今天老师上课讲到了异步编程。
一个异步过程调用发出后，调用者不能立刻得到结果。基于事件机制，实际处理这个调用的部件在完成后，通过状态、通知和回调来通知调用者。比如，你现在要有一批数据要大数据要入库，你又不想一边入库一边等待返回结果，你可以用异步，将大数据推入一个队列，然后另外一个线程来操作这个队列里面的数据入库，入完了，就通知一下主线程。这段时间你的主线程可以做任何事。
首先提到了Asynchromous Programming Model，异步编写模型是一种模式，该模式允许用更少的线程去做更多的操作，.NET
Framework很多类也实现了该模式，同时我们也可以自定义类来实现该模式，（也就是在自定义的类中实现返回类型为IAsyncResult接口的BeginXXX方法和EndXXX
方法）委托类型也定义了BeginInvoke和EndInvoke方法。
以FileStream 为例
同步方法
public override int Read(byte[] array,int offset, int count);
异步方法
public override IAsyncResult BeginRead(byte[] array, int offset, int numBytes, AsyncCallback userCallback, object stateObject);
结束异步的操作：
public override int EndRead(IAsyncResult asyncResult);
APM提供了四种方式供开发人员选择：
1. 在调用BeginXxx方法的线程上调用EndXxx方法来得到异步操作的结果，但是这种方式会阻塞调用线程，知道操作完成之后调用线程才继续运行
2. 查询IAsyncResult的syncWaitHandle属性，从而得到WaitHandle，然后再调用它的WaitOne方法来使一个线程阻塞并等待操作完成再调用EndXxx方法来获得操作的结果。
3. 循环查询IAsyncResult的IsComplete属性，操作完成后再调用EndXxx方法来获得操作返回的结果。
4. 使用 AsyncCallback委托来指定操作完成时要调用的方法，在操作完成后调用的方法中调用EndXxx操作来获得异步操作的结果。
推荐使用第四种方法：
using System;
using System.Net;
using System.Net.Sockets;
using System.Threading;
using System.Collections.Specialized;
using System.Collections;
namespace Examples.AdvancedProgramming.AsynchronousOperations
{
public class UseDelegateForAsyncCallback
{
static int requestCounter;
static ArrayList hostData = new ArrayList();
static StringCollection hostNames = new StringCollection();
static void UpdateUserInterface()
{
// Print a message to indicate that the application
// is still working on the remaining requests.
Console.WriteLine("{0} requests remaining.", requestCounter);
}
public static void Main()
{
// Create the delegate that will process the results of the
// asynchronous request.
AsyncCallback callBack = new AsyncCallback(ProcessDnsInformation);
string host;
do
{
Console.Write(" Enter the name of a host computer or <enter> to finish: ");
host = Console.ReadLine();
if (host.Length > 0)
{
// Increment the request counter in a thread safe manner.
Interlocked.Increment(ref requestCounter);
// Start the asynchronous request for DNS information.
Dns.BeginGetHostEntry(host, callBack, host);
}
} while (host.Length > 0);
// The user has entered all of the host names for lookup.
// Now wait until the threads complete.
while (requestCounter > 0)
{
UpdateUserInterface();
}
// Display the results.
for (int i = 0; i< hostNames.Count; i++)
{
object data = hostData [i];
string message = data as string;
// A SocketException was thrown.
if (message != null)
{
Console.WriteLine("Request for {0} returned message: {1}",
hostNames[i], message);
continue;
}
// Get the results.
IPHostEntry h = (IPHostEntry) data;
string[] aliases = h.Aliases;
IPAddress[] addresses = h.AddressList;
if (aliases.Length > 0)
{
Console.WriteLine("Aliases for {0}", hostNames[i]);
for (int j = 0; j < aliases.Length; j++)
{
Console.WriteLine("{0}", aliases[j]);
}
}
if (addresses.Length > 0)
{
Console.WriteLine("Addresses for {0}", hostNames[i]);
for (int k = 0; k < addresses.Length; k++)
{
Console.WriteLine("{0}",addresses[k].ToString());
}
}
}
}
// The following method is called when each asynchronous operation completes.
static void ProcessDnsInformation(IAsyncResult result)
{
string hostName = (string) result.AsyncState;
hostNames.Add(hostName);
try
{
// Get the results.
IPHostEntry host = Dns.EndGetHostEntry(result);
hostData.Add(host);
}
// Store the exception message.
catch (SocketException e)
{
hostData.Add(e.Message);
}
finally
{
// Decrement the request counter in a thread-safe manner.
Interlocked.Decrement(ref requestCounter);
}
}
}
}异步对可能起阻止作用的活动（例如，应用程序访问 Web 时）至关重要。 对Web 资源的访问有时很慢或会延迟。 如果此类活动在同步过程中受阻，则整个应用程序必须等待。 在异步过程中，应用程序可继续执行不依赖 Web 资源的其他工作，直至潜在阻止任务完成。
async Task<int> AccessTheWebAsync()
{
<span style="white-space:pre"> </span>HVpClient client = new HVpClient();
<span style="white-space:pre"> </span>Task<string> getStringTask = client.GetStringAsync("hVp://msdn.microsod.com");
<span style="white-space:pre"> </span>DoIndependentWork();
<span style="white-space:pre"> </span>string urlContents = await getStringTask;
<span style="white-space:pre"> </span>return urlContents.Length;
}HttpClient是接收HttpResponseMessages和发送HttpRequestMessages的主要类，如果你习惯了使用WebClient或者是HttpWebRequest， 需要注意HttpClient和他们
不同的地方：
1、在HttpClient实例上配置扩展，设置默认的头部，取消未完成的请求和更多的设置。
2、你通过一个单一的HttpClient实例，它有自己的连接池。
3、HttpClients不与特定的HTTP服务器绑定,你可以使用相同的HttpClient实例提交任何HTTP请求。
4、你可以用HttpClient为特定的站点创建特殊的Client
5、HttpClient采用新的型模式处理异步请求使它更容易管理和协调更多的请求。
HttpClient - Get
<pre name="code" class="csharp">client.GetAsync(_address).ConenueWith((requestTask)=>
{
//Get HTTP response from completed task.
HttpResponseMessage response = requestTask.Result;
//Check that response was successful or throw exception
response.EnsureSuccessStatusCode();
//Read response asynchronously as JsonValue and write out top facts for each country
response.Content.ReadAsAsync<JsonArray>().ConenueWith((readTask)=>{
Console.WriteLine("First 50 countries listed by The World Bank...");
foreach(var country in readTask.Result[1])
{
Console.WriteLine("{0},Country Code:{1},Capital:{2},Laetude:{3},Longitude:{4}",country.Value["name"],
country.Value["iso2Code"],country.Value["capitalCity"],country.Value["laetude"],country.Value["longitude"]);
}
});
});Uri serviceReq = new Uri(serviceAddress);
HttpClient client = new HttpClient();
HttpContent content = new StringContent(@"{ ""value"": ""test""}");
content.Headers.ContentType = new MediaTypeHeaderValue("application/json");
// Send a request asynchronously continue when complete
client.PostAsync(serviceReq, content).ContinueWith(
(requestTask) =>
{
// Get HTTP response from completed task.
HttpResponseMessage response = requestTask.Result;
// Check that response was successful or throw exception
response.EnsureSuccessStatusCode();
// Read response asynchronously as JsonValue and write out top facts for each country
response.Content.ReadAsAsync<string>().ContinueWith(
(readTask) =>
{
Console.WriteLine(readTask.Result);
});
});