实现异步请求
System.Net 类使用 .NET 框架的标准异步编程模型对 Internet 资源进行异步访问。WebRequest 类的 BeginGetResponse 和 EndGetResponse 方法分别启动和完成对 Internet 资源的异步请求。
** 注意 ** 在异步回调方法中使用同步调用可能会导致严重的性能降低。通过 ** WebRequest ** 及其子代实现的 Internet 请求必须使用 Stream.BeginRead 读取由 WebResponse.GetResponseStream 方法返回的流。
下面的 C# 示例程序说明如何通过 WebRequest 类使用异步调用。该示例是一个控制台程序，它从命令行获得 URI，请求此 URI 处的资源，然后在从 Internet 接收数据的过程中在控制台上打印数据。
该程序定义了两个供自己使用的类：一个是 ** RequestState ** 类，它在异步调用间传递数据；另一个是 ClientGetAsync 类，它实现对 Internet 资源的异步请求。
** RequestState ** 类在服务于请求的异步方法调用间保留请求的状态。在 RequestState 类中，有包含当前资源请求和收到的响应流的 WebRequest 和 Stream 实例、包含当前从 Internet 资源接收到的数据的缓冲区和包含整个响应的 StringBuilder 实例。当 AsyncCallback 方法向 WebRequest.BeginGetResponse 注册时， RequestState 实例 (ar) 作为 state 参数传递。
** ClientGetAsync ** 类实现对 Internet 资源的异步请求，并将结果响应写到控制台。此类包含以下列表中描述的方法和属性。
- allDone 属性包含 ManualResetEvent 类的一个实例，该实例发出信号表示请求已完成。
- Main() 方法读取命令行并开始对指定 Internet 资源的请求。此方法创建 WebRequest 实例 wreq 和 RequestState 实例 ar，调用 BeginGetResponse 开始处理请求，然后调用 allDone.WaitOne() 方法，以使应用程序在回调完成后才退出。读取来自 Internet 资源的响应后，Main() 将响应写到控制台，然后应用程序结束。
- showusage() 方法将示例命令行写到控制台。如果命令行中没有提供 URI，Main() 将调用此方法。
- RespCallBack() 方法为 Internet 请求实现异步回调方法。此方法创建包含来自 Internet 资源的响应的 WebResponse 实例，获取响应流，然后开始从该流中异步读取数据。
- ReadCallBack() 方法实现读取响应流的异步回调方法。它将从 Internet 资源接收的数据传输到 RequestState 实例的 ResponseData 属性中，然后对响应流启动另一个异步读取，直到不再有数据返回为止。读取完所有数据后，ReadCallBack() 关闭响应流，并调用 allDone.Set() 方法以指示 ResponseData 中的响应是完整的。
** 注意 ** 关闭所有网络流至关重要。如果没有将所有的请求和响应流都关闭，应用程序将用完服务器连接，而无法处理其他请求。
[C#]
using System;
using System.Net;
using System.Threading;
using System.Text;
using System.IO;
// The RequestState class passes data across async calls.
public class RequestState
{
const int BufferSize = 1024;
public StringBuilder RequestData;
public byte[] BufferRead;
public WebRequest Request;
public Stream ResponseStream;
// Create Decoder for appropriate enconding type.
public Decoder StreamDecode = Encoding.UTF8.GetDecoder();
public RequestState()
{
BufferRead = new byte[BufferSize];
RequestData = new StringBuilder(String.Empty);
Request = null;
ResponseStream = null;
}
}
// ClientGetAsync issues the async request.
class ClientGetAsync
{
public static ManualResetEvent allDone = new ManualResetEvent(false);
const int BUFFER_SIZE = 1024;
public static void Main(string[] args)
{
if (args.Length < 1)
{
showusage();
return;
}
// Get the URI from the command line.
Uri httpSite = new Uri(args[0]);
// Create the request object.
WebRequest wreq = WebRequest.Create(httpSite);
// Create the state object.
RequestState rs = new RequestState();
// Put the request into the state object so it can be passed around.
rs.Request = wreq;
// Issue the async request.
IAsyncResult r = (IAsyncResult) wreq.BeginGetResponse(
new AsyncCallback(RespCallback), rs);
// Wait until the ManualResetEvent is set so that the application
// does not exit until after the callback is called.
allDone.WaitOne();
Console.WriteLine(rs.RequestData.ToString());
}
public static void showusage() {
Console.WriteLine("Attempts to GET a URL");
Console.WriteLine("\r\nUsage:");
Console.WriteLine(" ClientGetAsync URL");
Console.WriteLine(" Example:");
Console.WriteLine(" ClientGetAsync http://www.contoso.com/");
}
private static void RespCallback(IAsyncResult ar)
{
// Get the RequestState object from the async result.
RequestState rs = (RequestState) ar.AsyncState;
// Get the WebRequest from RequestState.
WebRequest req = rs.Request;
// Call EndGetResponse, which produces the WebResponse object
// that came from the request issued above.
WebResponse resp = req.EndGetResponse(ar);
// Start reading data from the response stream.
Stream ResponseStream = resp.GetResponseStream();
// Store the response stream in RequestState to read
// the stream asynchronously.
rs.ResponseStream = ResponseStream;
// Pass rs.BufferRead to BeginRead. Read data into rs.BufferRead
IAsyncResult iarRead = ResponseStream.BeginRead(rs.BufferRead, 0,
BUFFER_SIZE, new AsyncCallback(ReadCallBack), rs);
}
private static void ReadCallBack(IAsyncResult asyncResult)
{
// Get the RequestState object from AsyncResult.
RequestState rs = (RequestState)asyncResult.AsyncState;
// Retrieve the ResponseStream that was set in RespCallback.
Stream responseStream = rs.ResponseStream;
// Read rs.BufferRead to verify that it contains data.
int read = responseStream.EndRead( asyncResult );
if (read > 0)
{
// Prepare a Char array buffer for converting to Unicode.
Char[] charBuffer = new Char[BUFFER_SIZE];
// Convert byte stream to Char array and then to String.
// len contains the number of characters converted to Unicode.
int len =
rs.StreamDecode.GetChars(rs.BufferRead, 0, BUFFER_SIZE, charBuffer, 0);
String str = new String(charBuffer, 0, len);
// Append the recently read data to the RequestData stringbuilder
// object contained in RequestState.
rs.RequestData.Append(
Encoding.ASCII.GetString(rs.BufferRead, 0, read));
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