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最近复习设计模式
拜读谭勇德的<<设计模式就该这样学>>
该书以java语言演绎了常见设计模式
本系列笔记拟采用golang练习之

迪米特法则

迪米特法则（Law of Demeter, LoD）又叫作最少知道原则（Least KnowledgePrinciple, LKP），指一个对象应该对其他对象保持最少的了解，尽量降低类与类之间的耦合。
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场景

• TeamLeader每天需要查看未完成的项目任务数
• TeamLeader指派TeamMember进行任务统计
• TeamMember提供对Task的汇总方法, 返回未完成的任务数
• 坏的设计:
  • Leader: 我需要统计未完成任务数
  • Member: 好的, 我可以统计, 但是任务清单在哪里呢?
  • Leader: … 我稍后给你吧
• 好的设计:
  • Leader: 我需要统计未完成任务数
  • Member: 好的. 任务清单我知道在那里, 我会搞定的
  • Leader: 好兵!

Task.go

定义任务信息, 以及加载任务清单的方法

package law_of_demeter
type TaskStatus int
const OPENING TaskStatus     = 0
const DONE TaskStatus         = 1
const CANCLED TaskStatus     = 2
const DENIED TaskStatus     = 3
type Task struct {
    iID int
    iStatus TaskStatus
}
func NewTask(id int, status TaskStatus) *Task {
    return &Task{
        id,
        status,
    }
}
func (me *Task) ID() int {
    return me.iID
}
func (me *Task) Status() TaskStatus {
    return me.iStatus
}
func LoadTaskList() []*Task {
    tasks := make([]*Task, 0)
    tasks = append(tasks, NewTask(1, OPENING))
    tasks = append(tasks, NewTask(2, DONE))
    tasks = append(tasks, NewTask(3, CANCLED))
    tasks = append(tasks, NewTask(4, DENIED))
    return tasks
}

ITeamLeader.go

定义TeamLeader的接口

package law_of_demeter
type ITeamLeader interface {
    CountOpeningTasks() int
}

BadTeamLeader.go

不好的ITeamLeader实现, 同时耦合了Task和BadTeamMember两个类

package law_of_demeter
import "fmt"
type BadTeamLeader struct {
    iID int
    sName string
}
func (me *BadTeamLeader) CountOpeningTasks() int {
    tasks := LoadTaskList()
    member := NewBadTeamMember(11, "王Member")
    sum := member.countOpeningTasks(tasks)
    fmt.Printf("%v CountOpeningTasks, got %v", me.sName, sum)
    return sum
}

BadTeamMember.go

不好的示例. 统计任务数的实现, 要求过多的参数, 增加调用方的耦合度和使用难度

package law_of_demeter
type BadTeamMember struct {
    iID int
    sName string
}
func NewBadTeamMember(id int, name string) *BadTeamMember {
    return &BadTeamMember{
        id,
        name,
    }
}
func (me *BadTeamMember) countOpeningTasks(lstTasks []*Task) int {
    sum := 0
    for _,it := range lstTasks {
        if it.Status() == OPENING {
            sum++
        }
    }
    return sum
}

GoodTeamLerader.go

更好的ITeamLeader实现, 只依赖了GoodTeamMember

package law_of_demeter
import "fmt"
type GoodTeamLeader struct {
    iID int
    sName string
}
func (me *GoodTeamLeader) CountOpeningTasks() int {
    member := NewGoodTeamMember(11, "王Member")
    sum := member.countOpeningTasks()
    fmt.Printf("%v CountOpeningTasks, got %v", me.sName, sum)
    return sum
}

GoodTeamMember.go

更好的TeamMember, 对外屏蔽了任务列表的获取细节

package law_of_demeter
type GoodTeamMember struct {
    iID int
    sName string
}
func NewGoodTeamMember(id int, name string) *GoodTeamMember {
    return &GoodTeamMember{
        id,
        name,
    }
}
func (me *GoodTeamMember) countOpeningTasks() int {
    sum := 0
    tasks := LoadTaskList()
    for _,it := range tasks {
        if it.Status() == OPENING {
            sum++
        }
    }
    return sum
}

law_of_demeter_test.go

单元测试

package main
import "testing"
import (lod "learning/gooop/principles/law_of_demeter")
func Test_LOD(t *testing.T) {
    bl := lod.NewBadTeamLeader(1, "张Leader")
    bl.CountOpeningTasks()
    gl := lod.NewGoodTeamLeader(2, "李Leader")
    gl.CountOpeningTasks()
}

测试输出

$ go test -v law_of_demeter_test.go 
=== RUN   Test_LOD
张Leader CountOpeningTasks, got 1
李Leader CountOpeningTasks, got 1
--- PASS: Test_LOD (0.00s)
PASS
ok      command-line-arguments  0.002s