andre.go

package main

import (
	"encoding/json"
	"fmt"
)

type Thing1 struct {
	String string `json:"meow"`
}

func (Thing1) DoThing() {}

type Thing2 struct {
	Number int    `json:"number"`
	String string `json:"string"`
}

func (Thing2) DoThing() {}

type Bleh interface {
	DoThing()
}

type Holder struct {
	Type string `json:"type"`
	Doer Bleh
}

func (h *Holder) UnmarshalJSON(data []byte) error {
	type HolderPayload Holder
	aux := &struct {
		Payload json.RawMessage `json:"payload"`
		*HolderPayload
	}{
		HolderPayload: (*HolderPayload)(h),
	}

	if err := json.Unmarshal(data, &aux); err != nil {
		return err
	}

	knownTypes := map[string]Bleh{
		"Thing1": &Thing1{},
		"Thing2": &Thing2{},
	}

	t, ok := knownTypes[aux.Type]
	if !ok {
		return fmt.Errorf("unknown type: %s", aux.Type)
	}

	if err := json.Unmarshal(aux.Payload, t); err != nil {
		return err
	}

	h.Doer = t
	return nil
}

func thing1() {
	payload := `{
		"type": "Thing2",
		"payload": {
			"number": 42,
			"string": "once upon a time in a galaxy far far away"
		}
	}`

	h := &Holder{}
	if err := json.Unmarshal([]byte(payload), h); err != nil {
		fmt.Println(err)
	}
	fmt.Println(h)
	fmt.Println(h.Doer)
}

func thing2() {
	payload := `{
		"type": "Thing1",
		"payload": {
			"meow": "lols"
		}
	}`

	h := &Holder{}
	if err := json.Unmarshal([]byte(payload), h); err != nil {
		fmt.Println(err)
	}
	fmt.Println(h)
	fmt.Println(h.Doer)
}

func main() {
	thing1()
	thing2()
}