ruby-metaprogramming

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Ruby Metaprogramming

Ruby元编程

Master Ruby's powerful metaprogramming capabilities to write code that writes code. Ruby's dynamic nature makes it exceptionally good at metaprogramming.

掌握Ruby强大的元编程能力，编写能生成代码的代码。Ruby的动态特性使其在元编程方面表现格外出色。

Dynamic Method Definition

动态方法定义

define_method

ruby

class Person
  [:name, :age, :email].each do |attribute|
    define_method(attribute) do
      instance_variable_get("@#{attribute}")
    end

    define_method("#{attribute}=") do |value|
      instance_variable_set("@#{attribute}", value)
    end
  end
end

person = Person.new
person.name = "Alice"
puts person.name  # "Alice"

ruby

class Person
  [:name, :age, :email].each do |attribute|
    define_method(attribute) do
      instance_variable_get("@#{attribute}")
    end

    define_method("#{attribute}=") do |value|
      instance_variable_set("@#{attribute}", value)
    end
  end
end

person = Person.new
person.name = "Alice"
puts person.name  # "Alice"

class_eval and instance_eval

class_eval 和 instance_eval

ruby

undefined

ruby

undefined

class_eval - Evaluates code in context of a class

class_eval - 在类的上下文中执行代码

class MyClass end

MyClass.class_eval do def hello "Hello from class_eval" end end

puts MyClass.new.hello

class MyClass end

MyClass.class_eval do def hello "Hello from class_eval" end end

puts MyClass.new.hello

instance_eval - Evaluates code in context of an instance

instance_eval - 在实例的上下文中执行代码

obj = Object.new obj.instance_eval do def greet "Hello from instance_eval" end end

puts obj.greet

undefined

obj = Object.new obj.instance_eval do def greet "Hello from instance_eval" end end

puts obj.greet

undefined

module_eval

ruby

module MyModule
end

MyModule.module_eval do
  def self.info
    "Module metaprogramming"
  end
end

puts MyModule.info

ruby

module MyModule
end

MyModule.module_eval do
  def self.info
    "Module metaprogramming"
  end
end

puts MyModule.info

Method Missing

Basic method_missing

基础method_missing

ruby

class DynamicFinder
  def initialize(data)
    @data = data
  end

  def method_missing(method_name, *args)
    if method_name.to_s.start_with?("find_by_")
      attribute = method_name.to_s.sub("find_by_", "")
      @data.find { |item| item[attribute.to_sym] == args.first }
    else
      super
    end
  end

  def respond_to_missing?(method_name, include_private = false)
    method_name.to_s.start_with?("find_by_") || super
  end
end

users = [
  { name: "Alice", age: 30 },
  { name: "Bob", age: 25 }
]

finder = DynamicFinder.new(users)
puts finder.find_by_name("Alice")  # {:name=>"Alice", :age=>30}

ruby

class DynamicFinder
  def initialize(data)
    @data = data
  end

  def method_missing(method_name, *args)
    if method_name.to_s.start_with?("find_by_")
      attribute = method_name.to_s.sub("find_by_", "")
      @data.find { |item| item[attribute.to_sym] == args.first }
    else
      super
    end
  end

  def respond_to_missing?(method_name, include_private = false)
    method_name.to_s.start_with?("find_by_") || super
  end
end

users = [
  { name: "Alice", age: 30 },
  { name: "Bob", age: 25 }
]

finder = DynamicFinder.new(users)
puts finder.find_by_name("Alice")  # {:name=>"Alice", :age=>30}

Const Missing

ruby

class DynamicConstants
  def self.const_missing(const_name)
    puts "Constant #{const_name} not found, creating it..."
    const_set(const_name, "Dynamic value for #{const_name}")
  end
end

puts DynamicConstants::SOMETHING  # "Dynamic value for SOMETHING"

ruby

class DynamicConstants
  def self.const_missing(const_name)
    puts "Constant #{const_name} not found, creating it..."
    const_set(const_name, "Dynamic value for #{const_name}")
  end
end

puts DynamicConstants::SOMETHING  # "Dynamic value for SOMETHING"

send and public_send

send 和 public_send

ruby

class Calculator
  def add(x, y)
    x + y
  end

  private

  def secret_method
    "This is private"
  end
end

calc = Calculator.new

ruby

class Calculator
  def add(x, y)
    x + y
  end

  private

  def secret_method
    "This is private"
  end
end

calc = Calculator.new

send can call any method (including private)

