Unmarshal json to reflected struct (continued)

Question

I want to write a gin middleware handler which gets data from c.Request.FormValue("data"), unmarshalls it into a structure (structures are rather different) and sets a variable in context (c.Set("Data",newP)). So I searched and wrote this:

package middleware

import (
    "reflect"
    "fmt"
    "github.com/gin-gonic/gin"
    "encoding/json"
)

//https://semaphoreci.com/community/tutorials/test-driven-development-of-go-web-applications-with-gin
//https://github.com/gin-gonic/gin/issues/420
func Data(t reflect.Type) gin.HandlerFunc {
    return func(c *gin.Context) {
        //https://stackoverflow.com/a/7855298/5257901
        //https://stackoverflow.com/a/45680060/5257901
        //t := reflect.TypeOf(orig)
        v := reflect.New(t.Elem())
        // reflected pointer
        newP := v.Interface()

        data:=c.Request.FormValue("data")
        fmt.Printf("%s data:%s\n",c.Request.URL.Path,data)
        if err:=json.Unmarshal([]byte(data),newP); err!=nil{
            fmt.Printf("%s data unmarshall %s, data(in quotes):\"%s\"",c.Request.URL.Path,err,data)
            c.Abort()
            return
        }
        ustr, _:=json.Marshal(newP)
        fmt.Printf("%s unmarshalled:%s\n",c.Request.URL.Path,ustr)
        c.Set("Data",newP)
        c.Next()
    }
}

and I use it like this:

func InitHandle(R *gin.Engine) {
    Plan := R.Group("/Plan")
    Plan.POST("/clickCreate",middleware.Data(reflect.TypeOf(new(tls.PlanTabel))), clickCreatePlanHandle)
}

and

var data = *(c.MustGet("Data").(*tls.PlanTabel))

which is rather heavy and ugly. I want to

middleware.Data(tls.PlanTabel{})

and

var data = c.MustGet("Data").(tls.PlanTabel)

In other words, omitting gin, I want a closure that eats i interface{} and returns a function (data string) (o interface{})

func Data(i interface{}) (func (string) (interface{})) {
    //some reflect magic goes here
    //extract the structure type from interface{} :

    //gets a reflect type pointer to it, like
    //t := reflect.TypeOf(orig)
    return func(data string) (o interface{}) {
        //new reflected structure (pointer?)
        v := reflect.New(t.Elem())
        //interface to it
        newP := v.Interface()
        //unmarshal
        json.Unmarshal([]byte(data),newP);
        //get the structure from the pointer back

        //returns interface to the structure

        //reflect magic ends
    }
}

Charlie Tumahai · Accepted Answer · 2018-02-06T06:33:14.373

2

The code in the question is close.

Try the following function. The resulting function returns the same type as the type of i:

func Data(i interface{}) func(string) (interface{}, error) {
    return func(data string) (interface{}, error) {
        v := reflect.New(reflect.TypeOf(i))
        err := json.Unmarshal([]byte(data), v.Interface())
        return v.Elem().Interface(), err
    }
}

Example use:

type Test struct {
    A string
    B string
}

f := Data((*Test)(nil))
v, err := f(`{"A": "hello", "B": "world"}`)
if err != nil {
    log.Fatal(err)
}
fmt.Printf("%#v\n", v) // v is a *Test

f = Data("")
v, err = f(`"Hello"`)
if err != nil {
    log.Fatal(err)
}
fmt.Printf("%#v\n", v)  // v is a string

If you want to return a struct value, then pass a struct value as the argument to Data:

f = Data(Test{})
v, err = f(`{"A": "hello", "B": "world"}`)
if err != nil {
    log.Fatal(err)
}
fmt.Printf("%#v\n", v) // v is a Test

Playground example

edited Feb 06 '18 at 06:33

answered Feb 06 '18 at 05:41

Charlie Tumahai

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Thanks. What was wrong with `return v.Elem().Interface()`? I liked it better. – Alexey Burdin Feb 06 '18 at 05:51
1

If the application is unmarshalling large `struct` values, then returning a pointer to the struct is usually better. I updated the question to show how to return the value as well. – Charlie Tumahai Feb 06 '18 at 05:53
1

@AlexeyBurdin See new and improved answer that return structs by value or by pointer depending on the argument to Data. – Charlie Tumahai Feb 06 '18 at 06:34

Unmarshal json to reflected struct (continued)

1 Answers1