Go实战--Design Patterns in Golang 之工厂模式(简单工厂、工厂方法、抽象工厂)

先看一下golang的Tiobe指数趋势：

可以看到在2017年7月，达到了最高点，之后略有下降。我坚信，2018年的7月，golang还会飙升。

生命不止，继续 go go go !!!

继续，golang中设计模式的探讨。
按照国际惯例，讲完单例模式，接下来就该轮到工厂模式。还是那句话，每个人对设计模式的理解都有所不同，欢迎各位探讨。

何为工厂模式

WIKI:
In class-based programming,the factory method pattern is a creational pattern that uses factory methods to deal with the problem of creating objects without having to specify the exact class of the object that will be created.

百度百科：
工厂模式是我们最常用的实例化对象模式了，是用工厂方法代替new操作的一种模式。

在面向对象的编程语言中（如java，C++）设计模式的概念广为人知,应用的也非常广泛。设计模式让我们的代码变得灵活起来，具有很强的扩展性。但在与C语言比肩的Go语言中，设计模式的概念并没有十分突出，甚至很少听到。在Go的开发中，借鉴design pattern的理念同样回味无穷我们的开发带来极大的便利。

c++中使用工厂模式
纯虚类：

class IAnimal
{
public:
    virtual int GetNumberOfLegs() const = 0;
    virtual void Speak() = 0;
    virtual void Free() = 0;
};

实现类：

// IAnimal implementations
class Cat : public IAnimal
{
public:
    int GetNumberOfLegs() const { return 4; }
    void Speak() { cout << "Meow" << endl; }
    void Free() { delete this; }

    static IAnimal * __stdcall Create() { return new Cat(); }
};

class Dog : public IAnimal
{
public:
    int GetNumberOfLegs() const { return 4; }
    void Speak() { cout << "Woof" << endl; }
    void Free() { delete this; }

    static IAnimal * __stdcall Create() { return new Dog(); }
};

class Spider : public IAnimal // Yeah it isn’t really an animal…
{
public:
    int GetNumberOfLegs() const { return 8; }
    void Speak() { cout << endl; }
    void Free() { delete this; }

    static IAnimal * __stdcall Create() { return new Spider(); }
};

class Horse : public IAnimal
{
public:
    int GetNumberOfLegs() const { return 4; }
    void Speak() { cout << "A horse is a horse,of course,of course." << endl; }
    void Free() { delete this; }

    static IAnimal * __stdcall Create() { return new Horse(); }
};

工厂类：

class AnimalFactory
{
private:
    AnimalFactory();
    AnimalFactory(const AnimalFactory &) { }
    AnimalFactory &operator=(const AnimalFactory &) { return *this; }

    typedef map FactoryMap;
    FactoryMap m_FactoryMap;
public:
    ~AnimalFactory() { m_FactoryMap.clear(); }

    static AnimalFactory *Get()
    {
        static AnimalFactory instance;
        return &instance;
    }

    void Register(const string &animalName,CreateAnimalFn pfnCreate);
    IAnimal *CreateAnimal(const string &animalName);
};

工厂类实现：

AnimalFactory::AnimalFactory()
{
    Register("Horse",&Horse::Create);
    Register("Cat",&Cat::Create);
    Register("Dog",&Dog::Create);
    Register("Spider",&Spider::Create);
}

void AnimalFactory::Register(const string &animalName,CreateAnimalFn pfnCreate)
{
    m_FactoryMap[animalName] = pfnCreate;
}

IAnimal *AnimalFactory::CreateAnimal(const string &animalName)
{
    FactoryMap::iterator it = m_FactoryMap.find(animalName);
    if( it != m_FactoryMap.end() )
 return it->second();
 return NULL;
}

使用：

int main( int argc,char **argv )
{
    IAnimal *pAnimal = NULL;
    string animalName;

    while( pAnimal == NULL )
    {
        cout << "Type the name of an animal or ‘q’ to quit: ";
        cin >> animalName;

        if( animalName == "q" )
        break;

        IAnimal *pAnimal = AnimalFactory::Get()->CreateAnimal(animalName);
        if( pAnimal )
        {
            cout << "Your animal has " << pAnimal->GetNumberOfLegs() << " legs." << endl;
            cout << "Your animal says: ";
            pAnimal->Speak();
        }
        else
        {
            cout << "That animal doesn’t exist in the farm! Choose another!" << endl;
        }
        if( pAnimal )
            pAnimal->Free();
        pAnimal = NULL;
        animalName.clear();
    }
    return 0;
}

Struct and Interface

我们知道，golang不是完全的面向对象语言，没有C++或是java中所谓的类。

但是，有struct和interface。这两个知识点是必须要掌握的，弄明白了他们还能理解如何在golang中使用设计模式。

struct：

type exampleStruct struct{
    num int
    s string
    flag bool
}

Go语言学习之struct(The way to go)

interface：

type myInterface interface { myFunction() float64 }

Go语言学习之interface(The way to go)

