Ruby委托/代理

Question

给出以下代码:

class ArrayProxy < BasicObject
  def initialize
    @array = []
  end

  def foo
    puts 'foo'
  end

  def method_missing(*args, &block)
    @array = @array.send(*args, &block)
  end

  def self.method_missing(*args, &block)
    new.send(*args, &block)
  end
end

为什么将'foo'的调用委托给数组？

ruby-1.9.2-p290 :018 > ArrayProxy.new << 1
 => [1] 
ruby-1.9.2-p290 :019 > ArrayProxy << 1
 => [1] 
ruby-1.9.2-p290 :020 > ArrayProxy.new.foo
foo
 => nil 
ruby-1.9.2-p290 :021 > ArrayProxy.foo
NoMethodError: undefined method `foo' for []:Array

Answer 1

正如注释中注释的Linux_iOS.rb.cpp.c.lisp.m.sh,__send__在这种情况下你应该使用方法,因为BasicObject没有定义实例方法send:

Object.instance_methods.grep /send/
# => [:send, :public_send, :__send__] 

BasicObject.instance_methods.grep /send/
# => [:__send__]

这也可以通过文档证明BasicObject.

类中缺少send实例方法会BasicObect产生以下调用链:

# initial call
ArrayProxy.foo

# there's no class method 'foo', so we go to class 'method_missing' method
ArrayProxy.method_missing :foo

# inside class 'method_missing' we delegate call to new instance using 'send'
ArrayProxy.new.send :foo

# there is no instance method 'send' in ArrayProxy class (and its parent class
# BasicObject) so instance 'method_missing' is called
ArrayProxy.new.method_missing :send, :foo

# instance 'method_missing' delegates call of 'send' method to @array
@array.send :send, :foo

# that is unfolded into regular call of 'send' on @array object
@array.send :foo

# and finally 'foo' is called for @array object
@array.foo
# => NoMethodError: undefined method `foo' for []:Array