Ruby 命令模式讲解和代码示例
命令是一种行为设计模式， 它可将请求或简单操作转换为一个对象。
此类转换让你能够延迟进行或远程执行请求， 还可将其放入队列中。
复杂度：
流行度：
使用示例： 命令模式在 Ruby 代码中很常见。 大部分情况下， 它被用于代替包含行为的参数化 UI 元素的回调函数， 此外还被用于对任务进行排序和记录操作历史记录等。
识别方法： 命令模式可以通过抽象或接口类型 （发送者） 中的行为方法来识别， 该类型调用另一个不同的抽象或接口类型 （接收者） 实现中的方法， 该实现则是在创建时由命令模式的实现封装。 命令类通常仅限于一些特殊行为。
概念示例
本例说明了命令设计模式的结构并重点回答了下面的问题：
- 它由哪些类组成？
- 这些类扮演了哪些角色？
- 模式中的各个元素会以何种方式相互关联？
main.rb: 概念示例
# The Command interface declares a method for executing a command. class Command # @abstract def execute raise NotImplementedError, "#{self.class} has not implemented method '#{__method__}'" end end # Some commands can implement simple operations on their own. class SimpleCommand < Command # @param [String] payload def initialize(payload) @payload = payload end def execute puts "SimpleCommand: See, I can do simple things like printing (#{@payload})" end end # However, some commands can delegate more complex operations to other objects, # called "receivers". class ComplexCommand < Command # Complex commands can accept one or several receiver objects along with any # context data via the constructor. def initialize(receiver, a, b) @receiver = receiver @a = a @b = b end # Commands can delegate to any methods of a receiver. def execute print 'ComplexCommand: Complex stuff should be done by a receiver object' @receiver.do_something(@a) @receiver.do_something_else(@b) end end # The Receiver classes contain some important business logic. They know how to # perform all kinds of operations, associated with carrying out a request. In # fact, any class may serve as a Receiver. class Receiver # @param [String] a def do_something(a) print "\nReceiver: Working on (#{a}.)" end # @param [String] b def do_something_else(b) print "\nReceiver: Also working on (#{b}.)" end end # The Invoker is associated with one or several commands. It sends a request to # the command. class Invoker # Initialize commands. # @param [Command] command def on_start=(command) @on_start = command end # @param [Command] command def on_finish=(command) @on_finish = command end # The Invoker does not depend on concrete command or receiver classes. The # Invoker passes a request to a receiver indirectly, by executing a command. def do_something_important puts 'Invoker: Does anybody want something done before I begin?' @on_start.execute if @on_start.is_a? Command puts 'Invoker: ...doing something really important...' puts 'Invoker: Does anybody want something done after I finish?' @on_finish.execute if @on_finish.is_a? Command end end # The client code can parameterize an invoker with any commands. invoker = Invoker.new invoker.on_start = SimpleCommand.new('Say Hi!') receiver = Receiver.new invoker.on_finish = ComplexCommand.new(receiver, 'Send email', 'Save report') invoker.do_something_important output.txt: 执行结果
Invoker: Does anybody want something done before I begin? SimpleCommand: See, I can do simple things like printing (Say Hi!) Invoker: ...doing something really important... Invoker: Does anybody want something done after I finish? ComplexCommand: Complex stuff should be done by a receiver object Receiver: Working on (Send email.) Receiver: Also working on (Save report.) 
