How to Deserialize JSON Websocket Messages as Union Types in Go?

Deserializing JSON Websocket Messages as Union Types in Go

In Go, the gorilla websocket library is commonly used for handling websocket connections. However, when using JSON for serialization and deserialization, handling incoming messages of varying types presents a challenge.

Consider the following example where you have structs for message types "Foo" and "Bar":

type Foo struct {
    A string `json:"a"`
    B string `json:"b"`
}

type Bar struct {
    C string `json:"c"`
    D string `json:"d"`
}

Gorilla's conn.ReadJSON function allows you to deserialize incoming JSON messages into specific structs. However, you would need to use separate conn.ReadJSON(Foo) and conn.ReadJSON(Bar) calls to handle messages of different types, which is inefficient and cumbersome.

To address this, you can use an intermediary struct that contains a control field and a field to hold the actual message data:

type Messages struct {
    Control string `json:"control"`
    X json.RawMessage
}

The Control field indicates the payload's type, and X holds the raw JSON data. To deserialize incoming messages using this approach:

var m Messages
err := c.ReadJSON(&m)
if err != nil {
    // Handle error
}

switch m.Control {
case "Foo":
    var foo Foo
    if err := json.Unmarshal([]byte(m.X), &foo); err != nil {
        // Handle error
    }
    // Do something with foo

case "Bar":
    // Follow the same pattern for handling Bar
}

This solution allows you to deserialize incoming messages regardless of their type, using the RawMessage interface type in json.RawMessage. The switch statement checks the control field to determine the actual message type and deserialize accordingly.

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