如何使用 Go 中接口数组中的值初始化结构？

Question

I'm getting a message from server like [0,"on",[6,1,5,"market",45.7]] and save it to []interface{} variable. I want to initialize struct with values of this array. I'm totally new in Go and try to do it like:

import "golang.org/x/net/websocket"
...
var msg []interface{}
// Server send response: `[0,"on",[6,1,5,"market",45.7]]`
if err := websocket.Message.Receive(ws, &msg); err != nil {
    logger.Println(err)
} else {
    type Order struct {
        ID int32,
        GID int32,
        CID int32,
        Type string,
        Amount float64
    }
    
    // here msg is [0,"on",[6,1,5,"market",45.7]]
    
    switch msg[1] {
    case "on":
        if rawOrder, ok := msg[2].([]interface{}); ok {
            order := Order{int32(rawOrder[0]), int32(rawOrder[1]), int32(rawOrder[2]), string(rawOrder[3]), float64(rawOrder[4])}
        }
}

But I'm getting an error "Cannot convert an expression of the type 'interface{}' to the type 'int32'" and the next step is use switch for every rawOrder[i] type, but it's toooo long. How can I do it easilly?

Answer 1

If you know that the codec used on the websocket will always be json, you can formally define Order and give it an UnmarshalJSON function to do the decoding for you.

import "golang.org/x/net/websocket"

type Order struct {
    ID, GID, CID int32
    Type         string
    Amount       float64
}

func (o *Order) UnmarshalJSON(data []byte) error {
    var first []json.RawMessage
    err := json.Unmarshal(data, &first)
    if err != nil {
        return fmt.Errorf("invalid order, must be array: %w", err)
    }
    if len(first) != 3 {
        return fmt.Errorf("invalid order, length must be 3, got %d", len(first))
    }

    var second string
    err = json.Unmarshal(first[1], &second)
    if err != nil {
        return fmt.Errorf("invalid order, second element must be string: %w", err)
    }

    switch second {
    case "on":
        var third []json.RawMessage
        err = json.Unmarshal(first[2], &third)
        if err != nil {
            return fmt.Errorf("invalid order, third element must be array: %w", err)
        }

        if len(third) != 5 {
            return fmt.Errorf("invalid order, element 3 length must be 5, got %d", len(third))
        }

        for i, f := range []interface{}{&o.ID, &o.GID, &o.CID, &o.Type, &o.Amount} {
            err = json.Unmarshal(third[i], f)
            if err != nil {
                return fmt.Errorf("invalid order, wrong type for element 3[%d]: %w", i, err)
            }
        }
        return nil
    }

    return fmt.Errorf("invalid order, unknown type %q", second)
}

...
var msg *Order
// Server send response: `[0,"on",[6,1,5,"market",45.7]]`
if err := websocket.JSON.Receive(ws, &msg); err != nil {
    logger.Println(err)
}

// msg is now an &Order{ID:6, GID:1, CID:5, Type:"market", Amount:45.7}

}

The reason the UnmarshalJSON function is huge is because your API is bad. If you control the Server; then you should avoid using mixed types in the same array, and you should avoid using arrays for relational data.