手撸 golang 架构设计准则 合成复用准则

缘起

最近温习设计模式
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
本系列笔记拟采纳 golang 练习之

合成复用准则

• 合成复用准则（Composite/Aggregate Reuse Principle, CARP）指尽量应用对象组合（has-a）或对象聚合（contanis-a）的形式实现代码复用，而不是用继承关系达到代码复用的目标。合成复用准则能够使零碎更加灵便，升高类与类之间的耦合度，一个类的变动对其余类造成的影响绝对较小。
• 继承，又被称为白箱复用，相当于把所有实现细节裸露给子类。组合 / 聚合又被称为黑箱复用，对类以外的对象是无奈获取实现细节的。

_

场景

• 某订单业务零碎, 须要连贯数据库对产品信息进行 CRUD 操作

• 不好的设计:

  • 定义 DBConnection 类, 实现对数据库的连贯和 SQL 执行
  • 定义 ProductDAO 类, 继承 DBConnection 类, 并封装对产品材料的增删改查
  • 问题: ProductDAO 对 DBConnection 的继承仅仅是为了代码复用, 不合乎合成复用准则

• 更好的设计:

  • 定义 IDBConnection 接口
  • 定义 MysqlConnection 类, 实现对 mysql 数据库的连贯和 SQL 执行
  • 定义 ProductDAO 类, 通过 Setter 办法注入 IDBConnection 实例

_

Product.go

产品实体类

package composite_reuse

type Product struct {
    ID int
    Name string
    Price float64
}

func NewProduct(id int, name string, price float64) *Product {
    return &Product{id, name, price,}
}

_

BadDBConnection.go

BadDBConnection 用于连贯数据库并执行 SQL 语句

package composite_reuse

import "fmt"

type BadDBConnection struct {
    sURL string
    sUID string
    sPWD string
}

func NewBadDBConnection(url string, uid string, pwd string) *BadDBConnection {
    return &BadDBConnection{url, uid, pwd,}
}

func (me *BadDBConnection) Execute(sql string, args... interface{}) (error, int) {fmt.Printf("BadDBConnection.Execute, sql=%v, args=%v\n", sql, args)
    return nil,0
}

BadProductDAO.go

不好的设计. 间接从 BadDBConnection 继承, 以获取拜访数据库的能力. 继承仅仅是为了代码复用, 而不是概念复用, 因而违反了合成复用准则.

package composite_reuse

type BadProductDAO struct {BadDBConnection}

func NewBadProductDAO(url string, uid string, pwd string) *BadProductDAO {
    return &BadProductDAO{*NewBadDBConnection(url, uid, pwd),
    }
}

func (me *BadProductDAO) Insert(it *Product) (error, int) {return me.Execute("insert into product(id,name,price) values(?, ?, ?)", it.ID, it.Name, it.Price)
}

func (me *BadProductDAO) Update(it *Product) (error, int) {return me.Execute("update product set name=? price=? where id=?", it.Name, it.Price, it.ID)
}

func (me *BadProductDAO) Delete(id int) (error, int) {return me.Execute("delete from product where id=?", id)
}

IGoodDBConnection.go

更好的设计, 将数据库连贯形象为接口, 以反对多种数据库

package composite_reuse

type IGoodDBConnection interface {Execute(sql string, args... interface{}) (error, int)
}

GoodMysqlConnection.go

更好的设计, GoodMysqlConnection 封装 MYSQL 数据库方言, 实现 IGoodDBConnection 接口

package composite_reuse

import "fmt"

type GoodMysqlConnection struct {
    sURL string
    sUID string
    sPWD string
}

func NewGoodMysqlConnection(url string, uid string, pwd string) IGoodDBConnection {
    return &GoodMysqlConnection{url, uid, pwd,}
}

func (me *GoodMysqlConnection) Execute(sql string, args... interface{}) (error, int) {fmt.Printf("GoodMysqlConnection.Execute, sql=%v, args=%v\n", sql, args)
    return nil, 0
}

GoodProductDAO.go

更好的设计, 通过 Setter 办法注入数据库方言实例(遵循了合成复用准则), 实现产品的 CRUD

package composite_reuse


type GoodProductDAO struct {mDBConnection IGoodDBConnection}

func NewGoodProductDAO() *GoodProductDAO {return &GoodProductDAO{}
}

func (me *GoodProductDAO) SetDBConnection(it IGoodDBConnection) {me.mDBConnection = it}

func (me *GoodProductDAO) Insert(it *Product) (error, int) {return me.mDBConnection.Execute("insert into product(id,name,price) values(?, ?, ?)", it.ID, it.Name, it.Price)
}

func (me *GoodProductDAO) Update(it *Product) (error, int) {return me.mDBConnection.Execute("update product set name=? price=? where id=?", it.Name, it.Price, it.ID)
}

func (me *GoodProductDAO) Delete(id int) (error, int) {return me.mDBConnection.Execute("delete from product where id=?", id)
}

composite_reuse_test.go

单元测试

package main

import "testing"
import (carp "learning/gooop/principles/composite_reuse")

func Test_CARP(t *testing.T) {p := carp.NewProduct(1, "手机", 1000)
    fnCallAndLog := func(fn func() (error, int)) {e,rows := fn()
        if e != nil {t.Errorf("error = %s", e.Error())
        } else {t.Logf("affected rows = %v", rows)
        }
    }

    // begin testing bad //////////////////////////////////////////////////////////////
    bd := carp.NewBadProductDAO("database connection url", "sa", "123")
    fnCallAndLog(func() (error, int) {return bd.Insert(p)
    })
    fnCallAndLog(func() (error, int) {return bd.Update(p)
    })
    fnCallAndLog(func() (error, int) {return bd.Delete(p.ID)
    })
    // end testing bad //////////////////////////////////////////////////////////////


    // begin testing good //////////////////////////////////////////////////////////////
    con := carp.NewGoodMysqlConnection("database connection url", "sa", "123")
    gd := carp.NewGoodProductDAO()
    gd.SetDBConnection(con)

    fnCallAndLog(func() (error, int) {return gd.Insert(p)
    })
    fnCallAndLog(func() (error, int) {return gd.Update(p)
    })
    fnCallAndLog(func() (error, int) {return gd.Delete(p.ID)
    })
    // end testing good //////////////////////////////////////////////////////////////
}

测试输入

$ go test -v composite_reuse_test.go 
=== RUN   Test_CARP
BadDBConnection.Execute, sql=insert into product(id,name,price) values(?, ?, ?), args=[1 手机 1000]
    composite_reuse_test.go:13: affected rows = 0
BadDBConnection.Execute, sql=update product set name=? price=? where id=?, args=[手机 1000 1]
    composite_reuse_test.go:13: affected rows = 0
BadDBConnection.Execute, sql=delete from product where id=?, args=[1]
    composite_reuse_test.go:13: affected rows = 0
GoodMysqlConnection.Execute, sql=insert into product(id,name,price) values(?, ?, ?), args=[1 手机 1000]
    composite_reuse_test.go:13: affected rows = 0
GoodMysqlConnection.Execute, sql=update product set name=? price=? where id=?, args=[手机 1000 1]
    composite_reuse_test.go:13: affected rows = 0
GoodMysqlConnection.Execute, sql=delete from product where id=?, args=[1]
    composite_reuse_test.go:13: affected rows = 0
--- PASS: Test_CARP (0.00s)
PASS
ok      command-line-arguments  0.004s