callback.call target, value
              end

              env
            end
          end
          private_class_method :conditional

          def self.simple(callback_sequence, user_callback)
            callback_sequence.after do |env|
              user_callback.call env.target, env.value

              env
            end
          end
          private_class_method :simple
        end
      end

      class Callback #:nodoc:#
        def self.build(chain, filter, kind, options)
          if filter.is_a?(String)
            raise ArgumentError, <<-MSG.squish
              Passing string to define a callback is not supported. See the `.set_callback`
              documentation to see supported values.
            MSG
          end

          new chain.name, filter, kind, options, chain.config
        end

        attr_accessor :kind, :name
        attr_reader :chain_config

        def initialize(name, filter, kind, options, chain_config)
          @chain_config = chain_config
          @name    = name
          @kind    = kind
          @filter  = filter
          @key     = compute_identifier filter
          @if      = Array(options[:if])
          @unless  = Array(options[:unless])
        end

        def filter; @key; end
        def raw_filter; @filter; end

        def merge_conditional_options(chain, if_option:, unless_option:)
          options = {
            if: @if.dup,
            unless: @unless.dup
          }

          options[:if].concat     Array(unless_option)
          options[:unless].concat Array(if_option)

          self.class.build chain, @filter, @kind, options
        end

        def matches?(_kind, _filter)
          @kind == _kind && filter == _filter
        end

        def duplicates?(other)
          case @filter
          when Symbol, String
            matches?(other.kind, other.filter)
          else
            false
          end
        end

        # Wraps code with filter
        def apply(callback_sequence)
          user_conditions = conditions_lambdas
          user_callback = CallTemplate.build(@filter, self)

          case kind
          when :before
            Filters::Before.build(callback_sequence, user_callback.make_lambda, user_conditions, chain_config, @filter)
          when :after
            Filters::After.build(callback_sequence, user_callback.make_lambda, user_conditions, chain_config)
          when :around
            callback_sequence.around(user_callback, user_conditions)
          end
        end

        def current_scopes
          Array(chain_config[:scope]).map { |s| public_send(s) }
        end

        private
          def compute_identifier(filter)
            case filter
            when String, ::Proc
              filter.object_id
            else
              filter
            end
          end

          def conditions_lambdas
            @if.map { |c| CallTemplate.build(c, self).make_lambda } +
              @unless.map { |c| CallTemplate.build(c, self).inverted_lambda }
          end
      end

      # A future invocation of user-supplied code (either as a callback,
      # or a condition filter).
      class CallTemplate # :nodoc:
        def initialize(target, method, arguments, block)
          @override_target = target
          @method_name = method
          @arguments = arguments
          @override_block = block
        end

        # Return the parts needed to make this call, with the given
        # input values.
        #
        # Returns an array of the form:
        #
        #   [target, block, method, *arguments]
        #
        # This array can be used as such:
        #
        #   target.send(method, *arguments, &block)
        #
        # The actual invocation is left up to the caller to minimize
        # call stack pollution.
        def expand(target, value, block)
          result = @arguments.map { |arg|
            case arg
            when :value; value
            when :target; target
            when :block; block || raise(ArgumentError)
            end
          }

          result.unshift @method_name
          result.unshift @override_block || block
          result.unshift @override_target || target

          # target, block, method, *arguments = result
          # target.send(method, *arguments, &block)
          result
        end

        # Return a lambda that will make this call when given the input
        # values.
        def make_lambda
          lambda do |target, value, &block|
            target, block, method, *arguments = expand(target, value, block)
            target.send(method, *arguments, &block)
          end
        end

        # Return a lambda that will make this call when given the input
        # values, but then return the boolean inverse of that result.
        def inverted_lambda
          lambda do |target, value, &block|
            target, block, method, *arguments = expand(target, value, block)
            ! target.send(method, *arguments, &block)
          end
        end

