在Ruby中设计Callback机制

阅读Paperclip源码，发现里面有一个不错的callback机制，如下：

 

def has_attached_file name, options = {}
      include InstanceMethods

      write_inheritable_attribute(:attachment_definitions, {}) if attachment_definitions.nil?
      attachment_definitions[name] = {:validations => []}.merge(options)

      after_save :save_attached_files
      before_destroy :destroy_attached_files

      define_paperclip_callbacks :post_process, :"#{name}_post_process"
      
      。。。。。。。
      
end

 

 

来看看define_paperclip_callbacks方法，在CallbackCompatability这个module中有定义：

 

module Rails3
      def self.included(base)
        base.extend(Defining)
        base.send(:include, Running)
      end

      module Defining
        def define_paperclip_callbacks(*callbacks)
          define_callbacks *[callbacks, {:terminator => "result == false"}].flatten
          callbacks.each do |callback|
            eval <<-end_callbacks
              def before_#{callback}(*args, &blk)
                set_callback(:#{callback}, :before, *args, &blk)
              end
              def after_#{callback}(*args, &blk)
                set_callback(:#{callback}, :after, *args, &blk)
              end
            end_callbacks
          end
        end
      end

      module Running
        def run_paperclip_callbacks(callback, opts = nil, &block)
          run_callbacks(callback, opts, &block)
        end
      end

 end

 

 

在看看set_callback是怎么被调用的

 

module Glue
    def self.included base #:nodoc:
      base.extend ClassMethods
      if base.respond_to?("set_callback")
        base.send :include, Paperclip::CallbackCompatability::Rails3
      else
        base.send :include, Paperclip::CallbackCompatability::Rails21
      end
    end
end

 

关键在于

run_callbacks(callback, opts, &block)

直接调用rails3的api，完毕后回调block，在block继续自己的业务逻辑