如何使用多种方法将中间件写入接口？

Question

I got a interface with more than one method. I wonder how to write a middleware for it.

I seek in Google but found all the answers are for interface with only one method. I found nothing for my problem. And I try to write a demo, but it does not work.

package main

import (
    "fmt"
    "strconv"
)

type ti interface {
    Say(int) string
    Eat(string) int
}

type Middleware func(ti) ti

func GetMiddleWare(t ti) ti {
    var tm ti
    t.Say = func(i int) string {
        fmt.Println("arg is " + strconv.Itoa(i))
        var ret string
        defer func() {
            fmt.Println("ret is " + ret)
        }()
        ret = t.Say(i)
        return ret
    }
    t.Eat = func(s string) int {
        fmt.Println("arg is " + s)
        var ret int
        defer func() {
            fmt.Println("ret is " + strconv.Itoa(ret))
        }()
        ret = t.Eat(s)
        return ret
    }
    return tm
}

it does not work

.\main.go:17:8: cannot assign to t.Say
.\main.go:26:8: cannot assign to t.Eat

So, how can I write a middleware for an interface with more than one method?

Answer 1

Define a type that wraps the value. Implement the interface methods on that type.

// Middleware wraps the value t with logging.
type Middleware struct {
    t ti
}

func (m Middleware) Say(i int) string {
    fmt.Println("arg is " + strconv.Itoa(i))
    var ret string
    defer func() {
        fmt.Println("ret is " + ret)
    }()
    ret = m.t.Say(i)
    return ret
}

func (m Middleware) Eat(s string) int {
    fmt.Println("arg is " + s)
    var ret int
    defer func() {
        fmt.Println("ret is " + strconv.Itoa(ret))
    }()
    ret = m.t.Eat(s)
    return ret
}

func GetMiddleWare(t ti) ti {
    return Middleware{t}
}