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// Exporting Monzo transactions to my YouNeedABudget.com (i.e. YNAB)
// account. YNAB unfortunately doesn't currently offer an Monzo integration. As
// a workaround and a practical excuse to learn Go, I decided to write one
// myself.
//
// This job is going to run N times per 24 hours. Monzo offers webhooks for
// reacting to certain types of events. I don't expect I'll need realtime data
// for my YNAB integration. That may change, however, so it's worth noting.
package main
import (
"encoding/json"
"fmt"
"log"
"net/http"
"net/url"
"os"
"os/exec"
)
////////////////////////////////////////////////////////////////////////////////
// Constants
////////////////////////////////////////////////////////////////////////////////
var (
clientId = os.Getenv("monzo_client_id")
clientSecret = os.Getenv("monzo_client_secret")
)
const (
redirectURI = "http://localhost:8080/authorization-code"
// TODO(wpcarro): Consider generating a random string for the state when the
// application starts instead of hardcoding it here.
state = "xyz123"
)
////////////////////////////////////////////////////////////////////////////////
// Business Logic
////////////////////////////////////////////////////////////////////////////////
// This is the response returned from Monzo when we exchange our authorization
// code for an access token. While Monzo returns additional fields, I'm only
// interested in AccessToken and RefreshToken.
type accessTokenResponse struct {
AccessToken string `json:"access_token"`
RefreshToken string `json:"refresh_token"`
}
// TODO(wpcarro): Replace http.PostForm and other similar calls with
// client.postForm. The default http.Get and other methods doesn't timeout, so
// it's better to create a configured client with a value for the timeout.
func getAccessToken(code string) {
res, err := http.PostForm("https://api.monzo.com/oauth2/token", url.Values{
"grant_type": {"authorization_code"},
"client_id": {clientId},
"client_secret": {clientSecret},
"redirect_uri": {redirectURI},
"code": {code},
})
failOn(err)
defer res.Body.Close()
payload := accessTokenResponse{}
json.NewDecoder(res.Body).Decode(&payload)
log.Printf("Access token: %s\n", payload.AccessToken)
log.Printf("Refresh token: %s\n", payload.AccessToken)
}
func listenHttp(sigint chan os.Signal) {
// Use a go-routine to listen for interrupt signals to shutdown our HTTP
// server.
go func() {
<-sigint
// TODO(wpcarro): Do we need context here? I took this example from the
// example on golang.org.
log.Println("Warning: I should be shutting down and closing the connection here, but I'm not.")
close(sigint)
}()
log.Fatal(http.ListenAndServe(":8080", http.HandlerFunc(func(w http.ResponseWriter, req *http.Request) {
// 1. Get authorization code from Monzo.
if req.URL.Path == "/authorization-code" {
params := req.URL.Query()
reqState := params["state"][0]
code := params["code"][0]
if reqState != state {
log.Fatalf("Value for state returned by Monzo does not equal our state. %s != %s", reqState, state)
}
// TODO(wpcarro): Add a more interesting authorization confirmation
// screen -- or even nothing at all.
fmt.Fprintf(w, "Authorized!")
// Exchange the authorization code for an access token.
getAccessToken(code)
return
}
log.Printf("Unhandled request: %v\n", *req)
})))
}
// Open a web browser to allow the user to authorize this application.
// TODO(wpcarro): Prefer using an environment variable for the web browser
// instead of assuming it will be google-chrome.
func authorizeClient() {
url := fmt.Sprintf("https://auth.monzo.com/?client_id=%s&redirect_uri=%s&response_type=code&state=%s", clientId, redirectURI, state)
exec.Command("google-chrome", url).Start()
}
func main() {
sigint := make(chan os.Signal, 1)
// TODO(wpcarro): Remove state here. I'm using as a hack to prevent my
// program from halting before I'd like it to. Once I'm more comfortable
// using channels, this should be a trivial change.
state := make(chan bool)
authorizeClient()
listenHttp(sigint)
<-state
}
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