        # Filters support:
        #
        #   Symbols:: A method to call.
        #   Strings:: Some content to evaluate.
        #   Procs::   A proc to call with the object.
        #   Objects:: An object with a <tt>before_foo</tt> method on it to call.
        #
        # All of these objects are converted into a CallTemplate and handled
        # the same after this point.
        def self.build(filter, callback)
          case filter
          when Symbol
            new(nil, filter, [], nil)
          when String
            new(nil, :instance_exec, [:value], compile_lambda(filter))
          when Conditionals::Value
            new(filter, :call, [:target, :value], nil)
          when ::Proc
            if filter.arity > 1
              new(nil, :instance_exec, [:target, :block], filter)
            elsif filter.arity > 0
              new(nil, :instance_exec, [:target], filter)
            else
              new(nil, :instance_exec, [], filter)
            end
          else
            method_to_call = callback.current_scopes.join("_")

            new(filter, method_to_call, [:target], nil)
          end
        end

        def self.compile_lambda(filter)
          eval("lambda { |value| #{filter} }")
        end
      end

      # Execute before and after filters in a sequence instead of
      # chaining them with nested lambda calls, see:
      # https://github.com/rails/rails/issues/18011
      class CallbackSequence # :nodoc:
        def initialize(nested = nil, call_template = nil, user_conditions = nil)
          @nested = nested
          @call_template = call_template
          @user_conditions = user_conditions

          @before = []
          @after = []
        end

        def before(&before)
          @before.unshift(before)
          self
        end

        def after(&after)
          @after.push(after)
          self
        end

        def around(call_template, user_conditions)
          CallbackSequence.new(self, call_template, user_conditions)
        end

        def skip?(arg)
          arg.halted || !@user_conditions.all? { |c| c.call(arg.target, arg.value) }
        end

        def nested
          @nested
        end

        def final?
          !@call_template
        end

        def expand_call_template(arg, block)
          @call_template.expand(arg.target, arg.value, block)
        end

        def invoke_before(arg)
          @before.each { |b| b.call(arg) }
        end

        def invoke_after(arg)
          @after.each { |a| a.call(arg) }
        end
      end

      class CallbackChain #:nodoc:#
        include Enumerable

        attr_reader :name, :config

        def initialize(name, config)
          @name = name
          @config = {
            scope: [:kind],
            terminator: default_terminator
          }.merge!(config)
          @chain = []
          @callbacks = nil
          @mutex = Mutex.new
        end

        def each(&block); @chain.each(&block); end
        def index(o);     @chain.index(o); end
        def empty?;       @chain.empty?; end

        def insert(index, o)
          @callbacks = nil
          @chain.insert(index, o)
        end

        def delete(o)
          @callbacks = nil
          @chain.delete(o)
        end

        def clear
          @callbacks = nil
          @chain.clear
          self
        end

        def initialize_copy(other)
          @callbacks = nil
          @chain     = other.chain.dup
          @mutex     = Mutex.new
        end

        def compile
          @callbacks || @mutex.synchronize do
            final_sequence = CallbackSequence.new
            @callbacks ||= @chain.reverse.inject(final_sequence) do |callback_sequence, callback|
              callback.apply callback_sequence
            end
          end
        end

        def append(*callbacks)
          callbacks.each { |c| append_one(c) }
        end

        def prepend(*callbacks)
          callbacks.each { |c| prepend_one(c) }
        end

        protected
          def chain; @chain; end

        private

          def append_one(callback)
            @callbacks = nil
            remove_duplicates(callback)
            @chain.push(callback)
          end

          def prepend_one(callback)
            @callbacks = nil
            remove_duplicates(callback)
            @chain.unshift(callback)
          end

          def remove_duplicates(callback)
            @callbacks = nil
            @chain.delete_if { |c| callback.duplicates?(c) }
          end

          def default_terminator
            Proc.new do |target, result_lambda|
              terminate = true
              catch(:abort) do
                result_lambda.call if result_lambda.is_a?(Proc)
                terminate = false
              end
              terminate
            end
          end
      end

      module ClassMethods
        def normalize_callback_params(filters, block) # :nodoc:
          type = CALLBACK_FILTER_TYPES.include?(filters.first) ? filters.shift : :before
          options = filters.extract_options!
          filters.unshift(block) if block
          [type, filters, options.dup]
        end

        # This is used internally to append, prepend and skip callbacks to the
        # CallbackChain.
        def __update_callbacks(name) #:nodoc:
          ([self] + ActiveSupport::DescendantsTracker.descendants(self)).reverse_each do |target|
            chain = target.get_callbacks name
            yield target, chain.dup
          end
        end

