How to Unmarshall JSON Data Dynamically Based on a Code Value?

Arbitrary Data Unmarshalling

In many scenarios, it's necessary to unmarshall JSON data in a flexible manner, allowing the unmarshalling process to adapt to different data structures based on a predetermined code value. This enables dynamic interpretation of data received from various sources.

For example, consider a JSON message that contains a "code" field, which specifies the type of data contained in the "payload" field. The "payload" field could represent a different data structure depending on the "code" value.

To achieve this, we can use the following technique:

<code class="go">package main

import (
    "encoding/json"
    "fmt"
)

type Message struct {
    Code    int
    Payload json.RawMessage // delay parsing until we know the code
}

// Define the possible data structures that can be unmarshalled from the "payload" field
type Range struct {
    Start int
    End   int
}
type User struct {
    ID   int
    Pass int
}

// Custom unmarshalling function
func MyUnmarshall(m []byte) {
    var message Message
    var payload interface{}
    json.Unmarshal(m, &message) // delay parsing until we know the color space

    // Determine the data structure based on the "code" field
    switch message.Code {
    case 3:
        payload = new(User)
    case 4:
        payload = new(Range)
    }

    // Unmarshall the "payload" field into the appropriate data structure
    json.Unmarshal(message.Payload, payload) //err check ommited for readability

    // Do something with the unmarshalled data
    fmt.Printf("\n%v%+v", message.Code, payload)
}

func main() {
    json := []byte(`{"Code": 4, "Payload": {"Start": 1, "End": 10}}`)
    MyUnmarshall(json)
    json = []byte(`{"Code": 3, "Payload": {"ID": 1, "Pass": 1234}}`)
    MyUnmarshall(json)
}</code>

In this example, the MyUnmarshall function receives a JSON message, stores the "code" and "payload" fields in a Message struct, and then dynamically creates a data structure instance based on the "code" value. It subsequently unmarshals the "payload" field into the appropriate structure and performs desired actions on the unmarshalled data.

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