Unmarshal JSON Messages

[swarmt]

Hi There! Excellent library.

Wondering if there is a simple way to unmarshal ROS messages in JSON format without modifying the autogenerated goroslib messages. The timestamp in the header needs to be converted.

{
  "header": {
    "seq": 30,
    "stamp": {
      "secs": 1645759057,
      "nsecs": 548703427
    },
    "frame_id": "world"
  },
}

type Header struct {
	msg.Package `ros:"std_msgs"`
	Seq         uint32
	Stamp       time.Time
	FrameId     string
}

[aler9]

You can convert any ROS message to the JSON format in this way:

type Header struct {
	msg.Package `ros:"std_msgs"`
	Seq         uint32
	Stamp       time.Time
	FrameId     string
}

b, err := json.Marshal(&Header{
	Seq: 33,
	Stamp: time.Now(),
	FrameId: "asd",
})
if err != nil {
	panic(err)
}

resulting in

{"Package":0,"Seq":30,"Stamp":"2022-02-25T03:17:37.548703Z","FrameId":"world"}

The problem of your JSON format is that:

• it uses keys in snake case instead of camel case
• it represents time in a non-standard way

Therefore, if you still want to use it, you have take into consideration these changes:

package main

import (
	"encoding/json"
	"fmt"
	"time"
	"unicode"

	"github.com/aler9/goroslib/pkg/msgs/std_msgs"
)

func snakeToCamel(in string) string {
	tmp := []rune(in)
	tmp[0] = unicode.ToUpper(tmp[0])
	for i := 0; i < len(tmp); i++ {
		if tmp[i] == '_' {
			tmp[i+1] = unicode.ToUpper(tmp[i+1])
			tmp = append(tmp[:i], tmp[i+1:]...)
			i--
		}
	}
	return string(tmp)
}

func main() {
	var b = []byte(`{` +
		`"seq": 30,` +
		`"stamp": {` +
		`  "secs": 1645759057,` +
		`  "nsecs": 548703427` +
		`},` +
		`"frame_id": "world"` +
		`}`)

	var m map[string]interface{}
	err := json.Unmarshal(b, &m)
	if err != nil {
		panic(err)
	}

	newm := map[string]interface{}{}
	for key, val := range m {
		// convert time from secs / nsec to RFC3339
		if key == "stamp" {
			secs := val.(map[string]interface{})["secs"].(float64)
			nsecs := val.(map[string]interface{})["nsecs"].(float64)
			val = time.UnixMicro(int64(secs*1000000 + nsecs/1000))
		}

		// convert keys from snake case to camel case
		newm[snakeToCamel(key)] = val
	}

	b, err = json.Marshal(newm)
	if err != nil {
		panic(err)
	}

	var h std_msgs.Header
	err = json.Unmarshal(b, &h)
	if err != nil {
		panic(err)
	}

	fmt.Println(h)
}

[swarmt]

Excellent thanks for the help.