我如何基于类型进行json.Unmarshal

Question

I am writing a go client for the Flowdock API. Their API for Message has a number of attributes two of which are Event and Content

When Event = message then Content is a string. When Event = comment Content is a JSON object.

I want to defer unmarhsalling Content until it is needed. To do this I map RawContent in my struct and define a Content() method on Message to return the correct object.

Here is the code to illustrate:

package main

import (
    "fmt"
    "encoding/json"
)

// Message represents a Flowdock chat message.
type Message struct {
    Event            *string          `json:"event,omitempty"`
    RawContent       *json.RawMessage `json:"content,omitempty"`
}


func (m *Message) Content() (content interface{}) {
    // when m.Event is a message the RawContent is a string
    // real code handles unmarshaling other types (removed for this example)
    return string(*m.RawContent)
}

func main() {
    var message = &Message{}
    rawJSON := `{"event": "message", "content": "Howdy-Doo @Jackie #awesome"}`
    if err := json.Unmarshal([]byte(rawJSON), &message); err != nil {
        panic(err.Error())
    }

    event := "message"
    rawMessage := json.RawMessage("Howdy-Doo @Jackie #awesome")
    want := &Message{
        Event: &event,
        RawContent: &rawMessage,
    }

    fmt.Println(message.Content(), want.Content())
}

The result of running this is: http://play.golang.org/p/eds_AA6Aay

"Howdy-Doo @Jackie #awesome" Howdy-Doo @Jackie #awesome

Note: message.Content() and want.Content() are different!

At first I did not expect the quotes to be included in message but it makes sense because of how the JSON is parsed. It is just a slice of the whole rawJSON string.

So my questions are:

1. Should I just strip off the quotes?
2. If so what is the simplest way in Go to strip? maybe this: http://play.golang.org/p/kn8XKOqF0z lines(6, 19)?
3. Is this even the right approach to handling JSON that can have different types for the same attribute?

Here is a more complete example showing how I handle a JSON RawContent: http://play.golang.org/p/vrBJ5RYcql

Answer 1

Question 1:

No, you should json.Unmarshal the content also when it only contains a string. Apart from the quotes, JSON strings may also contain backslash-escaped control characters.

Question 2:

Because of the answer in Question 1, do the following for the "message" case:

var s string        
if err := json.Unmarshal([]byte(*m.RawContent), &s); err != nil {
    panic(err)
}
return s

Question 3:

It is a good approach to Unmarshal the event type string and use RawMessage to store the rest of the JSON until you've evaluated the type and know what kind of structure the content has.

You might want to consider also having a specific type also for just simple string contents, eg.:

type MessageContent string

This way you are free to implement methods on the type, allowing the Content method to return some other interface than just the empty interface{}.

Note:

Beware that, if you also json.Unmarshal the message string as I suggested, your Playground example will panic when trying to Unmarshal your non-quoted want.RawContent string, because it is not valid JSON.