When I was working on gocoinex library for the first time, I needed some custom configuration on JSON unmarshalling process, I want to share my learnings with you here, but before that, we're going to have a recap on how we can unmarshall JSON in golang.
These are real world examples. (coinex exchange API)
First, we make our request like this:
raw_response, _ := http.Get("https://api.coinex.com/v1/market/list")
And we'll get a json object like this:
{
"code": 0,
"data": [
"LTCBCH",
"ETHBCH",
"ZECBCH",
"DASHBCH"
],
"message": "Ok"
}
But we should parse it,
There are different ways to parse it,
You can use NewDecoder or Unmarshal functions of json package,
and you can decode it to a struct or to a map[string]interface{}
It depends on your preferences, but in this case, I prefer NewDecoder and struct combination.
So we should make a struct like this:
type AllMarketList struct {
Code int `json:"code"`
Message string `json:"message"`
Data []string
}
Also we can have embedded structs, for example, we break last struct to two:
type GeneralResponse struct {
Code int `json:"code"`
Message string `json:"message"`
}
type AllMarketList struct {
GeneralResponse
Data []string
}
And there is no difference.
Finally, use NewDecoder to decode the raw_response to AllMarketList struct:
var allMarketList AllMarketList
json.NewDecoder(raw_response.Body).Decode(&allMarketList)
Completed code
package main
import (
"encoding/json"
"fmt"
"net/http"
)
type AllMarketList struct {
Code int `json:"code"`
Message string `json:"message"`
Data []string
}
func main() {
raw_response, _ := http.Get("https://api.coinex.com/v1/market/list")
var allMarketList AllMarketList
if err := json.NewDecoder(raw_response.Body).Decode(&allMarketList); err != nil {
fmt.Println(err)
}
defer raw_response.Body.Close()
fmt.Printf("%+v\n", allMarketList)
}
Example Two
Think we have a json like this:
{
"code": 0,
"data": {
"date": 1513865441609, # server time when returning
"ticker": {
"open": "10", # highest price
"last": "10.00", # latest price
"vol": "110" # 24H volume
}
},
"message" : "Ok"
}
We're going to improve some things in decoding process
- In this case, Unix timestamp can't be parsed, we should provide it a way.
- We want to remove "ticker" key and access to "open", "last", "vol" directly from "data".
- "last" should be exported to field named "Close"! (same happens with "vol" and "Volume".
- "open", "last", "vol" should be float, not string, but we'll leave them for next example.
Problems 1 and 2, can be solved by implementing UnmarshalJSON method on any struct we want to decode.
problem 3 will be easily solved with json tags. (I've mentioned it in the code below)
Our final struct should be like this:
// Final struct
type SingleMarketStatistics struct {
Code int `json:"code"`
Message string `json:"message"`
Data TickerData
}
// Inner struct that we should implement to solve problem 2
type TickerData struct {
ServerTime CTime `json:"date"` // CTime is short for CustomTime
Open float64 `json:"open"`
Close float64 `json:"last"` // Different Attribute name and tag name
Volume float64 `json:"vol"` // Different Attribute name and tag name
}
// Custome time
// Inner struct that we should implement to solve problem 1
type CTime struct {
time.Time
}
Custom time implementation
func (t *CTime) UnmarshalJSON(data []byte) error {
// Ignore null, like in the main JSON package.
if string(data) == "null" || string(data) == `""` {
return nil
}
// Fractional seconds are handled implicitly by Parse.
i, err := strconv.ParseInt(string(data), 10, 64)
update := time.UnixMilli(i)
*t = CTime{update}
return err
}
And we don't get an error anymore!
This method will be automatically used (thanks to interfaces!) whenever we want to decode a time to CTime!
Custom Data implementation
func (t *TickerData) UnmarshalJSON(data []byte) error {
if string(data) == "null" || string(data) == `""` {
return nil
}
// This is how this json really looks like.
var realTicker struct {
ServerTime CTime `json:"date"`
Ticker struct {
// tags also can be omitted when we're using UnmarshalJSON.
Open string `json:"open"`
Close string `json:"last"`
Volume string `json:"vol"`
} `json:"ticker"`
}
// Unmarshal the json into the realTicker struct.
if err := json.Unmarshal(data, &realTicker); err != nil {
return err
}
// Set the fields to the new struct,
// with any shape it has,
// or anyhow you want.
*t = TickerData{
ServerTime: realTicker.ServerTime,
Open: realTicker.Ticker.Open,
Close: realTicker.Ticker.Close,
Volume: realTicker.Ticker.Volume,
}
return nil
}
Now, just use NewDecoder as before, no change is needed.
var singleMarketStatistics SingleMarketStatistics
json.NewDecoder(raw_response.Body).Decode(&allMarketList)
Example Three
Imagine a JSON like this:
{
"asks": [ // This is a array of asks
[ // This is a array of ONE ask
"10.00", // Price of ONE ask
"0.999", // Amount of ONE ask
]
],
"bids": [ // Same structure as asks
[
"10.00",
"1.000",
]
]
}
As it's totally clear, in the non professional way, we should decode "asks" to [][]string and access first ask price like this asks[0][0] and amount asks[0][1]
Who remembers 0 is price and 1 is amount? What was which? 😄
So we'll manage them on the UnmarshalJSON method.
Also, we'll solve problem 4 of previous example that also exists here.
type BidAsk struct {
// Tags are not needed.
Price float64 `json:"price"` // Bid or Ask price
Amount float64 `json:"amount"` // Bid or Ask amount
}
func (t *BidAsk) UnmarshalJSON(data []byte) error {
// Ignore null, like in the main JSON package.
if string(data) == "null" || string(data) == `""` {
return nil
}
// Unmarshal to real type.
var bisask []string
if err := json.Unmarshal(data, &bisask); err != nil {
return err
}
// Change value type from string to float64.
price, err := strconv.ParseFloat(bisask[0], 64)
if err != nil {
return err
}
amount, err := strconv.ParseFloat(bisask[1], 64)
if err != nil {
return err
}
// Map old structure to new structure.
*t = BidAsk{
Price: price,
Amount: amount,
}
return err
}
type MarketDepth struct {
Asks []BidAsk `json:"asks"` // Ask depth
Bids []BidAsk `json:"bids"` // Bid depth
}
Again, we simply use:
var marketDepth MarketDepth
json.NewDecoder(raw_response.Body).Decode(&marketDepth)
And enjoy the beauty of your result:
for i, ask := range data.Data.Asks {
fmt.Printf("Ask %v\n", i)
fmt.Printf(" Price: %v\n", ask.Price) // here is the beauty
fmt.Printf(" Amount: %v\n", ask.Amount) // here is the beauty
fmt.Println()
}
for i, bid := range data.Data.Bids {
fmt.Printf("Bid %v\n", i)
fmt.Printf(" Price: %v\n", bid.Price) // here is the beauty
fmt.Printf(" Amount: %v\n", bid.Amount) // here is the beauty
fmt.Println()
}
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