简介

delegate 方法提供了一个 delegate 类方法，去将自己包含的对象轻易的暴露出来。

提供的 Options

  • :to => 指定一个对象
  • :prefix => 在对象方法前是否添加前缀
  • :allow_nil => 如果设置为 true, 则阻止 NoMethodError 抛出异常

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 # 指定暴露greeter的hello方法
end

Foo.new.hello   # => "hello"
Foo.new.goodbye # => NoMethodError: undefined method `goodbye' for #<Foo:0x1af30c>
多个关联
class Foo < ActiveRecord::Base
  belongs_to :greeter
  delegate :hello, :goodbye, to: :greeter
end

Foo.new.goodbye # => "goodbye"
方法可以委托给实例变量，类变量或常量提供了他们作为一个 symbols:
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
当然也可以 delegate 给自己的类方法，只需要使用:class
class Foo
  def self.hello
    "world"
  end

  delegate :hello, to: :class
end

Foo.new.hello # => "world"
下面是使用 prefix 的例子
class Invoice < Struct.new(:client)
  delegate :name, :address, to: :client, prefix: :customer
end

invoice = Invoice.new(john_doe)
invoice.customer_name    # => 'John Doe'
invoice.customer_address # => 'Vimmersvej 13'
如果你不希望返回 NoMethodError, 你可以使用 allow_nil: true
class User < ActiveRecord::Base
  has_one :profile
  delegate :age, to: :profile, allow_nil: true
end

User.new.age # nil

注意：如果对象不为空的话，allow_nil: true 仍然会抛出 NoMethodError

class Foo
  def initialize(bar)
    @bar = bar
  end

  delegate :name, to: :@bar, allow_nil: true
end

Foo.new("Bar").name # raises NoMethodError: undefined method `name'