send 可以调用任意方法（包括私有方法）

puts calc.send(:add, 3, 4) # 7 puts calc.send(:secret_method) # "This is private"

public_send only calls public methods

public_send 仅调用公共方法

puts calc.public_send(:add, 3, 4) # 7

calc.public_send(:secret_method) # NoMethodError

undefined

undefined

Class Macros

类宏

ruby

class ActiveModel
  def self.attr_with_history(attribute)
    define_method(attribute) do
      instance_variable_get("@#{attribute}")
    end

    define_method("#{attribute}=") do |value|
      history = instance_variable_get("@#{attribute}_history") || []
      history << value
      instance_variable_set("@#{attribute}_history", history)
      instance_variable_set("@#{attribute}", value)
    end

    define_method("#{attribute}_history") do
      instance_variable_get("@#{attribute}_history") || []
    end
  end
end

class Person < ActiveModel
  attr_with_history :name
end

person = Person.new
person.name = "Alice"
person.name = "Alicia"
puts person.name_history.inspect  # ["Alice", "Alicia"]

ruby

class ActiveModel
  def self.attr_with_history(attribute)
    define_method(attribute) do
      instance_variable_get("@#{attribute}")
    end

    define_method("#{attribute}=") do |value|
      history = instance_variable_get("@#{attribute}_history") || []
      history << value
      instance_variable_set("@#{attribute}_history", history)
      instance_variable_set("@#{attribute}", value)
    end

    define_method("#{attribute}_history") do
      instance_variable_get("@#{attribute}_history") || []
    end
  end
end

class Person < ActiveModel
  attr_with_history :name
end

person = Person.new
person.name = "Alice"
person.name = "Alicia"
puts person.name_history.inspect  # ["Alice", "Alicia"]

Singleton Methods

单例方法

ruby

obj = "hello"

ruby

obj = "hello"

Define method on single instance

为单个实例定义方法

def obj.shout self.upcase + "!!!" end

puts obj.shout # "HELLO!!!"

def obj.shout self.upcase + "!!!" end

puts obj.shout # "HELLO!!!"

Using define_singleton_method

使用 define_singleton_method

obj.define_singleton_method(:whisper) do self.downcase + "..." end

puts obj.whisper # "hello..."

undefined

obj.define_singleton_method(:whisper) do self.downcase + "..." end

puts obj.whisper # "hello..."

undefined

Eigenclass (Singleton Class)

Eigenclass（单例类）

ruby

class Person
  def self.species
    "Homo sapiens"
  end
end

ruby

class Person
  def self.species
    "Homo sapiens"
  end
end

Accessing eigenclass

访问 eigenclass

eigenclass = class << Person self end

puts eigenclass # #Class:Person

eigenclass = class << Person self end

puts eigenclass # #Class:Person

Adding class methods via eigenclass

通过 eigenclass 添加类方法

class Person class << self def count @@count ||= 0 end

def increment_count
  @@count ||= 0
  @@count += 1
end

end end

Person.increment_count puts Person.count # 1

undefined

class Person class << self def count @@count ||= 0 end

def increment_count
  @@count ||= 0
  @@count += 1
end

end end

Person.increment_count puts Person.count # 1

undefined

Reflection and Introspection

反射与自省

Object Introspection

对象自省

ruby

class MyClass
  def public_method; end
  protected
  def protected_method; end
  private
  def private_method; end
end

obj = MyClass.new

ruby

class MyClass
  def public_method; end
  protected
  def protected_method; end
  private
  def private_method; end
end

obj = MyClass.new

List methods

列出方法

puts obj.methods.include?(:public_method) puts obj.private_methods.include?(:private_method) puts obj.protected_methods.include?(:protected_method)

Check method existence

检查方法是否存在

puts obj.respond_to?(:public_method) # true puts obj.respond_to?(:private_method) # false puts obj.respond_to?(:private_method, true) # true (include private)

puts obj.respond_to?(:public_method) # true puts obj.respond_to?(:private_method) # false puts obj.respond_to?(:private_method, true) # true (包含私有方法)

