诗歌rails之 Delegate

Delegate是一种应用composite来代替extend的机制,可以有效地降低代码的耦合性。

Rails 2.2增加了delegate方法,可以十分方便地实现delegate机制。来看看源码吧:

01.def delegate(*methods)
02. options = methods.pop
03. unless options.is_a?(Hash) && to = options[:to]
04. raise ArgumentError, "Delegation needs a target. Supply an options hash with a :to key as the last argument (e.g. delegate :hello, :to => :greeter)."
05. end
06.
07. if options[:prefix] == true && options[:to].to_s =~ /^[^a-z_]/
08. raise ArgumentError, "Can only automatically set the delegation prefix when delegating to a method."
09. end
10.
11. prefix = options[:prefix] && "#{options[:prefix] == true ? to : options[:prefix]}_"
12.
13. methods.each do |method|
14. module_eval(<<-EOS, "(__DELEGATION__)", 1)
15. def #{prefix}#{method}(*args, &block)
16. #{to}.__send__(#{method.inspect}, *args, &block)
17. end
18. EOS
19. end
20.end

delegate方法首先检查传入的参数,正确参数形式为:method1, :method2, ..., :methodN, :to => klass[, :prefix => prefix]

delegate要求参数的最后必须是一个Hash,:to表示需要代理的类,:prefix表示代理的方法是否要加前缀,如果:prefix => true,则代理的方法名为klass_method1, klass_method2, ..., klass_methodN,如果:prefix => prefix (prefix为string),则代理的方法名为prefix_method1, prefix_method2, ..., prefix_methodN。

最终通过module_eval动态生成每个方法定义。通过__send__方法调用:to类的方法。

来看看调用的例子:

简单的调用:

01.class Greeter < ActiveRecord::Base
02. def hello() "hello" end
03. def goodbye() "goodbye" end
04.end
05.
06.class Foo < ActiveRecord::Base
07. delegate :hello, :goodbye, :to => :greeter
08.end
09.
10.Foo.new.hello # => "hello"
11.Foo.new.goodbye # => "goodbye"

增加:prefix => true:

1.class Foo < ActiveRecord::Base
2. delegate :hello, :goodbye, :to => :greeter, :prefix => true
3.end
4.
5.Foo.new.greeter_hello # => "hello"
6.Foo.new.greeter_goodbye # => "goodbye"

自定义前缀名:

1.class Foo < ActiveRecord::Base
2. delegate :hello, :goodbye, :to => :greeter, :prefix => :foo
3.end
4.
5.Foo.new.foo_hello # => "hello"
6.Foo.new.foo_goodbye # => "goodbye"

ruby的动态性再一次发挥了强大的功能!

posted @ 2010-04-21 17:45  麦飞  阅读(2979)  评论(0编辑  收藏  举报