        # 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 <tt>:scope</tt> 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
        #
        # * <tt>:if</tt> - A symbol, a string (deprecated) 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.
        # * <tt>:unless</tt> - A symbol, a string (deprecated) 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.
        # * <tt>:prepend</tt> - If +true+, the callback will be prepended to the
        #   existing chain rather than appended.
        def set_callback(name, *filter_list, &block)
          type, filters, options = normalize_callback_params(filter_list, block)

          if options[:if].is_a?(String) || options[:unless].is_a?(String)
            ActiveSupport::Deprecation.warn(<<-MSG.squish)
              Passing string to :if and :unless conditional options is deprecated
              and will be removed in Rails 5.2 without replacement.
            MSG
          end

          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+, <tt>:if</tt> or
        # <tt>:unless</tt> options may be passed in order to control when the
        # callback is skipped.
        #
        #   class Writer < Person
        #      skip_callback :validate, :before, :check_membership, if: -> { age > 18 }
        #   end
        #
        # An <tt>ArgumentError</tt> will be raised if the callback has not
        # already been set (unless the <tt>:raise</tt> option is set to <tt>false</tt>).
        def skip_callback(name, *filter_list, &block)
          type, filters, options = normalize_callback_params(filter_list, block)

          if options[:if].is_a?(String) || options[:unless].is_a?(String)
            ActiveSupport::Deprecation.warn(<<-MSG.squish)
              Passing string to :if and :unless conditional options is deprecated
              and will be removed in Rails 5.2 without replacement.
            MSG
          end

          options[:raise] = true unless options.key?(:raise)

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

              if !callback && options[:raise]
                raise ArgumentError, "#{type.to_s.capitalize} #{name} callback #{filter.inspect} has not been defined"
              end

              if callback && (options.key?(:if) || options.key?(:unless))
                new_callback = callback.merge_conditional_options(chain, if_option: options[:if], unless_option: options[:unless])
                chain.insert(chain.index(callback), new_callback)
              end

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

        # Remove all set callbacks for the given event.
        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

          set_callbacks(name, callbacks.dup.clear)
        end

        # Define sets of events in the object life cycle that support callbacks.
        #
        #   define_callbacks :validate
        #   define_callbacks :initialize, :save, :destroy
        #
        # ===== Options
        #
        # * <tt>:terminator</tt> - 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+.
        #
        # * <tt>:skip_after_callbacks_if_terminated</tt> - Determines if after
        #   callbacks should be terminated by the <tt>:terminator</tt> option. By
        #   default after callbacks are executed no matter if callback chain was
        #   terminated or not. This option makes sense only when <tt>:terminator</tt>
        #   option is specified.
        #
        # * <tt>:scope</tt> - 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
        #   <tt>Audit#before</tt> will be called. On the other hand
        #
        #     define_callbacks :save, scope: [:kind, :name]
        #
        #   would trigger <tt>Audit#before_save</tt> instead. That's constructed
        #   by calling <tt>#{kind}_#{name}</tt> 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 <tt>Audit#save</tt>.
        #
        # ===== Notes
        #
        # +names+ passed to +define_callbacks+ must not end with
        # <tt>!</tt>, <tt>?</tt> or <tt>=</tt>.
        #
        # Calling +define_callbacks+ multiple times with the same +names+ will
        # overwrite previous callbacks registered with +set_callback+.
        def define_callbacks(*names)
          options = names.extract_options!

          names.each do |name|
            name = name.to_sym

            set_callbacks name, CallbackChain.new(name, options)

            module_eval <<-RUBY, __FILE__, __LINE__ + 1
              def _run_#{name}_callbacks(&block)
                run_callbacks #{name.inspect}, &block
              end

              def self._#{name}_callbacks
                get_callbacks(#{name.inspect})
              end

              def self._#{name}_callbacks=(value)
                set_callbacks(#{name.inspect}, value)
              end

              def _#{name}_callbacks
                __callbacks[#{name.inspect}]
              end
            RUBY
          end
        end

        protected

          def get_callbacks(name) # :nodoc:
            __callbacks[name.to_sym]
          end

          def set_callbacks(name, callbacks) # :nodoc:
            self.__callbacks = __callbacks.merge(name.to_sym => callbacks)
          end
      end
  end
end