Golang を使用して Web アプリケーションに WeChat 認証ログインを実装する方法

https://open.weixin.qq.com/connect/oauth2/authorize?appid=APPID
&redirect_uri=REDIRECT_URI
&response_type=code
&scope=snsapi_userinfo
&state=STATE#wechat_redirect

func buildAuthURL(appID, redirectURI, state string) string {
    values := make(url.Values)
    values.Set("appid", appID)
    values.Set("redirect_uri", redirectURI)
    values.Set("response_type", "code")
    values.Set("scope", "snsapi_userinfo")
    values.Set("state", state)
    return "https://open.weixin.qq.com/connect/oauth2/authorize?" + values.Encode() + "#wechat_redirect"
}

func getAccessToken(appID, appSecret, code string) (string, error) {
    url := "https://api.weixin.qq.com/sns/oauth2/access_token?appid=" + appID +
        "&secret=" + appSecret + "&code=" + code + "&grant_type=authorization_code"

    resp, err := http.Get(url)
    if err != nil {
        return "", err
    }
    defer resp.Body.Close()

    body, err := ioutil.ReadAll(resp.Body)
    if err != nil {
        return "", err
    }

    var data struct {
        AccessToken string `json:"access_token"`
        ExpiresIn   int    `json:"expires_in"`
        OpenID      string `json:"openid"`
        Scope       string `json:"scope"`
    }

    if err := json.Unmarshal(body, &data); err != nil {
        return "", err
    }

    return data.AccessToken, nil
}

func getUserInfo(accessToken, openID string) (*userInfo, error) {
    url := "https://api.weixin.qq.com/sns/userinfo?access_token=" + accessToken + "&openid=" + openID

    resp, err := http.Get(url)
    if err != nil {
        return nil, err
    }
    defer resp.Body.Close()

    body, err := ioutil.ReadAll(resp.Body)
    if err != nil {
        return nil, err
    }

    var user userInfo

    if err := json.Unmarshal(body, &user); err != nil {
        return nil, err
    }

    return &user, nil
}

func wxLoginHandler(w http.ResponseWriter, r *http.Request) {
    appID := "your app id"
    appSecret := "your app secret"
    state := "random string"
    redirectURI := url.QueryEscape("http://your_server_url/callback")

    if r.Method == "GET" {
        // Redirect to Wechat login page
        http.Redirect(w, r, buildAuthURL(appID, redirectURI, state), 302)
    } else if r.Method == "POST" {
        // Get user info after login succeeds
        code := r.FormValue("code")
        if code == "" {
            http.Error(w, "Missing code parameter", http.StatusBadRequest)
            return
        }

        accessToken, err := getAccessToken(appID, appSecret, code)
        if err != nil {
            http.Error(w, "Failed to get access token", http.StatusInternalServerError)
            return
        }

        user, err := getUserInfo(accessToken, openID)
        if err != nil {
            http.Error(w, "Failed to get user info", http.StatusInternalServerError)
            return
        }

        // Do something with user info
        fmt.Fprintf(w, "Hello, %s!", user.Nickname)
    } else {
        http.Error(w, "Invalid request method", http.StatusMethodNotAllowed)
        return
    }
}