[英]Waiting on an IAsyncResult method that waits on another IAsyncResult (Chaining)
(只能使用.NET 3.5庫存,所以沒有任務,沒有Reactive Extensions)
我有,我認為是一個簡單的案例,但我很困惑。
缺點是,我將BeginGetRequestStream的IAsyncResult返回給BeginMyOperation()的調用者,我想真正發送回BeginGetResponse的IAsyncResult,它在調用EndGetRequestStream時調用。
所以我想知道,我該怎么做
public IAsyncResult BeginMyOperation(...)
{
HttpWebRequest webRequest = (HttpWebRequest)WebRequest.Create(requestUri);
webRequest.Method = "POST";
// This is the part, that puzzles me. I don't want to send this IAsyncResult back.
return webRequest.BeginGetRequestStream(this.UploadingStreamCallback, state);
}
// Only want this to be called when the EndGetResponse is ready.
public void EndMyOperation(IAsyncResult ar)
{
}
private IAsyncResult UploadingStreamCallback(IAsyncResult asyncResult)
{
using (var s = state.WebRequest.EndGetRequestStream(asyncResult))
{
using (var r = new BinaryReader(state.Request.RequestData))
{
byte[] uploadBuffer = new byte[UploadBufferSize];
int bytesRead;
do
{
bytesRead = r.Read(uploadBuffer, 0, UploadBufferSize);
if (bytesRead > 0)
{
s.Write(uploadBuffer, 0, bytesRead);
}
}
while (bytesRead > 0);
}
}
// I really want to return this IAsyncResult to the caller of BeginMyOperation
return state.WebRequest.BeginGetResponse(new AsyncCallback(state.Callback), state);
}
我認為解決這個問題的最簡單方法是使用Task
包裝器。 特別是,您可以在BeginGetResponse
完成時完成TaskCompletionSource
。 然后只返回TaskCompletionSource
的Task
。 請注意, Task
實現了IAsyncResult
,因此您的客戶端代碼不必更改。
就個人而言,我會更進一步:
Task
包裝BeginGetRequestStream
(使用FromAsync
)。 Task
創建一個繼續,該Task
處理請求並在Task
包裝BeginGetResponse
(同樣,使用FromAsync
)。 TaskCompletionSource
第二個Task
創建一個延續。 恕我直言,異常和結果值更自然地處理Task
總比IAsyncResult
。
我意識到這個問題差不多有一年了,但如果提問者的約束仍然存在,.NET 3.5上有一個選項可以輕松地組成異步操作。 看看Jeff Richter的PowerThreading庫 。 在Wintellect.PowerThreading.AsyncProgModel
命名空間中,您將找到AsyncEnumerator
類的幾種變體,您可以將它們與序列生成器一起使用來編寫異步代碼,就好像它是順序的一樣。
它的要點是您將異步代碼編寫為返回IEnumerator<int>
的序列生成器的主體,並且每當調用異步方法時,您都會發出一個帶有等待的異步操作數的yield return
。 圖書館處理血腥細節。
例如,要將一些數據發布到url並返回結果的內容:
public IAsyncResult BeginPostData(string url, string content, AsyncCallback callback, object state)
{
var ae = new AsyncEnumerator<string>();
return ae.BeginExecute(PostData(ae, url, content), callback, state);
}
public string EndPostData(IAsyncResult result)
{
var ae = AsyncEnumerator<string>.FromAsyncResult(result);
return ae.EndExecute(result);
}
private IEnumerator<int> PostData(AsyncEnumerator<string> ae, string url, string content)
{
var req = (HttpWebRequest)WebRequest.Create(url);
req.Method = "POST";
req.BeginGetRequestStream(ae.End(), null);
yield return 1;
using (var requestStream = req.EndGetRequestStream(ae.DequeAsyncResult()))
{
var bytes = Encoding.UTF8.GetBytes(content);
requestStream.BeginWrite(bytes, 0, bytes.Length, ae.end(), null);
yield return 1;
requestStream.EndWrite(ae.DequeueAsyncResult());
}
req.BeginGetResponse(ae.End(), null);
yield return 1;
using (var response = req.EndGetResponse(ae.DequeueAsyncResult()))
using (var responseStream = response.GetResponseStream())
using (var reader = new StreamReader(responseStream))
{
ae.Result = reader.ReadToEnd();
}
}
如您所見,私有PostData()
方法負責大部分工作。 有三種異步方法啟動,如三個yield return 1
語句所示。 使用此模式,您可以根據需要鏈接盡可能多的異步方法,並且仍然只將一個IAsyncResult
返回給調用者。
你要做的事情是可行的,但你需要創建一個新的IAsyncResult實現(類似“CompositeResult”,它監視第一個IAsyncResult,然后啟動第二個調用)。
但是,使用Reactive Extensions實際上這個任務實際上要容易得多 - 在這種情況下,您可以使用Observable.FromAsyncPattern將Begin / End方法轉換為返回IObservable的Func( 也代表異步結果),然后使用SelectMany鏈接它們:
IObservable<Stream> GetRequestStream(string Url);
IObservable<bool> MyOperation(Stream stream);
GetRequestStream().SelectMany(x => MyOperation(x)).Subscribe(x => {
// When everything is finished, this code will run
});
我真的不明白你想要實現什么,但我認為你應該重新考慮代碼。 IAsyncResult實例是允許處理異步方法調用的對象,它們是在通過BeginXXX執行異步調用時創建的。
在您的示例中,您基本上希望返回它尚不存在的IAsyncResult實例。
我真的不知道你要解決的問題是哪一個,但也許這些方法中的一種對你來說效果更好:
希望能幫助到你!
