使用Swift/SwiftyJSON解析JSON

我已经学习了一些关于这方面的教程,据我所知,我正在做的应该是有效的。我有以下来自api调用的json响应

{
    "Id": "1",
    "Name": "Test User",
    "Email": "test@email.com",
    "ProfileImage": null,
    "IsAdmin": true,
    "TakesJobs": false,
    "IsLocationUser": false,
    "IsCompanyAdmin": true,
    "LocationUsers": [],
    "CompanyUsers": [{
        "CompanyName": "Test Company",
        "Id": 6,
        "CompanyId": 5,
        "UserId": "1",
        "Admin": true,
        "TakesJobs": false,
        "UserName": null,
        "UserEmail": null,
        "AssignedJobs": null
    }]
}

本质上,我只想检查Id值是否为空。这是我使用的代码

res返回类型为JSON

ApiConnector.sharedInstance.login(emailText.text!, password: passwordText.text!) { (res) in
    if let id = res["Id"] as? String {
        if id != "" {

        }
        else
        {

        }

    }
}

我收到一条警告,说从“JSON”到无关类型“String”的转换总是失败。

要查看Id的值,我需要更改什么?

这是来自ApiConnector类的代码

func login(username: String, password: String, onCompletion: (JSON) -> Void) {
    let route = baseURL + "auth?Email=" + username + "&Password=" + password
    makeHTTPPostRequest(route, body: ["Email":username, "Password": password], onCompletion: { json, err in
        onCompletion(json as JSON)
        })
}