DAY 28：Command Pattern，将动作已指令一个一个完成

什麽是 Command Pattern？

将建立指令与实际执行分离

问题情境

PS5 有特定操作 CPU 指令的实作.ACommand()、.BCommand()，如果现在又多了新的特定操作，必须修改PS5{}，这不符合开闭原则，我们需要有方法满足开闭原则。

package main

import "fmt"

type CPU struct{}

func (CPU) ADoSomething() {
	fmt.Println("a do something")
}
func (CPU) BDoSomething() {
	fmt.Println("b do something")
}
func (CPU) CDoSomething() {
	fmt.Println("c do something")
}

type PS5 struct {
	cpu CPU
}

func (p PS5) ACommand() {
	p.cpu.ADoSomething()
	p.cpu.CDoSomething()
}
func (p PS5) BCommand() {
	p.cpu.ADoSomething()
	p.cpu.BDoSomething()
}
func main() {
	cpu := CPU{}
	ps5 := PS5{cpu}
	ps5.ACommand()
	ps5.BCommand()
}

解决方式

新增Commandinterface，里头有Execute()这个 function，并实作ACommand{}、BCommand{}，让指令的建立从PS5{}中分离出来，即满足开闭原则。

最後透过PS5{}.SetCommand()来将符合Commandinterface 的指令设定给 PS5，再透过PS5{}.DoCommand()选择要执行的指令。

package main

import "fmt"

type CPU struct{}

func (CPU) ADoSomething() {
	fmt.Println("a do something")
}
func (CPU) BDoSomething() {
	fmt.Println("b do something")
}
func (CPU) CDoSomething() {
	fmt.Println("c do something")
}

type Command interface {
	Execute()
}

type ACommand struct {
	cpu CPU
}

func (a ACommand) Execute() {
	a.cpu.ADoSomething()
	a.cpu.CDoSomething()
}

type BCommand struct {
	cpu CPU
}

func (b BCommand) Execute() {
	b.cpu.ADoSomething()
	b.cpu.BDoSomething()
}

type PS5 struct {
	commands map[string]Command
}

func (p *PS5) SetCommand(name string, command Command) {
	p.commands[name] = command
}

func (p *PS5) DoCommand(name string) {
	p.commands[name].Execute()
}

func main() {
	cpu := CPU{}
	aCommand := ACommand{cpu}
	bCommand := BCommand{cpu}
	ps5 := PS5{make(map[string]Command)}
	ps5.SetCommand("a", aCommand)
	ps5.SetCommand("b", bCommand)
	ps5.DoCommand("a")
	ps5.DoCommand("b")
}