将通用JSON对象解码为多种格式之一 [英] Decoding generic JSON objects to one of many formats

问题描述

我正在Go中基于通用JSON的消息传递协议上工作.我想做的是有一个BaseMessage，它具有诸如Type，timestamp等一般信息.但是同时，我希望能够为某些类型的数据定义更具体的消息结构. /p>

例如:

type Message struct {
    Type      string `json:type`
    Timestamp string `json:timestamp`

}

type EventMessage struct {
    Message
    EventType string
    EventCreator string
    EventData interface{}
}

我有一组处理程序，并确定哪个处理程序应处理该消息，我首先将JSON解码为常规的Message类型，以检查Type字段.对于此示例，我将获得与事件"消息类型关联的处理程序.

然后我想在结构上声明EventMessage类型时遇到问题.

下面的代码很粗糙，但希望它能显示我对如何处理消息的一般想法.

type Handler func(msg Message) Message
handlers := make(map[string]Handler)

var msg Message
decoder.Decode(&msg)
handler := handlers[msg.Type]
handler(msg)

我尝试使用interface{}，但是JSON解码器仅创建了一个映射，然后我无法断言这两种类型.我已经找到了使之成为可能的变通办法，但是它非常丑陋，可能效率不高，而且很容易出错.我想让事情简单明了，以便可以轻松维护此代码.

在Go中是否有一种处理通用JSON对象的方法，以便解码后的JSON可以是许多结构格式之一?

我也曾想过在主Message结构中的Data interface{}中包含更多特定信息，但是随后遇到了一个同样的问题，即无法在接口上声明任何类型.必须有一种更好的方式来处理我刚丢失的JSON格式.

解决方案

一种解决方法是使用

游乐场示例

I am working on a general JSON based message passing protocol in Go. What I would like to do is have a BaseMessage that has general information like Type, timestamp, etc. But at the same time I want to be able to define more specific message structures for certain types of data.

For example:

type Message struct {
    Type      string `json:type`
    Timestamp string `json:timestamp`

}

type EventMessage struct {
    Message
    EventType string
    EventCreator string
    EventData interface{}
}

I have a set of handlers and to determine which handler should process the message I decode the JSON to the general Message type first to check the Type field. For this example I would get the handler associated with an "Event" message type.

I run into problems when I then want to assert the EventMessage type onto the structure.

The following code is very rough, but hopefully it displays my general idea of how I am trying to handle the messages.

type Handler func(msg Message) Message
handlers := make(map[string]Handler)

var msg Message
decoder.Decode(&msg)
handler := handlers[msg.Type]
handler(msg)

I have tried to use an interface{} but then the JSON decoder just creates a map which I then cant assert either type on. I have figured out workarounds that make it possible, but its very ugly, probably not efficient, and most likely error prone. I would like to keep things simple and straightforward so this code can be easily maintained.

Is there a method of handling generic JSON objects in Go so that the decoded JSON could be one of many struct formats?

I have also played with the idea of having more specific info in a Data interface{} in the main Message struct, but then I run into the same problem of not being able to assert any types onto the interface. There has to be a better way to handle JSON formats that I am just missing.

解决方案

One way to handle this is to define a struct for the fixed part of the message with a json.RawMessage field to capture the variant part of the message. Decode the json.RawMessage to a type specific to the variant:

type Message struct {
  Type      string `json:type`
  Timestamp string `json:timestamp`
  Data      json.RawMessage
}  

type Event struct {
   Type    string `json:type`
   Creator string `json:creator`
}


var m Message
if err := json.Unmarshal(data, &m); err != nil {
    log.Fatal(err)
}
switch m.Type {
case "event":
    var e Event
    if err := json.Unmarshal([]byte(m.Data), &e); err != nil {
        log.Fatal(err)
    }
    fmt.Println(m.Type, e.Type, e.Creator)
default:
    log.Fatal("bad message type")
}

playground example

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