Unity调用Golang生成的Agora令牌时遇重复发送请求错误

场景还原 #

用Golang搭建本地服务生成Agora令牌，Chrome访问http://localhost:8080/rte/agora/publisher/uid/1234/能正常获取RTC和RTM令牌，但在Unity中调用接口时触发错误。

相关代码 #

令牌请求工具类 #

public static class HelperClass
{
    public static IEnumerator FetchToken(string url, string channel, int userId, Action<string> callback = null) 
    {
        Debug.Log("Hitting Web Request");
        UnityWebRequest request = UnityWebRequest.Get(string.Format("{0}/rtc/{1}/publisher/uid/{2}/", url, channel, userId));
           
        yield return request.SendWebRequest();

        if (request.isNetworkError || request.isHttpError) 
        {
            Debug.Log("request"+request.error);
            callback(null);
            yield break;
        }

       TokenObject tokenInfo = JsonUtility.FromJson<TokenObject>(request.downloadHandler.text);

        callback(tokenInfo.rtcToken);
    }
}

请求基础URL #

internal static string _tokenBase = "http://localhost:8080/rte/agora/publisher/uid/";

调用方式 #

StartCoroutine(HelperClass.FetchToken(_tokenBase, _channelName, 0, this.RenewOrJoinToken));

报错信息 #

InvalidOperationException: UnityWebRequest has already been sent; cannot begin sending the request again
UnityEngine.Networking.UnityWebRequest.SendWebRequest () (at /Users/bokken/buildslave/unity/build/Modules/UnityWebRequest/Public/UnityWebRequest.bindings.cs:266)
Agora.Util.HelperClass+<FetchToken>d__0.MoveNext () (at Assets/Agora-RTC-Plugin/API-Example/Tools/RequestToken.cs:27)
UnityEngine.SetupCoroutine.InvokeMoveNext (System.Collections.IEnumerator enumerator, System.IntPtr returnValueAddress) (at /Users/bokken/buildslave/unity/build/Runtime/Export/Scripting/Coroutines.cs:17)

问题原因 #

错误核心是同一个UnityWebRequest实例被多次调用SendWebRequest()，通常由以下两种情况导致：

1. FetchToken协程被重复启动，同一个协程实例被多次执行，导致内部的request被重复发送
2. 回调方法RenewOrJoinToken中再次触发了FetchToken调用，形成循环调用链

解决方案 #

1. 确保每次请求自动释放资源 #

使用using包裹UnityWebRequest，请求完成后自动释放资源，避免实例残留：

public static class HelperClass
{
    public static IEnumerator FetchToken(string url, string channel, int userId, Action<string> callback = null) 
    {
        Debug.Log("Hitting Web Request");
        // using自动释放request资源
        using (UnityWebRequest request = UnityWebRequest.Get(string.Format("{0}/rtc/{1}/publisher/uid/{2}/", url, channel, userId)))
        {
            yield return request.SendWebRequest();

            if (request.isNetworkError || request.isHttpError) 
            {
                Debug.Log("request"+request.error);
                callback?.Invoke(null);
                yield break;
            }

            TokenObject tokenInfo = JsonUtility.FromJson<TokenObject>(request.downloadHandler.text);
            callback?.Invoke(tokenInfo.rtcToken);
        }
    }
}

2. 避免协程重复启动 #

保存协程引用，启动新协程前先停止旧的，防止重复执行：

// 在调用协程的类中声明变量保存协程引用
private Coroutine _tokenFetchCoroutine;

// 调用时先停止旧协程
if (_tokenFetchCoroutine != null)
{
    StopCoroutine(_tokenFetchCoroutine);
}
_tokenFetchCoroutine = StartCoroutine(HelperClass.FetchToken(_tokenBase, _channelName, 0, this.RenewOrJoinToken));

3. 检查回调逻辑 #

排查RenewOrJoinToken方法，确保没有在回调中再次调用FetchToken，避免循环触发请求。

内容的提问来源于stack exchange，提问作者Ziva