Get method object

获取方法对象

method = obj.method(:public_method) puts method.class # Method

undefined

method = obj.method(:public_method) puts method.class # Method

undefined

Class Introspection

类自省

ruby

class Parent
  def parent_method; end
end

class Child < Parent
  def child_method; end
end

ruby

class Parent
  def parent_method; end
end

class Child < Parent
  def child_method; end
end

Inheritance chain

继承链

puts Child.ancestors # [Child, Parent, Object, Kernel, BasicObject]

Instance methods

实例方法

puts Child.instance_methods(false) # Only Child's methods

puts Child.instance_methods(false) # 仅Child的方法

Class variables and instance variables

类变量和实例变量

class Person @@count = 0 def initialize(name) @name = name end end

puts Person.class_variables # [:@@count] person = Person.new("Alice") puts person.instance_variables # [:@name]

undefined

class Person @@count = 0 def initialize(name) @name = name end end

puts Person.class_variables # [:@@count] person = Person.new("Alice") puts person.instance_variables # [:@name]

undefined

Hook Methods

钩子方法

Inheritance Hooks

继承钩子

ruby

class BaseClass
  def self.inherited(subclass)
    puts "#{subclass} inherited from #{self}"
    subclass.instance_variable_set(:@inherited_at, Time.now)
  end
end

class ChildClass < BaseClass
end

ruby

class BaseClass
  def self.inherited(subclass)
    puts "#{subclass} inherited from #{self}"
    subclass.instance_variable_set(:@inherited_at, Time.now)
  end
end

class ChildClass < BaseClass
end

Output: ChildClass inherited from BaseClass

输出: ChildClass inherited from BaseClass

undefined

undefined

Method Hooks

方法钩子

ruby

module Monitored
  def self.included(base)
    base.extend(ClassMethods)
  end

  module ClassMethods
    def method_added(method_name)
      puts "Method #{method_name} was added to #{self}"
    end

    def method_removed(method_name)
      puts "Method #{method_name} was removed from #{self}"
    end
  end
end

class MyClass
  include Monitored

  def my_method
  end
  # Output: Method my_method was added to MyClass
end

ruby

module Monitored
  def self.included(base)
    base.extend(ClassMethods)
  end

  module ClassMethods
    def method_added(method_name)
      puts "Method #{method_name} was added to #{self}"
    end

    def method_removed(method_name)
      puts "Method #{method_name} was removed from #{self}"
    end
  end
end

class MyClass
  include Monitored

  def my_method
  end
  # 输出: Method my_method was added to MyClass
end

included and extended

included 和 extended

ruby

module MyModule
  def self.included(base)
    puts "#{self} included in #{base}"
    base.extend(ClassMethods)
  end

  def self.extended(base)
    puts "#{self} extended by #{base}"
  end

  module ClassMethods
    def class_method
      "I'm a class method"
    end
  end

  def instance_method
    "I'm an instance method"
  end
end

class MyClass
  include MyModule  # Adds instance_method as instance method
end

class AnotherClass
  extend MyModule   # Adds instance_method as class method
end

ruby

module MyModule
  def self.included(base)
    puts "#{self} included in #{base}"
    base.extend(ClassMethods)
  end

  def self.extended(base)
    puts "#{self} extended by #{base}"
  end

  module ClassMethods
    def class_method
      "I'm a class method"
    end
  end

  def instance_method
    "I'm an instance method"
  end
end

class MyClass
  include MyModule  # 将instance_method添加为实例方法
end

class AnotherClass
  extend MyModule   # 将instance_method添加为类方法
end

DSL Creation

DSL创建

ruby

class RouteBuilder
  def initialize
    @routes = {}
  end

  def get(path, &block)
    @routes[path] = { method: :get, handler: block }
  end

  def post(path, &block)
    @routes[path] = { method: :post, handler: block }
  end

  def routes
    @routes
  end
end

ruby

class RouteBuilder
  def initialize
    @routes = {}
  end

  def get(path, &block)
    @routes[path] = { method: :get, handler: block }
  end

  def post(path, &block)
    @routes[path] = { method: :post, handler: block }
  end

  def routes
    @routes
  end
end

DSL usage

DSL使用示例

builder = RouteBuilder.new builder.instance_eval do get "/users" do "List of users" end

post "/users" do "Create user" end end

puts builder.routes

undefined

builder = RouteBuilder.new builder.instance_eval do get "/users" do "List of users" end

