Ruby元编程之类定义

2011-01-07 #Ruby #元编程

1.使用 Module#class_eval 打开一个类

class_eval 会同时修改 self 和当前类, 而instance_eval 仅仅会修改 self

class_eval 与 class 关键字相比, 使用的是扁平作用域

  def add_method_to(a_class)
    a_class.class_eval do
      def m; 'Hello!'; end
    end
  end

  add_method_to String
  "abc".m   # => "Hello!"

2.类实例变量, 类的实例对象的实例变量, 类变量

class MyClass
  @my_var = 1
end

class MyClass
  @my_var = 1

  def self.read; @my_var; end
  def write; @my_var = 2; end
  def read; @my_var; end
end

obj = MyClass.new
obj.write
obj.read            # => 2
MyClass.read        # => 1

# 类变量
class C
  @@v = 1  
end

class D < C
  def my_method; @@v; end
end

D.new.my_method  # => 1

@@v = 1

class MyClass
  @@v = 2
end
  
# 访问外部类变量
@@v  # => 2

3.通过 eigenclass 定义类属性, 类方法

class MyClass
  class << self
    attr_accessor :c
	
	def hello
	 "Hello"
	end
  end
end

MyClass.c = 'It works!'
MyClass.c               # => "It works!"
MyClass.hello           # => "Hello"

4.通过 Object#extend 和 include 来扩展类

module MyModule
  def my_method; 'hello'; end
end

obj = Object.new
obj.extend MyModule
obj.my_method       # => "hello"

class MyClass
  extend MyModule
end
MyClass.my_method   # => "hello"

module MyModule
  def my_method; 'hello'; end
end

class MyClass
  class << self
    include MyModule
  end
end

MyClass.my_method   # => "hello"

5.alias 环绕别名

class String
  # 先定义别名
  alias :real_length :length
  
  # 重定义length
  def length
    real_length > 5 ? 'long' : 'short'
  end
end

"War and Peace".length      # => "long"
"War and Peace".real_length # => 13


class Fixnum
  alias :old_plus :+
  
  def +(value)
    self.old_plus(value).old_plus(1)
  end
end

1 + 1    # => 3
1 - 1	 # => 0	

6.动态转发实现 deprecate 废弃方法

class Book
  def title # ...
  end

  def subtitle # ...
  end
  
  def lend_to(user)
    puts "Lending to #{user}"
    # ...
  end

  def self.deprecate(old_method, new_method)
    define_method(old_method) do |*args, &block|
      warn "Warning: #{old_method}() is deprecated. Use #{new_method}()."
      send(new_method, *args, &block)
    end
  end

  deprecate :GetTitle, :title
  deprecate :LEND_TO_USER, :lend_to
  deprecate :title2, :subtitle
end

b = Book.new
b.LEND_TO_USER("Bill")
# >> Warning: LEND_TO_USER() is deprecated. Use lend_to().
# >> Lending to Bill