HttpClient异步任务管理

Question

我正在创建一个通用加载程序，我想启动一个HttpClient SendAsync请求。但是，其中一些请求可能需要时间，因此我希望添加取消选项，并在完成后通知。

这似乎是一个标准的场景-- imho。

我不确定这是否是正确的方法，但根据我看过的一些例子，这是我的立场。如果您查看代码的底部，我的问题是--在这一点上，是否检查响应并引发成功或错误事件？

    public bool StartFetch()
    {
        if (IsFetching) return false;
        IsFetching = true;

        mCancellationTokenSource = new CancellationTokenSource();

        // this is not awaited, so execution should continue
        StartTask(request, mCancellationTokenSource.Token);
        return true;
    }

    public bool CancelFetch()
    {
        // send cancellation
        if (mCancellationTokenSource != null)
            mCancellationTokenSource.Cancel();

        Cleanup();
        return true;
    }


    private async Task StartTask(LFHttpRequest request, CancellationToken cancellationToken)
    {
        var message = new HttpRequestMessage(request.Method, request.Uri);
        var response = await HttpClient.SendAsync(message, cancellationToken); 

        // at this point, do I take a look at response and raise a custom OnSuccess or OnError event???   

       // or do I want to grab the task from `SendAsync`, check for completed or faulted?
    }

Answer 1

当您查看与任务相关的状态(如IsFetching )时，只公开Task本身通常更干净、更容易。

就像这样：

public Task<T> FetchTask { get; private set; }

public bool StartFetch()
{
    if (FetchTask != null) return false;

    mCancellationTokenSource = new CancellationTokenSource();
    FetchTask = FetchAsync(request, mCancellationTokenSource.Token);
    return true;
}

public bool CancelFetch()
{
    // send cancellation
    if (mCancellationTokenSource != null)
        mCancellationTokenSource.Cancel();

    FetchTask = null;
    return true;
}


private async Task<T> FetchAsync(LFHttpRequest request, CancellationToken cancellationToken)
{
    var message = new HttpRequestMessage(request.Method, request.Uri);
    var response = await HttpClient.SendAsync(message, cancellationToken); 
    response.EnsureSuccessStatusCode();
    var ret = // Convert response.Content into T.
    return ret;
}

Answer 2

如果InvalidOperationException状态无效，建议为StartFetch和CancelFetch操作抛出IsFetching s。这可能看起来很烦人，但它让您在程序员错误和线程问题成为一个更大、更隐蔽的问题之前就可以捕捉到它们。

至于异步方法，您的方法应该返回一个结果。所以也许类似于private async Task<MyHttpResult> StartTask(...)。您的结果应该包含确定成功、失败和取消的方法。

例如：

public sealed class MyHttpResult
{
  public HttpResponse Result { get; private set; }
  public Exception Error { get; private set; }
  public bool WasCancelled { get; private set; }

  public MyHttpResult(HttpResponse result, Exception error, bool wasCancelled)
  {
    this.Result = result;
    this.Error = error;
    this.WasCancelled = wasCancelled;
  }
}

如果取消了许多异步方法，它们将抛出一个TaskCanceledException，因此您可以使用catch来表示它，如下所示：

async Task<MyHttpResult> StartTask(LFHttpRequest request, CancellationToken cancellationToken)
{
  var message = new HttpRequestMessage(new HttpMethod(request.Method), request.Uri);

  HttpResponse response = null;
  Exception lastError = null;
  bool wasCancelled = false;

  try
  {
    response = await MessageInvoker.SendAsync(message, cancellationToken);
  }
  catch(TaskCanceledException)
  {
    wasCancelled = true;
  }
  catch(Exception ex)
  {
    lastError = ex;
  }

  var result = new MyHttpResult(response, lastError, wasCancelled);
  return result;
}

这都是假设您的观察者也是调用者，所以他们可以await这个方法。如果不是这样的话，您对EventHandler的想法是有意义的。您可以创建如下所示的自定义EventArgs类，而不是返回结果：

public delegate void TaskResultEventHandler<T>(object sender, TaskResultEventArgs<T> e);

public sealed class TaskResultEventArgs<T> : EventArgs
{
      public T Result { get; private set; }
      public Exception Error { get; private set; }
      public bool WasCancelled { get; private set; }

      public TaskResultEventArgs(T result, Exception error, bool wasCancelled)
      {
        this.Result = result;
        this.Error = error;
        this.WasCancelled = wasCancelled;
      }
}

然后，这只是一个公开一个TaskResultEventHandler<HttpResponse>和您的观察者订阅它的问题。您可以这样调用它：

var handler = this.HttpTaskCompleted;

if(handler != null)
  handler(this, new TaskResultEventArgs<HttpResponse>(response, lastError, wasCancelled));

Answer 3

在你等待http电话之后

var response = await HttpClient.SendAsync(message, cancellationToken); 

您应测试是否取消：

if(cancellationToken.IsCancellationRequested)
   //... do what you want, throw or return false or null, depending on how you want to handle this cancellation.

或者您可以在一个调用中检查并抛出Microsoft异常：

cancel.ThrowIfCancellationRequested();