异步任务取消c#xamarin

Question

我有搜索用户的功能.我提供了一个textview,并在textview上更改了方法,我正在触发一个从Web服务器获取数据的方法.但是当用户输入字母时我遇到问题,因为在异步任务中完成所有api命中.服务应该在100毫秒的等待后被击中,意味着如果用户键入字母"a"然后没有键入100毫秒然后我们必须点击服务.但是如果用户输入"a"然后"b"然后输入"c",那么应该为"abc"命中一个服务,而不是所有服务.

我按照官方链接,但它没有帮助我 https://msdn.microsoft.com/en-us/library/jj155759.aspx

所以基本上这是我的代码

textview.TextChange+= (sender,e) =>{
  CancellationTokenSource cts = new CancellationTokenSource();
      await Task.Delay(500); 
      // here some where I have to pass cancel token
      var lst = await APIClient.Instance.GetUserSearch("/user/get?searchTerm=" + newText, "application/json",cts);
      if (lst != null && lst.Count > 0){ 
        lstSearch.AddRange(lst);
      }
  }

这是GetUser的方法

 public async Task<JResponse> GetUserSearch<JResponse>(string uri, string contentType,CancellationToken cts)
    {
        try
        {
            Console.Error.WriteLine("{0}", RestServiceBaseAddress + uri);

            string url = string.Format("{0}{1}", RestServiceBaseAddress, uri); 

            var request = (HttpWebRequest)WebRequest.Create(url);
            request.ContentType = contentType;
            if (Utility.CurrentUser != null && !string.IsNullOrWhiteSpace(Utility.CurrentUser.AuthToken))
            {
                request.Headers.Add("api_key", Utility.CurrentUser.AuthToken);
            }
            request.Method = "POST";

            var payload = body.ToString();

            request.ContentLength = payload.Length;

            byte[] byteArray = Encoding.UTF8.GetBytes(body.ToString());
            request.ContentLength = byteArray.Length;

            using (var stream = await request.GetRequestStreamAsync())
            {
                stream.Write(byteArray, 0, byteArray.Length);
                stream.Close();
            }
            using (var webResponse = await request.GetResponseAsync())
            {
                var response = (HttpWebResponse)webResponse;
                using (var reader1 = new StreamReader(response.GetResponseStream()))
                {
                    Console.WriteLine("Finished : {0}", uri);
                    var responseStr = reader1.ReadToEnd();  
                    var responseObj = JsonConvert.DeserializeObject<JResponse>(
                        responseStr,
                        new JsonSerializerSettings()
                        {
                            MissingMemberHandling = MissingMemberHandling.Ignore,
                            NullValueHandling = NullValueHandling.Ignore
                        });
                    return responseObj;
                }
            }
        }
        catch (System.Exception ex)
        {
            Utility.ExceptionHandler("APIClient", "ProcessRequestAsync", ex);
        }

        return default(JResponse);
    }

Answer 1

在您的示例中,您正在创建CancellationTokenSource- 您需要保留对它的引用,以便下次调用处理程序时,可以取消先前的搜索.这是一个你应该能够运行的示例控制台应用程序,但重要的部分是在处理程序中.

private CancellationTokenSource _cts;

private async void TextChangedHandler(string text)   // async void only for event handlers
{
    try
    {
        _cts?.Cancel();     // cancel previous search
    }
    catch (ObjectDisposedException)     // in case previous search completed
    {
    }

    using (_cts = new CancellationTokenSource())
    {
        try
        {
            await Task.Delay(TimeSpan.FromSeconds(1), _cts.Token);  // buffer

            var users = await _userService.SearchUsersAsync(text, _cts.Token);
            Console.WriteLine($"Got users with IDs: {string.Join(", ", users)}");
        }
        catch (TaskCanceledException)       // if the operation is cancelled, do nothing
        {
        }
    }
}

一定要传递CancellationToken到所有的async方法,包括那些进行网络请求,这样,你的信号撤销权降到最低水平.