callbacks.rb

  module Callbacks
    def self.included(klass)
      klass.extend ClassMethods
    end
        
    def run_callbacks(kind, options = {})
      send("_run_#{kind}_callbacks")
    end
    
    class Callback
      @@_callback_sequence = 0
      
      def initialize(filter, kind, options, klass, name)
        @kind, @klass = kind, klass
        @name    = name
        @filter  = _compile_filter(filter)
        @options = _compile_options(options)
      end
      
      # This will supply contents for before and around filters, and no
      # contents for after filters (for the forward pass).
      def start
        # options[0] is the compiled form of supplied conditions
        # options[1] is the "end" for the conditional
        
        if @kind == :before || @kind == :around
          if @kind == :before
            # if condition    # before_save :filter_name, :if => :condition
            #   filter_name
            # end
            [@options[0], @filter, @options[1]].compact.join("\n")
          elsif @options[0]
            # Compile around filters with conditions into proxy methods
            # that contain the conditions.
            #
            # For `around_save :filter_name, :if => :condition':
            #
            # def _conditional_callback_save_17
            #   if condition
            #     filter_name do
            #       yield self
            #     end
            #   else
            #     yield self
            #   end
            # end
            name = "_conditional_callback_#{@kind}_#{@@_callback_sequence += 1}"
            txt = <<-RUBY
              def #{name}
                #{@options[0]}
                  #{@filter} do
                    yield self
                  end
                else
                  yield self
                end
              end
            RUBY
            @klass.class_eval(txt)
            "#{name} do"
          else
            "#{@filter} do"
          end
        end
      end
      
      # This will supply contents for around and after filters, but not
      # before filters (for the backward pass).
      def end
        if @kind == :around || @kind == :after
          # if condition    # after_save :filter_name, :if => :condition
          #   filter_name
          # end
          if @kind == :after
            [@options[0], @filter, @options[1]].compact.join("\n")
          else
            "end"
          end
        end
      end
      
      private
      # Options support the same options as filters themselves (and support
      # symbols, string, procs, and objects), so compile a conditional
      # expression based on the options
      def _compile_options(options)
        return [] unless options.key?(:if) || options.key?(:unless)
        
        conditions = []
        
        if options.key?(:if)
          filter, yield_self = _compile_filter(options[:if])
          conditions << filter
        end
        
        if options.key?(:unless)
          filter, yield_self = _compile_filter(options[:unless])
          conditions << "!#{filter}"
        end
        
        ["if #{conditions.join(" && ")}", "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 before_foo method on it to call
      #
      # All of these objects are compiled into methods and handled
      # the same after this point:
      #   Symbols:: Already methods
      #   Strings:: class_eval'ed into methods
      #   Procs::   define_method'ed into methods
      #   Objects:: 
      #     a method is created that calls the before_foo method
      #     on the object.
      def _compile_filter(filter)
        case filter
        when Symbol
          filter
        when Proc
          method_name = "_callback_#{@kind}_#{@@_callback_sequence += 1}"
          @klass.send(:define_method, method_name, &filter)
          method_name << (filter.arity == 1 ? "(self)" : "")
        when String
          method_name = "_callback_#{@kind}_#{@@_callback_sequence += 1}"
          @klass.class_eval <<-RUBY
            def #{method_name}
              #{filter}
            end
          RUBY
          method_name
        else
          method_name = "_callback_#{@kind}_#{@@_callback_sequence += 1}"
          kind, name = @kind, @name
          @klass.send(:define_method, method_name) do
            filter.send("#{kind}_#{name}", self)
          end
          method_name
        end
      end
    end
    
    # An Array with a compile method
    class CallbackChain < Array
      def compile
        method = []
        each do |callback|
          method << callback.start
        end
        method << "yield self"
        reverse_each do |callback|
          method << callback.end
        end
        method.compact.join("\n")
      end
    end
        
    module ClassMethods
      CHAINS = {:before => :before, :around => :before, :after => :after}
      
      # Make the _run_save_callbacks method. The generated method takes
      # a block that it'll yield to. It'll call the before and around filters
      # in order, yield the block, and then run the after filters.
      # 
      # _run_save_callbacks do
      #   save
      # end
      def _define_runner(symbol, str, options)
        self.class_eval <<-RUBY, __FILE__, __LINE__ + 1
          def _run_#{symbol}_callbacks
            #{str}
          end
        RUBY
        
        before_name, around_name, after_name = 
          options.values_at(:before, :after, :around)
      end
      
      # Define callbacks.
      #
      # Creates a <name>_callback method that you can use to add callbacks.
      #
      # Syntax:
      #   save_callback :before, :before_meth
      #   save_callback :after,  :after_meth, :if => :condition
      #   save_callback :around {|r| stuff; yield; stuff }
      #
      # The <name>_callback method also updates the _run_<name>_callbacks
      # method, which is the public API to run the callbacks.
      def define_callbacks(*symbols)
        symbols.each do |symbol|
          self.class_eval <<-RUBY, __FILE__, __LINE__ + 1
            class_inheritable_accessor :_#{symbol}_callbacks
            self._#{symbol}_callbacks = CallbackChain.new

            def self.#{symbol}_callback(type, *filters, &blk)
              options = filters.last.is_a?(Hash) ? filters.pop : {}
              filters.unshift(blk) if block_given?
              filters.map! {|f| Callback.new(f, type, options, self, :#{symbol})}
              self._#{symbol}_callbacks.push(*filters)
              _define_runner(:#{symbol}, self._#{symbol}_callbacks.compile, options)
            end
            
            #{symbol}_callback(:before)
          RUBY
        end
      end
    end
  end