您需要使用完成块而不是返回值，因为 dataTask 闭包从您的方法返回后，异步运行，即稍后运行。您不想尝试立即返回该值（因为您还没有它）。你想（a）改变这个函数不返回任何东西，但是（b）提供一个完成处理程序闭包，你将在dataTask闭包中调用它来构建responseString。

You need to use completion block instead of returning value because the dataTask closure is run asynchronously, i.e. later, well after you return from your method. You don't want to try to return the value immediately (because you won't have it yet). You want to (a) change this function to not return anything, but (b) supply a completion handler closure, which you will call inside the dataTask closure, where you build responseString.

例如，您可以这样定义：

For example, you might define it like so:

public func HTTPRequest(dir: String, param: [String:String]? = nil, completionHandler: @escaping (String?, Error?) -> Void) {
    var urlString = HOST + dir

    if let param = param {
        let parameters = param.map { return $0.key.percentEscaped() + "=" + $0.value.percentEscaped() }
        urlString += "?" + parameters.joined(separator: "&")
    }

    let url = URL(string: urlString)

    let task = URLSession.shared.dataTask(with: url!) { data, response, error in
        guard let data = data, error == nil else {
            completionHandler(nil, error)
            return
        }
        let responseString = String(data: data, encoding: .utf8)
        completionHandler(responseString, nil)
    }
    task.resume()
}

注意，我是使用以下内容转义参数字典中的值的百分比：

Note, I'm percent escaping the values in the parameters dictionary using something like:

extension String {

    /// Percent escapes values to be added to a URL query as specified in RFC 3986
    ///
    /// This percent-escapes all characters besides the alphanumeric character set and "-", ".", "_", and "~".
    ///
    /// http://www.ietf.org/rfc/rfc3986.txt
    ///
    /// - Returns: Returns percent-escaped string.

    func percentEscaped() -> String {
        let allowedCharacters = CharacterSet(charactersIn: "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789-._~")

        return self.addingPercentEncoding(withAllowedCharacters: allowedCharacters)!
    }

}

然后你就叫它像这样：

HTTPRequest(dir: directory, param: parameterDictionary) { responseString, error in
    guard let responseString = responseString else {
        // handle the error here
        print("error: \(error)")
        return
    }

    // use `responseString` here

    DispatchQueue.main.async {
        // because this is called on background thread, if updating
        // UI, make sure to dispatch that back to the main queue.
    }
}

// but don't try to use `responseString` here