简单工厂、工厂方法、抽象工厂

Stack Overflow：
https://stackoverflow.com/questions/13029261/design-patterns-factory-vs-factory-method-vs-abstract-factory

简单工厂
简单工厂模式的工厂类一般是使用静态方法，通过接收的参数的不同来返回不同的对象实例。

Simple Factory Pattern
Definition:
Creates objects without exposing the instantiation logic to the client.
Refers to the newly created object through a common interface

工厂方法
工厂方法是针对每一种产品提供一个工厂类。通过不同的工厂实例来创建不同的产品实例。
在同一等级结构中，支持增加任意产品。

Factory Method
Definition:
Defines an interface for creating objects,but let subclasses to decide which class to instantiate
Refers the newly created object through a common interface.

抽象工厂
抽象工厂是应对产品族概念的。比如说，每个汽车公司可能要同时生产轿车，货车，客车，那么每一个工厂都要有创建轿车，货车和客车的方法。

Abstract Factory
Definition:
Abstract Factory offers the interface for creating a family of related objects,without explicitly specifying their classes

golang中简单工厂模式

package main

import (
    "fmt"
)

type Operater interface {
    Operate(int,int) int
}

type AddOperate struct {
}

func (this *AddOperate) Operate(rhs int,lhs int) int {
    return rhs + lhs
}

type MultipleOperate struct {
}

func (this *MultipleOperate) Operate(rhs int,lhs int) int {
    return rhs * lhs
}

type OperateFactory struct {
}

func NewOperateFactory() *OperateFactory {
    return &OperateFactory{}
}

func (this *OperateFactory) CreateOperate(operatename string) Operater {
    switch operatename {
    case "+":
        return &AddOperate{}
    case "*":
        return &MultipleOperate{}
    default:
        panic("无效运算符号")
        return nil
    }
}

func main() {
    Operator := NewOperateFactory().CreateOperate("+")
    fmt.Printf("add result is %d\n",Operator.Operate(1, 2))
}

golang中工厂方法模式

package main

import (
    "fmt"
)

type Operation struct {
    a float64
    b float64
}

type OperationI interface {
    GetResult() float64
    SetA(float64)
    SetB(float64)
}

func (op *Operation) SetA(a float64) {
    op.a = a
}

func (op *Operation) SetB(b float64) {
    op.b = b
}

type AddOperation struct {
    Operation
}

func (this *AddOperation) GetResult() float64 {
    return this.a + this.b
}

type SubOperation struct {
    Operation
}

func (this *SubOperation) GetResult() float64 {
    return this.a - this.b
}

type MulOperation struct {
    Operation
}

func (this *MulOperation) GetResult() float64 {
    return this.a * this.b
}

type DivOperation struct {
    Operation
}

func (this *DivOperation) GetResult() float64 {
    return this.a / this.b
}

type IFactory interface {
    CreateOperation() Operation
}

type AddFactory struct {
}

func (this *AddFactory) CreateOperation() OperationI {
    return &(AddOperation{})
}

type SubFactory struct {
}

func (this *SubFactory) CreateOperation() OperationI {
    return &(SubOperation{})
}

type MulFactory struct {
}

func (this *MulFactory) CreateOperation() OperationI {
    return &(MulOperation{})
}

type DivFactory struct {
}

func (this *DivFactory) CreateOperation() OperationI {
    return &(DivOperation{})
}

func main() {
    fac := &(AddFactory{})
    oper := fac.CreateOperation()
    oper.SetA(1)
    oper.SetB(2)
    fmt.Println(oper.GetResult())
}

更具体的例子：
http://matthewbrown.io/2016/01/23/factory-pattern-in-golang/

golang中抽象工厂模式

package main

import "fmt"

type GirlFriend struct {
    nationality string
    eyesColor   string
    language    string
}

type AbstractFactory interface {
    CreateMyLove() GirlFriend
}

type IndianGirlFriendFactory struct {
}

type KoreanGirlFriendFactory struct {
}

func (a IndianGirlFriendFactory) CreateMyLove() GirlFriend {
    return GirlFriend{"Indian","Black","Hindi"}
}

func (a KoreanGirlFriendFactory) CreateMyLove() GirlFriend {
    return GirlFriend{"Korean","Brown","Korean"}
}

func getGirlFriend(typeGf string) GirlFriend {

    var gffact AbstractFactory
    switch typeGf {
    case "Indian":
        gffact = IndianGirlFriendFactory{}
        return gffact.CreateMyLove()
    case "Korean":
        gffact = KoreanGirlFriendFactory{}
        return gffact.CreateMyLove()
    }
    return GirlFriend{}
}

func main() {

    a := getGirlFriend("Indian")

    fmt.Println(a.eyesColor)
}