mapstructure 如何使用鉴别器来解码具体类型

Question

The docs ( https://github.com/mitchellh/mapstructure ) mention the benefits of mapstructure for decoding

{
  "type": "person",
  "name": "Mitchell"
}

I'm looking for an example that show the decoding process, eg to unmarshal either a Person or Pet class

Answer 1

That library only maps maps to structures. It does not do type-specific decoding. Quoting from its doc:

Perhaps we can't populate a specific structure without first reading the "type" field from the JSON. We could always do two passes over the decoding of the JSON (reading the "type" first, and the rest later). However, it is much simpler to just decode this into a map[string]interface{} structure, read the "type" key, then use something like this library to decode it into the proper structure.

All it offers is that you only have to parse the JSON source once, into a map, then you can decide yourself what type you need to map or unmarshal into. Then you can use the already constructed map to fill the type you need.

See this example:

type Person struct {
    Name string `json:"name"`
}

type Credentials struct {
    User     string `json:"user"`
    Password string `json:"password"`
}

func main() {
    for _, src := range []string{srcPerson, srcCredentials} {
        var m map[string]interface{}
        if err := json.Unmarshal([]byte(src), &m); err != nil {
            panic(err)
        }

        switch m["type"] {
        case "person":
            var p Person
            if err := mapstructure.Decode(m, &p); err != nil {
                panic(err)
            }
            fmt.Printf("%T %+v\n", p, p)
        case "credentials":
            var c Credentials
            if err := mapstructure.Decode(m, &c); err != nil {
                panic(err)
            }
            fmt.Printf("%T %+v\n", c, c)
        }
    }
}

const srcPerson = `{
  "type": "person",
  "name": "Mitchell"
}`

const srcCredentials = `{
  "type": "credentials",
  "user": "bob",
  "password": "secret"
}`

Output:

main.Person {Name:Mitchell}
main.Credentials {User:bob Password:secret}