4. 面对接口

一. duck typing 鸭子类型

Go语言的鸭子类型

• 同时具有python, c++的duck typing 的灵活性

def download(Retriver):
    return Retriver.get("www.gihub.com")

• 又有java的类型检查

<R extends Retriver>
String download(R r) {
    return r.get("www.github.com")
}

---

二. 接口的定义和实现

接口的定义

使用者download

实现者retriever

• 接口由使用者定义

type Retriever interface {
    Get(source string) string
}

func download(retriever Retriever) string {
    return retriever.Get("www.github.com")
}

接口的实现

• 接口实现是隐式的
• 只要实现接口里的方法

---

三. 接口的组合

• 可以将多个接口组合起来

type Retriever interface {
	Get(url string) string //不需要func关键字
}

type Poster interface {
	Post(url string,
		form map[string]string) string
}

type RetrieverPoster interface {    // 接口组合
	Retriever
	Poster
}

func session(s RetrieverPoster) string {   // 接口组合   可以调用每个接口的方法
	s.Post(url, map[string]string{
		"contents": "another faked imooc.com",
	})
	return s.Get(url)
}

代码实例

• 目录

├── main.go
├── mock
│    └── mockretriever.go
└── real
    └── retriever.go

• main.go

package main

import (
	"fmt"

	"time"

	"learngo/lang/retriever/mock"
	"learngo/lang/retriever/real"
)

type Retriever interface {
	Get(url string) string //不需要func关键字
}

type Poster interface {
	Post(url string,
		form map[string]string) string
}

const url = "http://www.imooc.com"

func download(r Retriever) string {
	return r.Get(url)
}

func post(poster Poster) {
	poster.Post(url,
		map[string]string{
			"name":   "cannon",
			"course": "golang",
		})
}

type RetrieverPoster interface {    // 接口组合
	Retriever
	Poster
}

func session(s RetrieverPoster) string {   // 接口组合   可以调用每个接口的方法
	s.Post(url, map[string]string{
		"contents": "another faked imooc.com",
	})
	return s.Get(url)
}

func main() {
	var r Retriever

	mockRetriever := mock.Retriever{
		Contents: "this is a fake imooc.com"}
	r = &mockRetriever
	inspect(r)

	r = &real.Retriever{
		UserAgent: "Mozilla/5.0",
		TimeOut:   time.Minute,
	}
	inspect(r)

	// Type assertion
	if mockRetriever, ok := r.(*mock.Retriever); ok {
		fmt.Println(mockRetriever.Contents)
	} else {
		fmt.Println("r is not a mock retriever")
	}

	fmt.Println(
		"Try a session with mockRetriever")
	fmt.Println(session(&mockRetriever))
}

func inspect(r Retriever) {
	fmt.Println("Inspecting", r)
	fmt.Printf(" > Type:%T Value:%v\n", r, r)
	fmt.Print(" > Type switch: ")
	switch v := r.(type) {
	case *mock.Retriever:
		fmt.Println("Contents:", v.Contents)
	case *real.Retriever:
		fmt.Println("UserAgent:", v.UserAgent)
	}
	fmt.Println()
}

• retirever.go

package real

import (
	"net/http"
	"net/http/httputil"
	"time"
)

type Retriever struct {
	UserAgent string
	TimeOut   time.Duration
}

func (r *Retriever) Get(url string) string {
	resp, err := http.Get(url)
	if err != nil {
		panic(err)
	}

	result, err := httputil.DumpResponse(
		resp, true)

	resp.Body.Close()

	if err != nil {
		panic(err)
	}

	return string(result)
}

• mockretriever.go

package mock

import "fmt"

type Retriever struct {
	Contents string
}

func (r *Retriever) String() string {    // 系统接口String
	return fmt.Sprintf(
		"Retriever: {Contents=%s}", r.Contents)
}

func (r *Retriever) Post(url string,
	form map[string]string) string {
	r.Contents = form["contents"]
	return "ok"
}

func (r *Retriever) Get(url string) string {
	return r.Contents
}

---

四. 常用系统接口

• Stringer
• Reader/Writer