这是一篇在Ruby和Rails中使用代理模式各种方法的总结文章。
delegate in Ruby
require 'delegate'
class Boss
def initialize(name)
@name = name
end
def take_ticket
p "(#{@name}) Help me take the ticket."
end
def plan
p "(#{@name}) Help me make a plan."
end
end
class Assistant < DelegateClass(Boss)
def initialize(boss)
super(boss)
end
def prepare_meeting
p "I am going to prepare the meeting."
end
end
boss = Boss.new("Anna")
anne = Assistant.new(boss)
anne.prepare_meeting
anne.take_ticket
anne.plan
anne 是 Assistant的实例, 是没有定义 take_ticket 和 plan 实例方法的。由于 Assistant 继承于 DelegateClass(Boss). 所以 anne = Assistant.new(boss) 这一步，让anne 能够调用 boos的实例 方法。这样anne 就能帮 boss 订票和制定计划了。
delegate in Rails
class Blog < ActiveRecord::Base
belongs_to :user
delegate :name, :address, :age, to: :user, prefix: true, allow_nil: true
end
@blog.user.name <=> @blog.user_name
@blog.user.address <=> @blog.user_address
@blog.user.age <=> @blog.user_age
@blog = Blog.new @blog 没有name实例方法，user 有name实例方法。如果 @blog想要调用user的name实例方法，一般情况是这样: @blog.user.name 现在，使用了: delegate :name, to: :user, prefix: true, allow_nil: true @blog可以直接调用: @blog.user_name 就表示@blog.user.name 。这就是delegate 的作用。
再看下面的例子:
class Greeter < ActiveRecord::Base
def hello
'hello'
end
def goodbye
'goodbye'
end
end
class Foo < ActiveRecord::Base
belongs_to :greeter
delegate :hello, to: :greeter
end
Foo.new.hello # => "hello"
Foo.new.goodbye # => NoMethodError: undefined method `goodbye' for #<Foo:0x1af30c>
按照道理来说， Foo 没有定义hello 实例方法， 而Foo.new.hello 确没有报错。这就是delegate的作用了。
其实 Foo.new.hello 等价于 Foo.new.greeter.hello。 调用的正式Greeter的 hello 实例方法。 用了代理，省去了 greeter的写法。 和上面例子的区别是 prefix的配置，没有配置前缀。但是，一定配置了前缀， 就需要带上前缀。当然，前缀还可以不写true，写true rails会帮你将前缀定义为 to 的对象。可以自己定义前缀的名称，
delegate :name, :address, to: :client, prefix: :customer
就变为 @blog.customer_name、@blog.customer_address
OK，我们继续看例子：
class Foo
CONSTANT_ARRAY = [0,1,2,3]
@@class_array = [4,5,6,7]
def initialize
@instance_array = [8,9,10,11]
end
delegate :sum, to: :CONSTANT_ARRAY
delegate :min, to: :@@class_array
delegate :max, to: :@instance_array
end
Foo.new.sum # => 6
Foo.new.min # => 4
Foo.new.max # => 11
这里Foo 配置了代理 sum，min，max 方法。并且 配置了to的对象。 我们知道，Foo并没有定义sum，min，max 这三个实例方法。 这三个实例方法其实就是Ruby数组库里面的方法，Ruby的数组对象，有sum，min，max三个实力方法。 所以，Foo代理就可以调用成功了。实际上，上面的代理操作等价于下面：
Foo.new::CONSTANT_ARRAY.sum # => 6
Foo.new.class_array.min # => 4
Foo.new.instance_array.max # => 11
你可以简单的认为 Foo.new 代理了 CONSTANT_ARRAY 、@@class_array 、@instance_array 三个数组
class Foo
def self.hello
"world"
end
delegate :hello, to: :class
end
Foo.new.hello # => "world"
还能直接代理类方法。
Struct的例子
Person = Struct.new(:name, :address)
# => Person
class Invoice < Struct.new(:client)
delegate :name, :address, :to => :client
end
#=> [:name, :address]
john_doe = Person.new("John Doe", "Vimmersvej 13")
#=> #<struct Person name="John Doe", address="Vimmersvej 13">
invoice = Invoice.new(john_doe)
#=> #<struct Invoice client=#<struct Person name="John Doe", address="Vimmersvej 13"invoice.name
#=> John Doe
invoice.address
#=>Vimmersvej 13
Forwardable
require 'forwardable'
class RecordCollection
attr_accessor :records
extend Forwardable
def_delegator :@records, :[], :record_number
def_delegator :@records, :sum, :my_sum
def_delegators :@records, :sum
end
r = RecordCollection.new
r.records = [4,5,6]
r.record_number(0) # => 4
r.my_sum # => 15
r.sum # => NoMethodError: undefined method `sum' for #<RecordCollection:0x0000000dc3ee90 @records=[4, 5, 6]>
r 为 RecordCollection 的实例。RecordCollection的实例有一个属性 records.
