如何访问json解码上的接口字段？

Question

I have a json document and I'm using a client which decodes the document in an interface (instead of struct) as below:

var jsonR interface{}
err = json.Unmarshal(res, &jsonR)

How can I access the interface fields? I've read the go doc and blog but my head still can't get it. Their example seem to show only that you can decode the json in an interface but doesn't explain how its fields can be used.

I've tried to use a range loop but it seems the story ends when I reach a map[string]interface. The fields that I need seem to be in the interface.

for k, v := range jsonR {
    if k == "topfield" {
        fmt.Printf("k  is %v, v is %v", k, v)

    }

}

Answer 1

The value inside the interface depends on the json structure you're parsing. If you have a json dictionary, the dynamic type of jsonR will be: map[string]interface{}.

Here's an example.

package main

import (
    "encoding/json"
    "fmt"
    "log"
)

func main() {
    a := []byte(`{"topfield": 123}`)
    var v interface{}
    if err := json.Unmarshal(a, &v); err != nil {
        log.Fatalf("unmarshal failed: %s", err)
    }
    fmt.Printf("value is %v", v.(map[string]interface{})["topfield"])
}