c# - 如何在返回前等待回调

Question

我正在尝试将代码从 .Net 移植到 Unity C#，但我坚持使用包含回调的语法。

基本上，我必须用 this one 替换 .Net 'HttpClient' 库，也称为“HttpClient”。但是“获取”语法不一样，它使用回调。我对 C# 和 Http 查询还很陌生，不知道如何处理这种语法以获得预期的返回。

用.Net写的原函数:

internal static JObject GetToBackOffice(string action)
{
    var url = backOfficeUrl;
    url = url + action;

    HttpClient httpClient = new HttpClient();

    var response =
        httpClient.GetAsync(url
        ).Result;
    if (response.StatusCode == System.Net.HttpStatusCode.OK)
    {
        var idProcess = Newtonsoft.Json.Linq.JObject.Parse(response.Content.ReadAsStringAsync().Result);

        return idProcess;
    }
    else
        return null;

我正在为 Unity 编写的 C# 代码:

internal class Utils
{ 
    internal static JObject GetToBackOffice(string action)
    {
        var url = backOfficeUrl;
        url = url + action;

        HttpClient httpClient = new HttpClient();

        JObject idProcess = new JObject();

        httpClient.GetString(new Uri(url),
            (response) =>
            {
                // Raised when the download completes
                if (response.StatusCode == System.Net.HttpStatusCode.OK)
                {
                    idProcess = Newtonsoft.Json.Linq.JObject.Parse(response.Data);
                }
                else
                {
                    idProcess = null;
                }
            });
        // Here I would like to wait for the response, so that idProcess is filled with the received data before returning
        return idProcess;
    } 

public class Action
{
     public bool SendData(string id, string secretKey, FakeData data)
    {
        var idProcess = Utils.GetToBackOffice(String.Format("Events/{0}/infos",id));
        //...I do then something with idProcess
        //Currently, when I use idProcess here, it is still empty since the GetString response hasn't been received yet when this line is executed

        return true;
    }
}

public class EventHubSimulator : MonoBehaviour
{
    void Start()
    {
        //Fill the parameters (I skip the details)
        string oId = ...;
        string secretKey = ...;
        var vh = ...;

        Action action = new Action();
        action.SendData(oId, secretKey, vh);
    }
}

我的问题是，在 GetToBackOffice 函数之后，我的代码直接将“idProcess”用于其他用途，但该对象为空，因为尚未收到响应。我想在我的函数返回之前等待响应。

希望我说得足够清楚。我知道已经发布了类似的问题，但找不到针对我的具体问题的解决方案。

---

编辑:

最后我按照 Nain 的建议使用了协程，但无法按照他所说的方式得到我期望的结果。这种方法似乎可行(尽管这可能不是一个好方法)。

public class EventHubSimulator : MonoBehaviour
    {
        void Start()
        {
            //Fill the parameters (I skip the details)
            string oId = ...;
            string secretKey = ...;
            var vh = ...;

            Utils utils = new Utils();
            StartCoroutine(utils.SendData(oId, secretKey, vh));
        }
    }

public class Utils: MonoBehaviour
{
    private const string backOfficeUrl = "http://myurl/api/";

    public CI.HttpClient.HttpResponseMessage<string> response;

    public IEnumerator SendData(string id, string secretKey, FakeData data)
    {
        response = null;
        yield return GetToBackOffice(String.Format("Events/{0}/infos", id)); //Make a Http Get request
        //The next lines are executed once the response has been received
        //Do something with response
        Foo(response);
    }

    IEnumerator GetToBackOffice(string action)
    {
        var url = backOfficeUrl;
        url = url + action;

        //Make a Http Get request
        HttpClient httpClient = new HttpClient();
        httpClient.GetString(new Uri(url), (r) =>
        {
            // Raised when the download completes
            if (r.StatusCode == System.Net.HttpStatusCode.OK)
            {
                //Once the response has been received, write it in the global variable
                response = r;
                Debug.Log("Response received : " + response);
            }
            else
            {
                Debug.Log("ERROR =============================================");
                Debug.Log(r.ReasonPhrase);
                throw new Exception(r.ReasonPhrase);
            }
        });

        //Wait for the response to be received
        yield return WaitForResponse();
        Debug.Log("GetToBackOffice coroutine end ");
    }

    IEnumerator WaitForResponse()
    {
        Debug.Log("WaitForResponse Coroutine started");
        //Wait for response to become be assigned
        while (response == null) 
        {
            yield return new WaitForSeconds(0.02f);
        }
        Debug.Log("WaitForResponse Coroutine ended");
    }
}    

Answer 1

一种解决方案是在 Nain 作为答案提交时轮询完成。如果您不想轮询，可以使用 TaskCompletionSource。 This Q&A更深入地探讨了原因和方式。

你的代码可以这样写:

async Task CallerMethod()
{
    JObject result = await GetToBackOffice(...);
    // Do something with result
}

internal static Task<JObject> GetToBackOffice(string action)
{
    var tsc = new TaskCompletionSource<JObject>();
    var url = backOfficeUrl;
    url = url + action;

    HttpClient httpClient = new HttpClient();

    JObject idProcess = new JObject();

    httpClient.GetString(new Uri(url),
        (response) =>
        {
            // Raised when the download completes
            if (response.StatusCode == System.Net.HttpStatusCode.OK)
            {
                tsc.SetResult(Newtonsoft.Json.Linq.JObject.Parse(response.Data));
            }
            else
            {
                tsc.SetResult(null);
            }
        });

    return tsc.Task;
}

另请参阅 this 的异步方法调用协程并等待完成部分msdn 博文。

注意 任务和异步/等待支持仅在 beta functionality 时可用在团结中。另见 this发布。