异步任务取消 C# Xamarin

我有一个搜索用户的功能。我提供了一个文本视图，在该文本视图更改方法上，我正在触发一种从 Web 服务器获取数据的方法。但是当用户键入字母时，我遇到了问题，因为所有 api 命中都是在异步任务中完成的。服务应该在等待 100 毫秒后命中，这意味着如果用户键入字母"a"然后 100 毫秒内不输入，那么我们必须命中服务。但是，如果用户键入"a"然后键入"b"然后键入"c"，则应为"abc"命中一个服务，而不是针对所有服务。

我关注了官方链接，但它对我没有帮助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);
      }
  }

这是我获取用户的方法

 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);
    }

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
        {
        }
    }
}

textview.TextChange += (sender,e) => 
{
   if (_fieldChangeTimer == null)
     _fieldChangeTimer = new Timer(delegate
     {
                PerformeSearch();
     }, null, 700, Timeout.Infinite);
   else
   {
     _fieldChangeTimer.Change(700, Timeout.Infinite);
   }
};

private void cancelButton_Click(object sender, RoutedEventArgs e)  
{  
    if (cts != null)  
    {  
        cts.Cancel();  
    }  
}