How to tell the main thread that a URLSessionDataTask has finished

If I understood correctly you might try this solution:

spinner.startAnimation(self)btnOk.isEnabled = falsebtnCancel.isEnabled = falseattemptPost { (success) in    DispatchQueue.main.async {        spinner.stopAnimation(self)        btnOk.isEnabled = true        btnCancel.isEnabled = true    }   // UI wise, eventually you can do something with 'success'}func attemptPost(_ completion:@escaping (Bool)->())    let url = Constants.SERVICE_URL + "account/post"    let body: [String : Any] =        ["firstName": txtFirstName.stringValue,         "lastName": txtLastName.stringValue,         "email": txtEmail.stringValue,         "password": txtPassword.stringValue];    let req = Request.create(urlExtension: url, httpVerb: Constants.HTTP_POST, jsonBody: body)    let task = URLSession.shared.dataTask(with: req) { data, response, err in        guard let data = data, err == nil else {            completion(false)            return        }        if let resp = try? JSONSerialization.jsonObject(with: data) {            completion(true)        }    }    task.resume()}

so the idea is executing from attemptPost a block which will run asynchronously into the main thread your UI stuff