def_delegator :@records, :[], :record_number
def_delegator :@records, :sum, :my_sum
这样就表示， 对records(@records)这个属性的[]、sum 的方法分别用 record_number、my_sum代替表示。原方法表示去除。 所以就出现了：
r.sum # => NoMethodError: undefined method `sum' for #<RecordCollection:0x0000000dc3ee90 @records=[4, 5, 6]>
如果我们也不想让 sum方法被my_sum 代替而去除。 想要sum 和 my_sum两个方法都能起作用，ruby页考虑道了这点。
require 'forwardable'
class RecordCollection
attr_accessor :records
extend Forwardable
def_delegator :@records, :[], :record_number
def_delegator :@records, :sum, :my_sum
def_delegators :@records, :sum
end
r = RecordCollection.new
r.records = [4,5,6]
r.record_number(0) # => 4
r.my_sum # => 15
r.sum # => 15
用def_delegators 将sum方法 加回来就可以了。
官方另一个例子：
class Queue
extend Forwardable
def initialize
@q = [ ] # prepare delegate object
end
# setup preferred interface, enq() and deq()...
def_delegator :@q, :push, :enq
def_delegator :@q, :shift, :deq
# support some general Array methods that fit Queues well
def_delegators :@q, :clear, :first, :push, :shift, :size
end
q = Queue.new
q.enq 1, 2, 3, 4, 5
q.push 6
q.shift # => 1
while q.size > 0
puts q.deq
end
q.enq "Ruby", "Perl", "Python"
puts q.first
q.clear
puts q.first
delegate method form rails API documentation
class Module
# Delegate method
# It expects an array of arguments that contains the methods to be delegated
# and a hash of options
def delegate(*methods)
# Pop up the options hash from arguments array
options = methods.pop
# Check the availability of the options hash and more specifically the :to option
# Raises an error if one of them is not there
unless options.is_a?(Hash) && to = options[:to]
raise ArgumentError, "Delegation needs a target. Supply an options hash with a :to key as the last argument (e.g. delegate :hello, :to => :greeter)."
end
# Make sure the :to option follows syntax rules for method names
if options[:prefix] == true && options[:to].to_s =~ /^[^a-z_]/
raise ArgumentError, "Can only automatically set the delegation prefix when delegating to a method."
end
# Set the real prefix value
prefix = options[:prefix] && "#{options[:prefix] == true ? to : options[:prefix]}_"
# Here comes the magic of ruby :)
# Reflection techniques are used here:
# module_eval is used to add new methods on the fly which:
# expose the contained methods' objects
methods.each do |method|
module_eval("def #{prefix}#{method}(*args, &block)\n#{to}.__send__(#{method.inspect}, *args, &block)\nend\n", "(__DELEGATION__)", 1)
end
end
end
links：
http://ruby-doc.org/stdlib-2.3.1/libdoc/forwardable/rdoc/Forwardable.html
http://blog.khd.me/ruby/delegation-in-ruby/