首先,從Jeffrey Richter的MSDN雜志文章“ 實現CLR異步編程模型 (2007年3月號)”中獲取AsyncResultNoResult
和AsyncResult<TResult>
實現代碼。
擁有這些基類后,您可以相對輕松地實現自己的異步結果。 在此示例中,我將使用您的基本代碼來啟動Web請求,然后將響應作為由多個內部異步操作組成的單個異步操作來獲取。
// This is the class that implements the async operations that the caller will see
internal class MyClass
{
public MyClass() { /* . . . */ }
public IAsyncResult BeginMyOperation(Uri requestUri, AsyncCallback callback, object state)
{
return new MyOperationAsyncResult(this, requestUri, callback, state);
}
public WebResponse EndMyOperation(IAsyncResult result)
{
MyOperationAsyncResult asyncResult = (MyOperationAsyncResult)result;
return asyncResult.EndInvoke();
}
private sealed class MyOperationAsyncResult : AsyncResult<WebResponse>
{
private readonly MyClass parent;
private readonly HttpWebRequest webRequest;
private bool everCompletedAsync;
public MyOperationAsyncResult(MyClass parent, Uri requestUri, AsyncCallback callback, object state)
: base(callback, state)
{
// Occasionally it is necessary to access the outer class instance from this inner
// async result class. This also ensures that the async result instance is rooted
// to the parent and doesn't get garbage collected unexpectedly.
this.parent = parent;
// Start first async operation here
this.webRequest = WebRequest.Create(requestUri);
this.webRequest.Method = "POST";
this.webRequest.BeginGetRequestStream(this.OnGetRequestStreamComplete, null);
}
private void SetCompletionStatus(IAsyncResult result)
{
// Check to see if we did not complete sync. If any async operation in
// the chain completed asynchronously, it means we had to do a thread switch
// and the callback is being invoked outside the starting thread.
if (!result.CompletedSynchronously)
{
this.everCompletedAsync = true;
}
}
private void OnGetRequestStreamComplete(IAsyncResult result)
{
this.SetCompletionStatus(result);
Stream requestStream = null;
try
{
stream = this.webRequest.EndGetRequestStream(result);
}
catch (WebException e)
{
// Cannot let exception bubble up here as we are on a callback thread;
// in this case, complete the entire async result with an exception so
// that the caller gets it back when they call EndXxx.
this.SetAsCompleted(e, !this.everCompletedAsync);
}
if (requestStream != null)
{
this.WriteToRequestStream();
this.StartGetResponse();
}
}
private void WriteToRequestStream(Stream requestStream) { /* omitted */ }
private void StartGetResponse()
{
try
{
this.webRequest.BeginGetResponse(this.OnGetResponseComplete, null);
}
catch (WebException e)
{
// As above, we cannot let this exception bubble up
this.SetAsCompleted(e, !this.everCompletedAsync);
}
}
private void OnGetResponseComplete(IAsyncResult result)
{
this.SetCompletionStatus(result);
try
{
WebResponse response = this.webRequest.EndGetResponse(result);
// At this point, we can complete the whole operation which
// will invoke the callback passed in at the very beginning
// in the constructor.
this.SetAsCompleted(response, !this.everCompletedAsync);
}
catch (WebException e)
{
// As above, we cannot let this exception bubble up
this.SetAsCompleted(e, !this.everCompletedAsync);
}
}
}
}
有些事情需要注意:
異步編程並不是最簡單的事情,但一旦理解了概念,它就會非常強大。
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