post "/users" do "Create user" end end

puts builder.routes

undefined

Dynamic Class Creation

动态类创建

ruby

undefined

ruby

undefined

Create class dynamically

动态创建类

MyClass = Class.new do define_method :greet do "Hello from dynamic class" end end

puts MyClass.new.greet

MyClass = Class.new do define_method :greet do "Hello from dynamic class" end end

puts MyClass.new.greet

Create class with inheritance

创建带继承关系的类

Parent = Class.new do def parent_method "From parent" end end

Child = Class.new(Parent) do def child_method "From child" end end

child = Child.new puts child.parent_method puts child.child_method

undefined

Parent = Class.new do def parent_method "From parent" end end

Child = Class.new(Parent) do def child_method "From child" end end

child = Child.new puts child.parent_method puts child.child_method

undefined

Object Extension

对象扩展

ruby

module Greetable
  def greet
    "Hello!"
  end
end

obj = Object.new
obj.extend(Greetable)
puts obj.greet  # "Hello!"

ruby

module Greetable
  def greet
    "Hello!"
  end
end

obj = Object.new
obj.extend(Greetable)
puts obj.greet  # "Hello!"

Only this instance has the method

只有这个实例拥有该方法

another_obj = Object.new

another_obj.greet # NoMethodError

undefined

undefined

Binding and eval

Binding 和 eval

ruby

def get_binding(param)
  local_var = "local value"
  binding
end

b = get_binding("test")

ruby

def get_binding(param)
  local_var = "local value"
  binding
end

b = get_binding("test")

Evaluate code in the binding context

在binding上下文中执行代码

puts eval("param", b) # "test" puts eval("local_var", b) # "local value"

instance_eval with binding

结合binding使用instance_eval

class MyClass def initialize @value = 42 end end

obj = MyClass.new puts obj.instance_eval { @value } # 42

undefined

class MyClass def initialize @value = 42 end end

obj = MyClass.new puts obj.instance_eval { @value } # 42

undefined

TracePoint

ruby

trace = TracePoint.new(:call, :return) do |tp|
  puts "#{tp.event}: #{tp.method_id} in #{tp.defined_class}"
end

trace.enable

def my_method
  "Hello"
end

my_method

trace.disable

ruby

trace = TracePoint.new(:call, :return) do |tp|
  puts "#{tp.event}: #{tp.method_id} in #{tp.defined_class}"
end

trace.enable

def my_method
  "Hello"
end

my_method

trace.disable

Best Practices

最佳实践

Use metaprogramming sparingly - it can make code hard to understand
Always implement respond_to_missing? when using method_missing
Prefer define_method over class_eval when possible
Document metaprogramming heavily - it's not obvious what's happening
Use public_send over send to respect visibility
Cache metaprogrammed methods to avoid repeated definition
Test metaprogrammed code thoroughly - bugs can be subtle

谨慎使用元编程 - 它会让代码难以理解
使用method_missing时务必实现respond_to_missing?
可能的话优先使用define_method而非class_eval
为元编程代码添加详细文档 - 其逻辑并不直观
优先使用public_send而非send 以遵循可见性规则
缓存元编程生成的方法 避免重复定义
全面测试元编程代码 - 潜在的bug可能很隐蔽

Anti-Patterns

反模式

❌ Don't overuse method_missing - it's slow and hard to debug ❌ Don't use eval with user input - major security risk ❌ Don't metaprogram when simple code works - clarity over cleverness ❌ Don't forget to call super in method_missing ❌ Don't create methods without documenting them - IDE support breaks

❌ 不要过度使用method_missing - 速度慢且难以调试 ❌ 不要将eval与用户输入结合使用 - 存在重大安全风险 ❌ 能用简单代码实现时不要用元编程 - 清晰胜于技巧 ❌ 在method_missing中不要忘记调用super ❌ 不要在未文档化的情况下创建方法 - 会破坏IDE支持

Common Use Cases

常见用例

ORMs (ActiveRecord) - Dynamic finders, associations
DSLs - Route definitions, configurations
Decorators - Method wrapping and enhancement
Mocking/Stubbing - Test frameworks
Attribute definition - Custom accessors with behavior

ORM框架（如ActiveRecord）- 动态查找器、关联关系
DSL - 路由定义、配置
装饰器 - 方法包装与增强
Mock/Stub - 测试框架
属性定义 - 带自定义行为的访问器