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go语言的duck typing：

1. 具有python，cpp的duck typing的灵活性
2. 又具有java的类型检查

接口变量里有什么?

1. 实现者的类型
2. 实现者的值（或指针指向实现者）

注意：

1. 接口变量里自带指针，接口变量本身采用值传递，几乎不需要使用接口的指针
2. 指针接收者实现只能以指针方式使用；值接收者都可以

查看接口变量的两种方法：

1. Type Assertion
2. Type Switch

golang中表示任何类型：interface{}

1

1.1 retriever/mock/retriever.go:

package mock

import "fmt"

type Retriever struct {
    Contents string
}

func (r *Retriever) String() 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
}

1.2 retriever/real/retriever.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)
}

1.3 retriever/main.go:

package main

import (
    "fmt"
    "learngo/retriever/mock"
    "learngo/retriever/real"
    "time"
)

// 接口里面全是函数，不用加func关键字
type Retriever interface {
    Get(url string) string
}

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":   "ccmouse",
            "course": "golang",
        })
}

type RetrieverPoster interface {
    Retriever
    Poster
}

func session(s RetrieverPoster) string {
    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()
}

1.4 Output:

Inspecting Retriever: {Contents=this is a fake imooc.com}
 > Type:*mock.Retriever Value:Retriever: {Contents=this is a fake imooc.com}
 > Type switch: Contents: this is a fake imooc.com

Inspecting &{Mozilla/5.0 1m0s}
 > Type:*real.Retriever Value:&{Mozilla/5.0 1m0s}
 > Type switch: UserAgent: Mozilla/5.0

r is not a mock retriever
Try a session with mockRetriever
another faked imooc.com